Download PHOTOSYNTHESIS AND PHOTORESPIRATION IN THREE C4

UNIVERSIDADE DE LISBOA
FACULDADE DE CIÊNCIAS
DEPARTAMENTO DE BIOLOGIA VEGETAL
PHOTOSYNTHESIS AND PHOTORESPIRATION IN THREE
C4 GRASSES OF DIFFERENT METABOLIC SUB-TYPES,
UNDER WATER STRESS
Ana Elizabete do Carmo Silva
DOUTORAMENTO EM BIOLOGIA
(Fisiologia e Bioquímica)
2008
UNIVERSIDADE DE LISBOA
FACULDADE DE CIÊNCIAS
DEPARTAMENTO DE BIOLOGIA VEGETAL
PHOTOSYNTHESIS AND PHOTORESPIRATION IN THREE
C4 GRASSES OF DIFFERENT METABOLIC SUB-TYPES,
UNDER WATER STRESS
Ana Elizabete do Carmo Silva
DOUTORAMENTO EM BIOLOGIA
(Fisiologia e Bioquímica)
Tese orientada pela Professora Doutora Maria Celeste Arrabaça
2008
The experimental work leading to this PhD was done in collaboration
with
ROTHAMSTED RESEARCH
Department of Plant Sciences
Under the supervision of Professor Dr. Martin A. J. Parry
And close advice of Dr. Alfred J. Keys
In addition to the supervision and orientation of Professor Celeste Arrabaça, Professor Martin
Parry and Dr. Alfred Keys, the work here presented involved the collaboration with other
Professors and Researchers. The four chapters of results contained in the Thesis (II, III, IV and
V) correspond to the integral copy or extended versions of manuscripts submitted (or in final
phase of preparation to be submitted) for publication in peer-reviewed journals. The relative
contributions of each author apart from myself and the supervisors and advisors, who were
involved in the planning of the experiments, discussion of the results and elaboration of
manuscripts, are mentioned in the beginning of each chapter.
Para os devidos efeitos no n.º 2 do Art.8º do Decreto-Lei 388/70, o autor da tese declara que
participou na execução do trabalho experimental descrito, bem como na análise e interpretação
dos resultados e na redação dos textos e manuscritos submetidos para publicação.
Ana Elizabete do Carmo Silva
Março de 2008
Acknowledgements / Agradecimentos
ACKNOWLEDGEMENTS / AGRADECIMENTOS
I am grateful to Fundação para a Ciência e a Tecnologia (Portugal) for financial support (PhD
grant SFRH/BD/13730/2003), and to Faculdade de Ciências da Universidade de Lisboa
(Portugal) and Rothamsted Research (UK) for receiving me and providing the conditions to do
the work here presented. The grass seeds used were kindly provided by AgResearch, Margot
Forde Forage Germplasm Centre, New Zealand and Alípio Dias & Irmão, Lda., Portugal.
Many people have somehow contributed to the successful realization of this PhD – many thanks
to all!!! Here I can only express a special acknowledgement to a few of them!... Muitos foram
aqueles que de alguma forma, directa ou indirectamente, contribuíram para a realização deste
Doutoramento a bom termo – a todos, um grande bem-haja! Aqui, posso apenas agradecer a
apenas algumas dessas pessoas!
Professora Doutora Maria Celeste Arrabaça pela sugestão do tema deste trabalho, pelo incentivo
à sua execução e pelo acolhimento no Centro de Engenharia Biológica da FCUL. Agradeço-lhe a
orientação e supervisão do trabalho, as demais lições científicas e afins, incluindo a da
preserverança e a importância de manter a calma, e a disponibilidade que sempre mostrou para
discutir os mais diversos aspectos que surgiram ao longo da sua realização. Agradeço-lhe
também toda a amizade e o carinho.
Professor Dr. Martin Parry for receiving me at Rothamsted Research and showing me other faces
of Science. I have learnt so many lessons while there, I have grown so much! I will always be
thankful for the support, for the pushes applied when needed and for all the words and lessons –
it did make a difference!
Dr. Alfred Keys, Alf… How can I find words that express the gratefulness for the many things
you did for me? You were the main reason that made me to go to the UK and I will always owe
you so much! The support, care and friendship, all the advices and the scientific or life-related
discussions, the tears and the smiles… It is hard to imagine life far from you but (like with other
parental relations) children must have wings and use them to fly around the world!
Todos os demais Professores e colegas do Departamento de Biologia Vegetal e do Centro de
Engenharia Biológica da FCUL! Nomeadamente (e jamais exclusivamente!)... Prof. João
Arrabaça pelo grande carinho (às vezes bem disfarçado!) com que me acolheu, pelo despertar do
interesse pela cultura e pela transmissão de tão diversos e surpreendentes conhecimentos à
“joven cientista”! Prof. Anabela Bernardes da Silva, Belucha, por me ter puxado para a
Fisiologia e Bioquímca, pela amizade, pelos abraços e seus substitutos, pela transmissão de
valores e conhecimentos tão importantes e pelas longas discussões - enzimáticas e afins! Prof.
Jorge Marques da Silva, por todos os incentivos que transmitiu, directa ou indirectamente, pelos
conhecimentos de stress hídrico e de fisiologia - alguns um tanto ou quanto fluorescentes! Dra.
Ana Rita Matos pela lufada de ar fresco que trouxe ao CEB e pela capacidade de “apaziguação”.
Ana Sofia Soares pelas aventuras e alegrias partilhadas no início de um percurso que acabou por
tomar rumos diferentes! Prof. Cristina Cruz pela energia contagiante e pela recepção de braços
abertos nas rápidas “visitas” ao laboratório da Ecologia! Prof. Cristina Máguas e Dr. Rodrigo
Maia pelo apoio na determinação de composições isotópicas. Prof. Lia Ascensão pelo
acolhimento no “cantinho da Microscopia” e pela transmissão de valores morais e científicos!
Prof. Renata Meira (visitante do Brazil!) pela ajuda sempre pronta e pelo bom humor
contagiante! Prof. Rui Malhó, Rui, pelo despertar de uma energia conquistadora e pela forma
diferente de iniciar o dia, com sorrisos partilhados!! Manuela Lucas, Manelita, por ser uma
segunda mãe, por tomar conta de mim, por todas as ajudas e por ser a maior mestra na “lei do
desenrasque” – arranja sempre uma solução para o problema do próximo!
i
Acknowledgements / Agradecimentos
All the researchers and colleagues at Rothamsted Research! John Andralojc for all the jokes and
the help with Rubisco-related measurements and Pippa Madgwick for having a nice and
comforting word always ready! Jeroni Galmés for sharing the hot lab, for the “paella” and for the
company in beautiful Scotland! Marcela Baúdo, for being another Speedy Gonzalez (!!!) and for
the deep breaths shared while planning drought experiments... Guy Kiddle for the advices on the
HPLC, on the amino acid identification and also the ones on scientific life! Till Pellny for the
technical, scientific and life advices and, most of all, for having fun teasing me!!! Simon Driscoll
for the technical support on the gas-exchange measurements and for the knowledge transmitted
on the subject. Also for sharing his life story and his music! Riekert van Heerden for the advices
on proline estimation, on lab working and energy level maintenance!!! Richard Parkinson, Kate
Rydlewski and Steve Harvey, who helped me to take care of my “little girls” in the greenhouse
28, compartment 106!!! Stephen Powers for all the statistical analysis and advices, the highhorses and the long (but productive) discussions, the support and comforting words and,
especially, for Believing me as a scientist from the very first moment!
Amigos que conheci na FCUL! Entre os quais… André, pela loucura constante, Márcia pela boa
disposição e energia positiva e Elsita pelos mimos tão gostosos! Cátia pela partilha de suspiros
durante frases escritas e outros rabiscos que tal! Céu pelo melhor sorriso da manhã na FCUL!
All the good friends I met abroad! Namely… Rui for being my salvation at the Manor! Isabel for
being a good listener and a good advisor, on PhDs and Life! Marta for sharing the experience on
stress studies, Anneke for being a company in the late hours, Hiro for being pure, Francesca for
being like a sister, Alexia for the joy! Maria Paz for being crazy, Nisreen for being a good friend,
Ivânia and Luis for making part of the Portuguese mafia at RRes! Salvador for the sports
company and for the friendship! Duncan for being such a good listener!!! My chocolate supplies
will always be there… Eleonora for the smile and the NB copies! Tanya for the contagious
energy!!!
Amigos em Casa... Sílvia Duarte, conselheira de saúde! Catarina Oliveira, assitente de imagem:
a beleza é um conceito relativo! Bruno Serrão, assistente de informática – o que seria do meu
portátil sem as tuas consultas terapêuticas!!! Ana Francisco, Anita, mesmo que distantes, não nos
ausentamos de todo, estamos lá e sabemos disso! Foi um prazer enfrentar o microscópio
contigo!!! Ana Catarino, Nocas, pela vontade contagiante de vencer nesta conquista por um lugar
ao sol e de conhecer o mundo lá fora! Inês Bruno, por estar sempre a meu lado neste percurso
que temos feito pela Vida - de alguma forma sei que vais estar sempre lá!
Friends at my second Home! Petra Bleeker, Peet, fofa, for being my older sister!!! Thanks for all
the advices!!! Samuel Doufur, Sam, for all the Chemistry explanations, for the secret supplies…
Also for the culture lessons and cooking sessions, for the constant company and support, even
during the worst hours… and, most of all, for being such a nice friend!
Miguel pelos bons momentos que passámos juntos, o percurso de vida que caminhámos lado a
lado, muitas vezes afastados... O apoio incontestável em todas as decisões difíceis, mesmo que
muito discutidas e refutadas! A lição de vida que ficou e o carinho que permanece, sempre.
À minha família agradeço sobretudo a compeensão. Não é fácil colocar em palavras o quão
importante foi para mim o apoio constante que recebi da vossa parte e a força que me dá, todos
os dias, o Amor que nos une! A Cat e a Ju, pataca e pataquinha, que são crianças a sério,
percebem o quanto as amo e aceitam que vá “lá para a Inglaterra”! O Daniel partilha uma certa
estória e tem crescido muito rápido nesta Vida! Mãe, unidas pela distância e partilhando aquela
forma especial de ver e de sentir! Pai… o que sou hoje resulta em grande parte daquilo que me
ensinaste a Ser!
OBRIGADA!
ii
“Wisdom, Happiness and Courage
Are not waiting somewhere (…) at the end of a straight line;
They’re part of a continuous cycle that begins right here.
They’re not only the ending, but the beginning as well.
The more it snows, the more it goes on snowing.”
“A Sabedoria, a Felicidade e a Coragem
Não estão à espera algures (…) ao fundo de uma linha recta;
São parte de um ciclo contínuo que começa aqui mesmo.
Não são apenas o fim, mas também o princípio.
Quanto mais neva, mais continua a nevar.”
Benjamin Hoff in The Tao of Pooh
iii
Contents
CONTENTS
Summary ........................................................................................................................................ 1
Resumo .......................................................................................................................................... 3
Chapter I. General introduction and objectives ........................................................................... 7
C4 photosynthetic metabolism ................................................................................................. 17
Drought stress .......................................................................................................................... 23
Objectives of the Thesis ........................................................................................................... 31
References ................................................................................................................................ 32
Chapter II. Water relations and leaf anatomy of C4 grasses ...................................................... 47
Abstract .................................................................................................................................... 49
Introduction .............................................................................................................................. 50
Material and methods .............................................................................................................. 53
Results ...................................................................................................................................... 59
Discussion ................................................................................................................................ 71
References ................................................................................................................................ 79
Chapter III. Photorespiration and C4 photosynthesis under drought stress ............................... 85
Abstract .................................................................................................................................... 87
Introduction .............................................................................................................................. 88
Material and methods .............................................................................................................. 91
Results ...................................................................................................................................... 98
Discussion .............................................................................................................................. 107
References .............................................................................................................................. 115
Chapter IV. C4 enzymes in drought-stressed grasses................................................................ 121
Abstract .................................................................................................................................. 123
Introduction ............................................................................................................................ 124
Material and methods ............................................................................................................ 127
Results .................................................................................................................................... 131
Discussion .............................................................................................................................. 136
References .............................................................................................................................. 140
Chapter V. Rubisco from C4 grasses under drought stress .. .................................................... 145
Abstract .................................................................................................................................. 147
Introduction ............................................................................................................................ 148
Material and methods ............................................................................................................ 152
Results .................................................................................................................................... 159
Discussion .............................................................................................................................. 167
References .............................................................................................................................. 172
Chapter VI. General discussion and conclusions ..................................................................... 179
Concluding remarks ............................................................................................................... 192
Future perspectives ................................................................................................................ 193
References .............................................................................................................................. 194
v
List of Symbols and Abbreviations Used
LIST OF SYMBOLS AND ABBREVIATIONS USED
ABA
ACC
ADP
Ala
AlaAT
AMP
Asn
Asp
AspAT
ATP
Bicine
BS
Ca
Ci
Cm
Cs
CA
CA1P
CABP
Chl
net CO2 assimilation rate
net CO2 assimilation rate calculated from quadratic expression for enzyme-limited
photosynthesis
net CO2 assimilation rate calculated from the asymptotic exponential curve applied to the
variation of A with Ci
abscisic acid
1-aminocyclopropane-1- carboxylic acid
adenosine 5’-diphosphate
alanine
alanine aminotransferase
adenosine monophosphate
asparagine
aspartate
aspartate aminotransferase
adenosine 5’-triphosphate
N,N-bis(2-hydroxy-ethyl)glycine
bundle sheath
atmospheric CO2 concentration (in the gas phase)
CO2 concentration in the intercellular air-spaces
CO2 concentration in the mesophyll cells
CO2 concentration in the bundle sheath cells
carbonic anhydrase
2-carboxyarabinitol-1-phosphate
2-carboxyarabinitol-1,5-bisphosphate
chlorophyll
δ
DTT
DW
EDTA
E4P
isotope composition
1,4-dithiothreitol
dry weight
ethylenediaminetetraacetic acid
erythrose-4-phosphate
γ*
gbs
gi
gswa
Gln
Glu
Gly
Hepes
HNV
FW
GOGAT
Kc
half the reciprocal of Rubisco specificity
bundle sheath conductance to CO2
mesophyll conductance to CO2
stomatal conductance to water vapour
glutamine
glutamate
glycine
4-(2-hydroxy-ethyl)-1-piperazine-ethanesulfonic acid
5-hydroxy-L-norvaline (or 2-amino-5-hydroxypentanoic acid)
fresh weight
ferredoxin (Fd-) or NADH-glutamate synthase
Michaellis-Menten constant of Rubisco for CO2
A
Ac
An
vii
List of Symbols and Abbreviations Used
Ko
Kp
KRuBP
LN2
LSD
LWP
M
MDH
MEA
Met
MS
NAD-ME
NAD+
NADH
NADP-ME
NADP+
NADPH
n.d.
O
O2
OAA
P
PDBP
PEG
PEP
PEPC
PEPCK
PG
PGA
Phe
Pi
PPdK
PPFD
PPi
Pr
PVP
Rd
Rm
R2
REML
ROS
Rubisco
Michaellis-Menten constant of Rubisco for O2
Michaellis-Menten constant of PEPC for CO2
Michaellis-Menten constant of Rubisco for RuBP
liquid nitrogen
least significant difference
leaf water potential
mesophyll
malate dehydrogenase
monoethanolamine
methionine
moderate drought stress
NAD-malic enzyme
nicotinamide-adenine dinucleotide (oxidized)
nicotinamide-adenine dinucleotide (reduced)
NADP- malic enzyme
nicotinamide-adenine dinucleotide phosphate (oxidized)
nicotinamide-adenine dinucleotide phosphate (reduced)
not determined
O2 partial pressure in the bundle sheath and mesophyll cells
oxygen
oxaloacetate
probability or level of significance
D-glycero-2,3-diulose-1,5-bisphosphate
polyethylene glycol
phosphoenolpyruvate
phosphoenolpyruvate carboxylase
phosphoenolpyruvate carboxykinase
2-phosphoglycolate
3-phosphoglycerate
phenylalanine
orthophosphate
pyruvate,orthophosphate dikinase
photosynthetic photon flux density
pyrophosphate
rate of photorespiration
polyvinylpyrrolidone
leaf mitochondrial respiration
mesophyll mitochondrial respiration
percentage of variance accounted for by a model
Residual Maximum Likelihood
reactive oxygen species
ribulose-1,5-bisphosphate carboxylase/oxygenase
Ru5P
ribulose-5-phosphate
RuBP
RWC
ribulose-1,5-bisphosphate
leaf relative water content
viii
List of Symbols and Abbreviations Used
S
s2
s.e.
SED
Ser
SLA
SS
SWC
TCA
TFA
TW
Vc
Vcmax
Vmax
Vo
Vomax
Vp
Vpmax
Vphysiol
WUE
WWP
non-watered or drought-stressed
residual mean square
standard error of the mean
standard error of differences
serine
specific leaf area
severe drought stress
soil water content
tricarboxylic acid
trifluoroacetic acid
turgid weight
rate of Rubisco carboxylation
maximum Rubisco carboxylation activity
maximal activity
rate of Rubisco oxygenation
maximum Rubisco oxygenation activity
rate of PEPC carboxylation
maximum PEPC carboxylation activity
physiological activity
water use efficiency
water weight in pot
ix
Summary
SUMMARY
Drought stress is one of the major constraints to plant photosynthetic performance. With
increasing water scarcity in many areas of the world the understanding of fundamental droughtstress physiology and biochemistry is crucial to optimize the exploitation of plant responses and
improve water use efficiency. The CO2-concentrating mechanism present in C4 plants limits
photorespiration and increases their photosynthetic performance. The physiological and
biochemical responses to gradually-induced water deficit of three C4 grasses with different
decarboxylating mechanisms were compared. Paspalum dilatatum Poir. (NADP-ME) was less
resistant to drought than Cynodon dactylon Pers. (NAD-ME) and Zoysia japonica Steudel
(PEPCK). The faster leaf dehydration in the first species reflects the higher water requirement
associated with its high productivity. Leaf structure of all three species, especially C. dactylon
and Z. japonica, showed advantageous characteristics to cope with xeric environments. In all
three species photosynthesis and stomatal conductance decreased under water deficit but showed
continued CO2 assimilation even when leaf dehydration was severe. Photorespiration, as
evaluated by CO2 exchange rates at different concentrations of CO2 and O2, by mechanistic
modelling of C4 photosynthesis and by changes in amino acids in a 30 s dark period, remained
slow under drought conditions, supporting the photosynthetic efficiency of the CO2concentrating mechanism. Moderate drought stress did not affect dramatically the carboxylating
and decarboxylating enzyme activities but caused changes in the regulation of PEPC and
Rubisco. Considerable activity of PEPCK was found in all three species suggesting its
involvement in the C4 photosynthetic pathway, acting as a secondary decarboxylase in P.
dilatatum and C. dactylon, or in other non-photosynthetic processes occurring in the leaves of
the three C4 grasses. An unusual hydroxylated amino acid was drought-inducible and its
potential benefits for drought resistance should be further investigated.
KEYWORDS:
Water deficit, C4 photosynthesis, Photorespiration, Paspalum dilatatum, Cynodon dactylon,
Zoysia japonica
1
Resumo
RESUMO
O défice hídrico é um dos factores que mais limitam a produtividade vegetal. A escassez de água
é um problema que atinge proporções cada vez maiores em muitas áreas do planeta e tende a ser
exacerbado devido às reconhecidas alterações climáticas. A implementação de práticas de rega
mais correctas, envolvendo monitorização das condições hídricas do solo e/ou das plantas, bem
como a utilização de espécies e variedades de plantas melhor adaptadas a condições de baixa
disponibilidade de água poderão contribuir para uma mais eficiente gestão dos recursos hídricos.
O conhecimento da fisiologia e bioquímica em condições de stress são fundamentais para que as
respostas adaptativas das plantas a condições xéricas possam ser exploradas e a eficiência do uso
da água melhorada.
A pressão exercida sobre os diferentes utilizadores de água é assim cada vez maior e
afecta diversas áreas, incluindo a agricultura e o turismo. O crescente número de campos de
golfe em Portugal implica gastos hídricos adicionais que são muitas vezes criticados, sobretudo
nos verões em que a seca é mais severa e a necessidade de água para a produção agrícola torna o
desporto uma necessidade supérflua. A utilização de espécies e variedades de relvas mais
adequadas (que gastem menos água mantendo elevada performance) aliada a práticas de rega que
envolvam, por exemplo, reutilização de águas residuais irá contribuir para uma maior eficiência
do uso da água disponível.
O mecanismo de concentração de CO2 presente nas plantas com metabolismo
fotossintético em C4 torna-as menos sensíveis ao oxigénio e a resultante limitação da
fotorrespiração nestas plantas contribui para o aumento da eficiência do uso de água. As relvas
C4 estão, por esse motivo, geralmente associadas a uma maior resistência ao défice hídrico, mas
não há necessariamente uma relação directa entre eficiência de uso de água e resistência à seca.
A distribuição das espécies C3 e C4 sugere que a temperatura é o principal factor que se
correlaciona com a ocorrência de cada uma das variantes fotossintéticas. A precipitação, por
outro lado, parece estar relacionada coma distribuição diferencial dos vários subtipos de
fotossíntese em C4. As espécies do subtipo NADP-ME (enzima málico dependente do NADP)
são geralmente mais abundantes em áreas com maior precipitação e as espécies do subtipo NADME (enzima málico dependente do NAD) estão mais representadas em zonas mais secas,
enquanto que a distribuição das espécies do subtipo PEPCK (fosfoenolpiruvato carboxicinase)
em relação aos gradientes de precipitação apresenta um padrão mais incerto.
As respostas fisiológicas e bioquímicas ao défice hídrico foram estudadas em três
espécies de relvas C4 dos diferentes subtipos metabólicos: Paspalum dilatatum Poir. (NADP3
Resumo
ME), Cynodon dactylon Pers. (NAD-ME) e Zoysia japonica Steudel (PEPCK). As três espécies
foram comparadas em termos da presença de mecanismos de resistência ao stress envolvendo a
manutenção dos conteúdos hídricos da planta ou a tolerância a condições de desidratação foliar.
Uma abordagem inicial envolveu o estudo das relações hídricas e da estrutura foliar, a que se
seguiu o estudo das trocas gasosas, da fotorrespiração e de outros aspectos do metabolismo
fotossintético relativos aos processos de carboxilação e descarboxilação.
As plantas de P. dilatatum foram menos resistentes ao défice hídrico, observando-se uma
diminuição mais rápida do teor hídrico relativo das suas folhas que nas plantas de C. dactylon e
Z. japonica. As folhas das três espécies são caracterizadas por uma estrutura foliar especializada,
com anatomia foliar Kranz, que permite uma maior eficiência da utilização dos recursos hídricos.
A espécie Z. japonica, em particular, tem paredes celulares mais rígidas e possui maior
quantidade de esclerênquima, o que se traduz numa maior rigidez e dureza das folhas. A rigidez
celular permite a diminuição da pressão de turgescência quando o teor hídrico começa a diminuir,
promovendo o abaixamento do potencial hídrico das folhas, que foi mais acentuado em Z.
japonica e pode ser visto como uma estratégia que permite o aumento da tomada de água do solo
à medida que a sua disponibilidade diminui. A capacidade de acumulação de osmólitos,
incluindo a prolina e outros aminoácidos, permite aumentar a pressão osmótica e contribuem
assim para a diminuição do potencial hídrico. No entanto, o aumento de aminoácidos como a
prolina, metionina, valina, fenilalanina, isoleucina e leucina sugere também o seu envolvimento
directa ou indirectamente (através da produção de compostos do metabolismo secundário) em
mecanismos de defesa, nomeadamente protecção contra espécies reactivas de oxigénio e
utilização de poder redutor em excesso. Nas folhas de C. dactylon e Z. japonica foi identificado
um aminoácido hidroxilado não-proteico que não tinha sido previamente descrito como estando
presente em folhas e que apresenta características que devem ser exploradas. O aminoácido 5hidroxi-L-norvalina (HNV) ocorre em folhas de plantas de Z. japonica bem irrigadas e o seu
conteúdo aumenta com o défice hídrico. Em C. dactylon HNV não está presente nas folhas de
plantas bem irrigadas mas surge e o seu conteúdo aumenta acentuadamente com a desidratação
foliar. Em P. dilatatum o aminoácido não está presente e não é induzido pelo stress. Associando
este comportamento à elevada resistência à seca de C. dactylon e Z. japonica sugere-se que este
aminoácido poderá estar intimamente associado ao mecanismo de defesa destas plantas.
A anatomia foliar das plantas das três gramíneas não foi muito alterada em condições de
stress e sugere que as folhas destas espécies apresentam características intrínsecas que lhes
permitem uma melhor adaptação a condições de reduzida disponibilidade de água, conferindolhes uma maior eficiência no uso de água. O aumento da proporção de células do mesófilo em
4
Resumo
relação às células da bainha perivascular em P. dilatatum e Z. japonica em condições de stress
sugere alguma capacidade de adaptação ao défice hídrico, mas a ausência de maiores alterações
morfológicas induzidas pelo stress reflecte a baixa plasticidade fenotípica das três espécies.
O mecanismo de concentração de CO2 característico do metabolismo fotossintético em
C4 limita a reacção oxigenativa da Rubisco e, consequentemente, a taxa de fotorrespiração. No
entanto, o fecho dos estomas em condições de défice hídrico pode levar à diminuição da
concentração intercelular de CO2 e ao aumento da fotorrespiração. Nas três gramíneas estudadas
houve uma diminuição da taxa de fotossíntese e da condutância estomática com a desidratação
foliar, mas as trocas gasosas continuaram mesmo quando o teor hídrico diminuiu para valores
inferiores a 80%, correspondente a cerca de 10 dias sem irrigação e um reduzido conteúdo de
água no solo. O decréscimo da fotossíntese com o aumento da concentração de oxigénio e a
diminuição do conteúdo em glicina ao fim de um período de 30 s em escuridão evidenciaram a
presença de taxas de fotorrespiração, embora muito mais lentas nestas três espécies C4 que em
plantas C3. A análise conjunta das trocas gasosas de CO2 a diferentes concentrações de CO2 e O2,
incluindo o ponto de compensação para o CO2 e sensibilidade da fotossíntese ao oxigénio e a
aplicação de um modelo de fotossíntese em C4 aos dados experimentais revelaram a manutenção
de taxas de fotorrespiração lentas em condições de défice hídrico. A variação do conteúdo em
aminoácidos, especialmente glicina, na fracção solúvel das folhas, suportou a capacidade das três
espécies estudadas para limitar a velocidade de oxigenação da RuBP mesmo em condições de
défice hídrico. O aumento do conteúdo em glicina, serina e etanolamina com a desidratação
foliar bem como os resultados comparativos da modelação das trocas gasosas em condições
controlo e de stress forneceram os únicos indícios para um ligeiro aumento do metabolismo
fotorrespiratório em relação à fotossíntese.
Para uma maior eficiência do metabolismo fotossintético em C4 é necessário haver uma
boa coordenação entre a carboxilação primária do CO2, levada a cabo pela PEPC no mesófilo, e
a subsequente descarboxilação dos ácidos C4 na bainha perivascular. As actividades da PEPC,
NADP-ME, NAD-ME e PEPCK não foram muito afectadas pelo défice hídrico e consideradas
no seu conjunto sugeriram uma maior eficiência fotossintética em C. dactylon que nas outras
duas espécies. O estado de fosforilação da PEPC aumentou, revelando uma potencial resposta
adaptativa das três gramíneas C4 ao stress no sentido de tornar a carboxilação primária do CO2
mais eficiente em condições que promovem o fecho dos estomas, reduzindo a disponibilidade de
CO2 a nível intercelular. Curiosamente, observaram-se consideráveis actividades da PEPCK nas
três espécies, pertencentes a cada um dos subtipos metabólicos de plantas C4, revelando que a
enzima poderá actuar como descarboxilase alternativa nas plantas de P. dilatatum e C. dactylon
5
Resumo
(subtipos NADP-ME e NAD-ME, respectivamente) ou desempenhar funções no metabolismo
não-fotossintético das três espécies. As potencialidades associadas à presença de elevadas
actividades da PEPCK em gramíneas C4 dos vários subtipos metabólicos devem ser investigadas
em maior detalhe no sentido de aumentar a compreensão do metabolismo fotossintético em C4.
Em condições de défice hídrico, a diminuição das taxas de assimilação de CO2 implica
que a actividade da Rubisco seja regulada de modo a manter o equilíbrio entre os diferentes
processos do metabolismo fotossintético. A diminuição do conteúdo em RuBP nas folhas
desidratadas das três espécies sugere que a regeneração deste substrato é afectada em condições
de défice hídrico, possivelmente devido a danos ao nível do ciclo de Calvin ou reduzida síntese
de ATP, limitando a assimilação de CO2. O aumento de inibidores nas mesmas folhas sugere que
estes se ligam à Rubisco em condições de stress protegendo a enzima contra o dano proteolítico,
não se observando alterações no conteúdo em Rubisco.
A aplicação de modelos de fotossíntese para simular os efeitos induzidos por alterações
ambientais tem sido comprometida pela falta de conhecimento de vários parâmetros
fundamentais e pela quantidade de pressupostos inerentes á sua utilização. As constantes
cinéticas para as actividades carboxilativa e oxigenativa da Rubisco parcialmente purificada de
cada uma das três gramíneas C4 estudadas irá permitir aumentar o rigor e a precisão com que se
fazem simulações baseadas nestes modelos. Em comparação com a espécie C3 utilizada como
modelo, o trigo, a Rubisco de P. dilatatum, C. dactylon e Z. japonica é caracterizada por factores
de especificidade mais baixos, e constantes de Michaelis-Menten para o CO2 e o O2 e
velocidades de carboxilação máxima mais elevadas.
As três espécies de gramíneas C4 estudadas apresentam elevada eficiência fotossintética,
sobretudo C. dactylon, e elevada resistência ao défice hídrico, sobretudo C. dactylon e Z.
japonica. O metabolismo fotossintético é regulado de modo adaptativo e a limitação da taxa de
fotorrespiração mantém-se em condições de stress. As potencialidades associadas ao aumento do
conteúdo em HNV com a desidratação foliar e à presença de elevadas actividades da PEPCK nas
três espécies pertencentes a diferentes subtipos metabólicos devem ser exploradas. As constantes
cinéticas da Rubisco determinadas para gramíneas C4 permitirão o rigor na simulação das
respostas do metabolismo fotossintético a diversas condições ambientais.
PALAVRAS-CHAVE:
Défice hídrico, Fotossíntese C4, Fotorrespiração Paspalum dilatatum, Cynodon dactylon, Zoysia
japonica
6
Chapter I.
GENERAL INTRODUCTION AND OBJECTIVES
General Introduction and Objectives
GENERAL INTRODUCTION AND OBJECTIVES
Climate change and water shortage – the problem!
Water is one of the most important factors limiting plant growth, development and survival. In
particular in Mediterranean climates, which are characterized by long hot and dry periods, water
availability is the major limitation to plant productivity (Turner 2004). The decrease of fresh
water availability is one of the most serious environmental problems of the planet, as recognized
by the United Nations Environment Programme (Global Environment Outlook: Environment for
Development, Geo-4 report 2007). The number of dry days per year is expected to raise in many
areas of the globe (Petit et al. 1999), exacerbating the problem.
The efficient management of water resources by agricultural and recreational systems is
therefore an outstanding priority in many regions of the world and there is increasing pressure on
the different irrigators to increase the efficiency of water use by crops and pastures or lawns.
Golf courses are probably the most criticized water-users, especially during summer-droughts, in
the USA and in some European countries, including Portugal. This is mostly because the game is
not considered a ‘primary need’. Even considering the enormous importance in terms of tourism
and development, serious concerns arise in terms of water consumption. The tourism sector has
been recognised as one of the key water users in Europe where there is big potential for water
savings, including the use of more efficient irrigation techniques and rain water harvesting in
golf courses (MAOTDR 2007b). In America great investments have been made in order to
promote increased water use efficiency at the level of the golf industry (Snow 2001), namely
through the use of varieties of turfgrass that require less water, better irrigation techniques and
management practices (e.g. monitoring the soil moisture for scheduling irrigation), and the use of
alternative water sources.
Deficit irrigation techniques have been successfully applied in the field to improve water
use efficiency by some crops and might have further application in water-limited environments
(Costa et al. 2007; Fereres & Soriano 2007). For instance, the strategy of partial rootzone drying,
which involves simultaneous exposition of the roots to wet and drying soil, allows the decrease
of stomatal conductance, decreasing water loss, without affecting plant performance (Davies et
al. 2002; Santos et al. 2003). The use of secondary, non-potable waters for the irrigation of
turfgrasses has been suggested as another possible alternative (Snow 2001; Marcum 2006) and
water re-utilization is already implemented in many golf courses in the South of Portugal
9
Chapter I.
(MAOTDR 2007a). Irrigation systems controlled by soil moisture sensors can also be used to
minimize water waste by matching turfgrass requirements (Pathan et al. 2007).
The use of plant species and cultivars better suited to the environment can also improve
water use efficiency and plant production and yields in water-limited regions (Turner 2004). As
recently proposed by Marcum (2006), turfgrasses with high salinity tolerance allow the use of
saline, non-potable waters for irrigation. Snow (2001) reported the use of stress-tolerant
bermudagrass (Cynoodn dactylon) cultivars with low water requirements yielding considerable
water savings in American golf courses, and also the successful irrigation of the extremely salttolerant and highly productive seashore paspalum (Paspalum vaginatum) with salted waters. It is
also important to note that, in what concerns the golf courses in particular, there are different
areas associated with different game functions and the turfgrasses used in each of these should
correspond to the functional needs associated (Bernardes da Silva et al. 2008).
The grass family and the C4 photosynthetic pathway
The multifaceted contribution of the Poaceae to the world economy is the base for the superlative
importance of this plant family. The grass family includes the cereals (such as wheat, rice, maize
and sorghum), the most important source of sucrose in the world (sugarcane), and the forage and
turf grasses, which are the backbone of sustainable agriculture (Jones 1985). The paramount
importance of the forage and turf grasses arises from the economic benefits associated with its
use in golf courses and other recreational areas, the sustainability of live stocks and wild animals,
and from their contribution to the soil conservation and environment protection.
Grasses are commonly classified as cool-season and warm-season, reflecting low (ca.
15º-25ºC) or high (ca. 25º-35ºC) optimal daytime growth temperatures, respectively. The distinct
physiological characteristics between the two types of grasses result from the presence of the C4
photosynthetic pathway that elevates the CO2 concentration at the site of carboxylation in the
warm-season (C4) grasses (Furbank & Hatch 1987). C4 photosynthesis occurs in nearly half of
the species in the Poaceae (Hattersley 1988; Sage et al. 1999a) and some of the world’s most
important crop and turf species are C4 grasses (see Jones 1985; and Brown 1999). Warm-season
C4 grasses, by opposition to cool-season C3 species, are characterized by performing better at
high irradiance levels and high temperatures (Johnston 1996), showing higher rates of
photosynthesis than their C3 counterparts in full sunlight and at temperatures above 30ºC (Brown
1999). C4 grasses have generally higher water use efficiency (WUE) due to an efficient
assimilation of CO2 in combination with lower transpiration rates (Edwards et al. 1985). A
10
General Introduction and Objectives
comparison by Gherbin et al. (2007) revealed that warm-season C4 grasses produce higher yields
than cold-season C3 grasses in warm dry summer conditions like those observed in the
Mediterranean climate and in some regions of the USA and Australia.
The distribution of C3 and C4 grasses in several regions of the world suggests that
temperature is the climatic variable best correlated with the relative occurrence of grass species
of the two photosynthetic pathways (Henderson et al. 1995; Sage et al. 1999b). As shown by
Cabido et al. (1997), C4 species tend to dominate in areas characterised by warmer temperatures
and their distribution is therefore dependent on a latitudinal and altitudinal gradient. Sage et al.
(1999b) noted that aridity is not a prerequisite for C4 dominance over C3 and the success of C4
species depends essentially on the presence of warm temperatures and high light intensities,
resulting in a high representation of this photosynthetic pathway in tropical and subtropical
regions, where more than two thirds of all grasses are C4.
The presence of a CO2-concentrating mechanism makes C4 photosynthesis more
competitive in conditions that promote photorespiration, like high temperatures and low
intercellular CO2 concentrations. Consequently, it is generally assumed that C4 species will
perform better in warm habitats and conditions that promote stomatal closure, including
decreased water availability. Importantly, Sage et al. (1999b) noted that the seasonality of
precipitation plays an important role and its occurrence during the warm season tends to favour
C4 dominance. Nevertheless, several reports suggest that C4 grasses may dominate in areas
where precipitation decreases in the warmer summer months (see Cabido et al. 2008) and their
exploitation can therefore bring advantage in terms of water savings.
The diversification of the anatomical-biochemical variants of C4 photosynthesis might be
related with natural selection pressures of changes in rainfall (Hattersley & Watson 1992), as
precipitation gradients seem to be the major determinant for the relative distribution of the three
classical variants of C4 photosynthesis. These are named after the main decarboxylating enzyme
in each pathway (Gutierrez et al. 1974; Hatch et al. 1975): NADP-malic enzyme (NADP-ME),
NAD-malic enzyme (NAD-ME) and phosphoenolpyruvate carboxykinase (PEPCK). The biogeographical distribution of C4 species with different decarboxylation mechanisms in several
regions of the world (e.g. Hattersley 1992; Cabido et al. 2008) shows that NADP-ME species are
relatively more abundant in areas with higher annual rainfall whereas NAD-ME species
predominate in arid zones, and PEPCK seem to have a less clear pattern of association with
precipitation gradients. Brown (1999) refers that most of the cultivated C4 species with
agronomic importance are NADP-ME, possibly as a consequence of their occurrence in wetter
areas, the first to be exploited during colonization. In the current climate conditions and
11
Chapter I.
assuming the future perspectives, the implementation of some potentially more drought-resistant
NAD-ME species might bring considerable advantage in terms of better use of water resources.
The positive and negative correlation of NAD-ME and NADP-ME species with aridity,
respectively, may be related with enhanced water use efficiency in the former species
(Ghannoum et al. 2002). However, it is unclear if this is due to the functional differences
between the two variants of C4 photosynthesis. Most grasses from the subfamily Chloridoideae
are C4 and belong to the subtypes NAD-ME or PEPCK, whereas the subfamily Panicoideae is
photosynthetically more variable, with representatives of C3, intermediate C3-C4 and C4
photosynthesis (Hattersley & Watson 1992). All three biochemical subtypes of C4 grasses occur
in the Panicoideae, but the great majority of C4 species in this subfamily are NADP-ME (Taub
2000). The distribution of the subfamilies Panicoideae and Chloridoideae is also strongly
correlated with the precipitation gradients (Taub 2000; Cabido et al. 2008), suggesting that
characteristics other than the biochemistry of photosynthesis may be responsible for the
geographical patterns observed, possibly reflecting some divergent patterns associated with the
multiple origins of C4 grasses (Kellogg 1999).
In addition to the specialised photosynthetic biochemistry, the leaves of most C4 grasses
show anatomical modifications associated with the functionality of the CO2-concentrating
mechanism. These characteristics are referred to as Kranz anatomy (see review by Dengler &
Nelson 1999). The term ‘Kranz’ refers to a wreath of cells surrounding the vascular tissues and
was first used by Haberlandt (1882) who initially referred to the presence of distinct leaf
anatomies and recognized that grasses could be divided into two groups and these were related
with their ecological adaptation. A suite of subtype-specific anatomical characteristics has been
further associated with each of the decarboxylation mechanisms (Prendergast & Hattersley 1987;
Dengler et al. 1994). Most C4 grasses belong to the ‘classical’ biochemical-anatomical subtypes
(Hattersley & Watson 1992), but some variations occur in nature (e.g. Prendergast et al. 1987).
As suggested by Hattersley (1992), differences in leaf structure could possibly be associated with
differential ability of different grass species to cope with decreased water availability.
C4 species and climate change
Although C4 plants represent less than 4% of the terrestrial plant species (Sage et al. 1999a) they
contribute about one-quarter of the primary productivity of the planet and a large fraction of the
primary production consumed by humans, directly or not, is derived from C4 crops and pastures
(Brown 1999). The specialized leaf structure, with a high density of vascular bundles, makes C4
grasses less digestible and tougher, and thus less appealing to herbivores, than C3 grasses
12
General Introduction and Objectives
(Wilson & Hattersley 1989; Scheirs et al. 2001), but there is no consistent selection against C4
and in some areas C4 grasses are actually preferred in comparison to C3 (see Heckathorn et al.
1999). The increased interest in C4 grass pastures comes from the fact that they are more
productive and may be more drought resistant (Blaikie et al. 1988). The same reasons provide
evidence for their preferential use as turf in recreational areas. Additionally, the potential use of
C4 grasses as biofuels in energy production systems was recently outlined (Samson et al. 2005).
Climate change has remarkable consequences on biodiversity, species’ distribution and
their relative abundance. Water availability, in particular, is one of the most relevant
environmental factors affecting plant survival, productivity and distribution. The effects of
increasing atmospheric CO2 on growth and photosynthesis in C3 and C4 plants are still
controversial and associated with a great level of uncertainty (Chen et al. 1996; Campbell &
Smith 2000; Ainsworth & Long 2005; Korner 2006; Long et al. 2006; Soares et al. 2008), but
the predicted temperature rise is likely to favour C4 photosynthetic performance and
competitiveness and result in increased dominance of grasses with this photosynthetic pathway
(Henderson et al. 1995; Sage & Kubien 2003). Changes in rainfall scenarios are not as easy to
predict as those of temperature and involve more uncertainty but are likely to affect the relative
distribution of C4 grass species, with increased NAD-ME proportional occurrence in drier areas.
The comprehensive understanding of the C4 photosynthetic metabolism and the response
of plants with the C4 pathway to the environment are crucial if the best advantage is to be taken
from their potentialities. Recent progress has been made on the understanding of C4
photosynthesis in dicotyledon species e.g. (Voznesenskaya et al. 2007) and suggestion has been
made to adopt a species from the genus Cleome as a C4 model system (Brown et al. 2005). None
the less, there is great advantage on the use of C4 grass species. It is difficult to outcome the
economic, agronomic and ecological importance of the Poaceae. The C4 photosynthetic pathway
was first discovered in the grass family and photosynthetic variation within the Poaceae is
comprehensively understood (see review by Hattersley & Watson 1992). Most C4 grasses have
been biochemically typed in terms of photosynthetic variant and a checklist has been provided
by Hattersley some twenty years ago (Hattersley 1988). Therefore, grasses provide the ideal
system for the study of C4 photosynthesis under changing environments.
13
Chapter I.
The C4 grasses Paspalum dilatatum, Cynodon dactylon and Zoysia japonica
The three warm-season C4 grasses studied in the present work (Figure I.1) have been previously
classified as belonging to each of the different biochemical subtypes of C4 photosynthesis.
Paspalum dilatatum Poir. is a NADP-ME species (Usuda et al. 1984), Cynodon dactylon Pers. is
a NAD-ME species (Hatch & Kagawa 1974) and Zoysia japonica Steudel is a PEPCK species
(Gutierrez et al. 1974). P. dilatatum belongs to the subfamily Panicoideae, while C. dactylon and
Z. japonica belong to the subfamily Chloridoideae (Watson & Dallwitz 1992). The genus
Paspalum originated in South America, Cynodon in Africa and Zoysia in Southeastern Asia
(Brown 1999).
Paspalum dilatatum
Cynodon dactylon
Zoysia japonica
Figure I.1. Plants of the three C4 grass species in the greenhouse during the early stages of development.
Photograph was taken ca. two (P. dilatatum and C. dactylon) or four (Z. japonica) weeks after sowing.
Dallisgrass (P. dilatatum), bermudagrass (C. dactylon) and zoysiagrass (Z. japonica) are
warm-season perennial species used for turfgrass purposes throughout the world (Brown 1999).
Additionally, the first two species are important forage and cultivated pasture grasses and C.
dactylon is also one of the world’s most serious weeds (Jones 1985; Brown 1999).
The species P. dilatatum, native from South America, is an important forage grass in the
subtropical and warm regions of the world, mainly due to its high nutritive value (Venuto et al.
2003). Since long ago, dallisgrass has been a dominant pasture in Australia, especially during the
summer season (Stockdale 1983). Its great value is also derived from its cold tolerance and
ability to survive frosts in winter (Rowley 1976). Andrews & Crofts (1979a) evaluated the
possibility of replacing pastures of dallisgrass by bermudagrass in order to increase the growing
14
General Introduction and Objectives
season recognizing the value of the latter in areas with low frost incidence. However, both the
highly productive, wide-temperature- and grazing-tolerant dallisgrass as well as the promising
bermudagrass were outyelded by an improved cultivar of Pennisetum clandestinum (Pearson et
al. 1985). None the less, dallisgrass is still an important pasture forage and progress has recently
been observed in the improvement of its forage yields (Venuto et al. 2007). In some modern
prairies, dallisgrass can become a weed to bermudagrass turf (see Henry et al. 2007).
The outstanding economic importance of C. dactylon results from the wide distribution
throughout tropical and subtropical areas and the enormous variability of the species (Taliaferro
1995). The recognition of ecotypes with different characteristics of establishment and persistence
(Andrews & Crofts 1979a) and of digestibility (Andrews & Crofts 1979b), suggested these could
be used most favourably to different purposes (e.g. control of soil erosion, feedstock, etc).
Bermudagrass, native from Africa, is the most widely used turfgrass (in lawns, golf courses and
other sports fields) in tropical and subtropical regions of the world (Brosnan & Deputy 2008) and
is used as forage for livestock (Starks et al. 2006). Additionally, C. dactylon can also be applied
in the stabilisation of soils (Vignolio et al. 2002; Moreno-Espindola et al. 2007) and has been
recently suggested as a potential energy crop for biofuel production (Boateng et al. 2007).
However, C. dactylon is sensitive to shading and requires full sun for best performance
(Guglielmini & Satorre 2002; Tegg & Lane 2004; Brosnan & Deputy 2008). The species is also
cold-sensitive, but different lines are now being developed with improved freeze tolerance
(Anderson et al. 2007). Bermudagrass is a target for genetic engineering for turf quality
improvement (Li et al. 2005; Wang & Ge 2005).
The species Z. japonica, native from Japan, is sometimes called Japanese or Korean lawn
grass (Duble 2002). Despite its slow growth rate, zoysiagrasses are shade tolerant and perform
well as lawns, being widely used in golf courses and other sports fields (Deputy & Hensley
1999). Zoysia japonica is widely used in Japan and other countries of Asia as a turfgrass for
sports fields and as a forage grass (see Cai et al. 2005 for references in Japanese!). Slow
establishment of lawns is one of the major barriers for the use of zoysiagrass, but it was recently
shown that different genotypes may establish faster (Patton et al. 2007). This grass species
requires watering during lawn establishment but afterwards it is one of the most drought- and
heat-resistant warm-season grasses (Deputy & Hensley 1999). Some contradictory references for
its cold resistance are related with differences in freeze tolerance among genotypes of
zoysiagrass (Patton & Reicher 2007) and the winter hardiness and high temperature tolerance
suggest that zoysiagrass can adapt to a wide range of environmental changes (see White et al.
2001). Zoysiagrass is characterized by very stiff leaf blades due to high content in silica (Duble
15
Chapter I.
2002) and is nearly as salt tolerant as bermudagrass due to the presence of salt secretion glands
in their leaves (Marcum 1999). Additionally, zoysiagrass seems to be less sensitive to low
nutrient supply than bermudagrass (Menzel & Broomhall 2006), with associated potential
savings in fertilizers. Its recognized turf value makes zoysiagrass subject of genetic
transformation (e.g. Ge et al. 2006).
The remarkable characteristics of P. dilatatum, C. dactylon and Z. japonica provide
evidence for the potentialities associated with the understanding of the responses of these grass
species to the environment, particularly to conditions of decreased water availability, and the
functional significance of their C4 photosynthetic pathways under these conditions.
16
General Introduction and Objectives
C4 PHOTOSYNTHETIC METABOLISM
Biochemistry and anatomy of C4 photosynthesis
The C4 photosynthetic pathway, with specialised biochemical and anatomical characteristics,
results in elevated CO2 concentrations at the site of the carboxylating enzyme, ribulose-1,5bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39), thereby increasing photosynthetic
efficiency in conditions promoting high rates of photorespiration. The main features associated
with C4 photosynthesis were identified in the two decades that followed the discovery of the C4
dicarboxylic acid pathway in the mid-1960’s (see historical overview by Hatch 1999).
In C4 grass leaves with Kranz anatomy, primary fixation of inorganic carbon by
phosphoenolpyruvate (PEP) carboxylase (PEPC; EC 4.1.1.31) occurs in the mesophyll (M) cells.
The C4 acids formed are then transported to the bundle sheath (BS) cells, where they undergo
decarboxylation and the released CO2 is subsequently assimilated by Rubisco in the C3 pathway
(Hatch 1987; Kanai & Edwards 1999). PEPC is confined to the cytosol of M cells and Rubisco is
confined to the chloroplasts of BS cells (Edwards et al. 2001). The specialized leaf structure (see
Dengler & Nelson 1999), namely the chemical modification and increased thickness of the BS
cell walls and the reduction of the exposure of BS surface area to intercellular spaces, decrease
leakage of CO2 back to M cells so that CO2 accumulates (Furbank et al. 1989; Brown & Byrd
1993; Evans & von Caemmerer 1996; Jenkins 1997; Kiirats et al. 2002). C4 photosynthesis
saturates at lower CO2 concentrations than in C3 plants, essentially because the affinity of PEPC
for HCO3- is much higher than the affinity of Rubisco for CO2 (Kanai & Edwards 1999). The use
of C4 photosynthesis mutants provided evidence that the flux through this pathway is controlled
by several enzymes, with special emphasis on Rubisco, PEPC pyruvate Pi dikinase (PPdK,
responsible for the regeneration of PEP in the M cells) (see review by Lea et al. 1999).
Three biochemical subtypes of the C4 photosynthetic pathway have been classically
defined according to the main enzymes responsible for the decarboxylation step (Gutierrez et al.
1974; Hatch et al. 1975): NADP-malic enzyme (NADP-ME, EC 1.1.1.40), NAD-malic enzyme
(NAD-ME, EC 1.1.1.39) and PEP carboxykinase (PEPCK, EC 4.1.1.49). The three mechanisms
of C4 photosynthesis were first described by Hatch (1987) and are summarised in Figure I.2.
These subtypes are distinguished by several aspects of leaf biochemistry and anatomy (see
reviews by Dengler & Nelson 1999; Kanai & Edwards 1999), which are discussed in further
detail in Chapter IV. The regulation of the C4 pathway, including the different subtypes, was
comprehensively revised by Leegood and Walker (1999) and details on the regulation of the
17
Chapter I.
A
NADP-ME
CHLOROPLAST
CHLOROPLAST
NADP+
NADPH
OAA
OAA
Malate
MDH
Malate
Pi
NADP+
CO2
Calvin
CA
PEPC
Cycle
NADPH
NADP-ME
Rubisco
HCO3AMP
+ PPi
PEP
PEP
PPdK
Pyruvate
Pyruvate
MESOPHYLL CELL
B
RuBP
CO2
ATP
+ Pi
BUNDLE SHEATH CELL
NAD-ME
CHLOROPLAST
MITOCHONDRION
AspAT
OAA
Aspartate
Aspartate
AspAT
(NH2)
Pi
Alanine
OAA
Alanine
MDH
(NH2)
CO2
CA
AlaAT
PEPC
Pyruvate
CHLOROPLAST
HCO3-
AlaAT
Malate
NAD+
Cycle
AMP
+ PPi
PEP
PEP
PPdK
Rubisco
NAD-ME
ATP
+ Pi
MESOPHYLL CELL
CO2
Pyruvate
Pyruvate
Pyruvate
BUNDLE SHEATH CELL
Figure I.2. (See legend on next page.)
18
Calvin
NADH
RuBP
General Introduction and Objectives
C
PEPCK
AspAT
OAA
Aspartate
Alanine
(NH2)
Aspartate
Alanine
AspAT
CHLOROPLAST
(NH2)
AlaAT
CO2
PEPC
CA
NADPH
OAA
HCO3-
PEP
PEP
Malate
NAD+
ATP
Electron
transport
NADH
NAD-ME
Pyruvate
ADP
O2
CO2
Pyruvate
MESOPHYLL CELL
RuBP
H2O
Malate
ATP
+ Pi
PPdK
Rubisco
MITOCHONDRION
AlaAT
AMP
+ PPi
Calvin
Cycle
CO2
NADP+
MDH
PEP
ATP ADP
Pyruvate
CHLOROPLAST
PEPCK
OAA
Pi
Pyruvate
BUNDLE SHEATH CELL
Figure I.2. Generalised scheme for the CO2-concentrating mechanisms of the three biochemical subtypes
of C4 photosynthetic pathway (adapted from Kanai & Edwards 1999). In all subtypes, primary CO2
fixation by PEPC in the cytosol of the M cells results in the formation of C4 acids that are transported to
the BS and decarboxylated increasing the CO2 concentration in these cells, where Rubisco and the C3
cycle is located. The C4 cycle is complete once PEP has been regenerated and becomes available for
carboxylation in the mesophyll. In the NADP-ME subtype (A, previous page), malate is the C4 acid
transported to the BS and decarboxylation occurs in the chloroplast of these cells; in the NAD-ME
subtype (B, previous page), aspartate is the C4 acid transported to the BS where it is converted into malate
that is decarboxylated in the mitochondria; in the PEPCK subtype (C, this page), aspartate is the main C4
acid transported to the BS where it is converted into oxaloacetate before decarboxylation in the cytosol
but, concomitantly, malate is transported from the M chloroplasts to the BS mitochondria where its
decarboxylation by NAD-ME provides the energy required for PEPCK (involving use of reducing power
for the formation of ATP in the respiratory electron transport system) and contributes to enhance the CO2
concentration available for assimilation through the C3 cycle. Grey-dashed arrows indicate metabolite
transport. Abbreviations used: AlaAT, alanine aminotransferase; AspAT, aspartate aminotransferase; CA,
carbonic anhydrase; MDH, malate dehydrogenase; NAD-ME, NAD-malic enzyme; NADP-ME, NADPmalic enzyme; PEP, phosphoenolpyruvate; PEPC, PEP carboxylase; PEPCK, PEP carboxykinase; Pi,
orthophosphate; PPdK, pyruvate,orthophosphate dikinase; PPi, pyrophosphate; RuBP, ribulose-1,5bisphosphate.
19
Chapter I.
enzymes involved in the carboxylating and decarboxylating reactions are given in Chapter IV
(PEPC and decarboxylating enzymes) and Chapter V (Rubisco).
The coordinate function between the M and BS cells (and thus the C3 and C4 cycles) is
crucial for C4 photosynthesis efficiency. Photosynthesis at high irradiance is limited by the
activities of PEPC and Rubisco (von Caemmerer & Furbank 1999) and a ratio of two between
PEPC and Rubisco carboxylation activities is enough to saturate CO2 assimilation. Reduction of
PEPC activity results in low assimilation rates and increased photorespiration rates at high light
(Maroco et al. 1998; Bailey et al. 2000), due to the decreased production of C4 acids and,
consequently, of CO2 released in the BS cells. Reduction of Rubisco activity leads to reduced
assimilation rates, increased CO2 concentration in BS cells and increased leakage from the BS
(Furbank et al. 1996; Siebke et al. 1997; von Caemmerer et al. 1997). Because the C4 pathway
has energy costs, associated with the regeneration of PEP (Hatch 1987; Kanai & Edwards 1999),
the leakage of CO2 back to the M cells increases the ATP used per mole of CO2 fixed and hence
decreases photosynthetic efficiency. At low irradiance, C4 photosynthesis is mostly limited by
the regeneration of RuBP and PEP, dependent on ATP and NADPH availability (von
Caemmerer & Furbank 1999).
The higher water use efficiency (WUE) of C4 plants relative to C3 results from the
combination of an efficient assimilation of CO2 through the C4 pathway, namely because the
CO2-concentrating mechanism limits photorespiration, with lower transpiration rates (Edwards et
al. 1985). As reported by Hatch (1987), the WUE in C4 plants corresponds to the double of that
observed in C3 plants and the disparity is likely to increase with temperature, essentially as result
of increased photorespiration in C3 plants.
Photorespiration in C4 plants
Rubisco properties limit the photosynthetic efficiency in C3 plants (Parry et al. 2007), essentially
because the enzyme acts both as a carboxylase and an oxygenase. When CO2 is used as a
substrate for the reaction with RuBP, the product 3-phosphoglycerate (PGA) is metabolised
through the Calvin cycle and carbon is assimilated into useful products. Conversely, reaction of
RuBP with O2 produces 2-phosphoglycolate (PG) and initiates the process of photorespiration,
which results in the loss of fixed carbon and consumption of energy (Kumarasinghe et al. 1977),
thereby decreasing the efficiency of CO2 assimilation. The photorespiratory carbon cycle (Figure
I.3) occurs in three subcellular compartments, the chloroplast, peroxisome and mitochondrion,
and results in the release of CO2 and NH3. The remaining carbon is recycled to PGA that reenters Calvin cycle and the ammonia released in the mitochondria must be re-assimilated in the
20
General Introduction and Objectives
photorespiratory nitrogen cycle (Keys et al. 1978). The involvement of common intermediates in
these pathways reflects the great level of integration and interdependence of the photorespiratory
carbon oxidation cycle, the photorespiratory nitrogen cycle and the photosynthetic carbon
reduction cycle (Keys 1999). The biochemistry of photorespiration and its regulation have been
comprehensively reviewed by Leegood et al. (1995).
O2
RuBP
amino acid keto acid
Rubisco
PG
P-glycolate
oxidase
Glyoxylate
H2O
Pi
Glycolate
Aminotransferase
Glycine
Glycine (2)
H2O2
Glycolate
oxidase
O2
NAD+
Glycolate
Calvin
Cycle
NADH
Glycine decarboxylase
&
Serine hydroxymethyl
transferase
Hydroxypyruvate
NADH
Aminotransferase
NAD+
ADP ATP
PGA
Glycerate
cinase
Glycerate
CHLOROPLAST
CO2
Glycine
NH3
Glyoxylate
Reductase
Glycerate
PEROXISOME
Serine
Serine
MITOCHONDRION
Figure I.3. Simplified scheme of the photorespiratory carbon cycle. The oxygenase reaction of Rubisco
produces PG and PGA. The latter (not shown) is metabolised in the Calvin cycle and PG initiates the
photorespiratory cycle. Glutamate and serine are the most common amino group donors for the amination
of glyoxylate to glycine in the peroxisome. The subsequent conversion of two glycines into serine in the
mitochondrion results in the release of both CO2 and NH3 and the remaining carbon is recycled to PGA
that enters the Calvin cycle in the chloroplast. The regeneration of PGA from glycerate consumes ATP.
Grey-dashed arrows indicate metabolite transport. Abbreviations used: PG, 2-phosphoglycolate; PGA, 3phosphoglycerate; Pi, orthophosphate; RuBP, ribulose-1,5-bisphosphate.
The C4 photosynthetic pathway elevates the CO2 concentration in BS cells and the
resultant high CO2 to O2 ratio offsets the low affinity of Rubisco for CO2 and largely inhibits its
oxygenase activity and, hence, photorespiratory rates in air. Photorespiration rates correspond to
about one-quarter of CO2 assimilation rates in C3 species under normal chloroplast conditions at
25ºC (Keys 1986; Sharkey 1988), whilst in C4 plants photorespiration is likely to correspond to
less than 2% of the photosynthetic rate (Hatch 1987). Oxygen inhibits RuBP carboxylation but
21
Chapter I.
has no effect on PEP carboxylation in both maize and soybean leaf extracts (Bowes & Ogren
1972). Because CO2 and O2 are competing alternative substrates for reaction with RuBP
catalysed by Rubisco, the relative specificity of Rubisco and the concentrations of the gases will
determine the flow of carbon between the photorespiratory and the photosynthetic carbon cycles.
An increase in photorespiration is therefore expected when the intercellular CO2 decreases, as
may occur under stress conditions promoting stomatal closure.
The CO2-concentrating mechanism has energy costs (Hatch 1987) and the competitive
advantage of C4 plants in relation to C3 depends on the temperature, mostly because higher
temperatures result in increased photorespiration relative to photosynthesis in C3 plants due to
the enhanced ratio of O2 to CO2 in conditions that decrease the solubility of CO2 more than that
of O2 (see Edwards et al. 1985 and references therein).
The estimation of true rates of photosynthesis and photorespiration is difficult because of
the complexity of the CO2 and O2 exchanges involved (Sharkey 1988; Keys 1999). The refixation of photorespired CO2 within the leaves (Loreto et al. 1999; Kiirats et al. 2002) further
compromises the estimation of photorespiration through CO2 exchange. Measurement of
photorespiration in C4 plants is especially complex given the level of biochemical and structural
specialization. The mathematical modelling of C4 photosynthesis by von Caemmerer and
Furbank (1999) uses basic equations to represent carbon fluxes in C4 plants. The model assumes
a steady-state balance between the release of CO2 in the BS through the C4 pathway, the
assimilation of CO2 by the C3 cycle and the leakage of CO2 from BS cells. At high irradiance,
CO2 fixation will be limited by PEPC and Rubisco activities and, the CO2 concentration in the
BS cells and, ultimately, the photorespiration rate can be estimated from the analysis of net
photosynthesis response to intercellular CO2. Additionally, amino acids can be used as
metabolite markers for the photorespiratory pathway (Foyer et al. 2003) and glycine and serine
contents were found to be well correlated with photorespiration (Novitskaya et al. 2002). Further
considerations on photorespiration rates in C4 plants are detailed in Chapter III.
22
General Introduction and Objectives
DROUGHT STRESS
Drought and water deficit
Stress can be defined as “a deviation from the optimal condition of life” (Larcher 2003) and
elicits responses that may be adaptive or cause damage. Water stress occurs when water
availability is scarce (water deficit) or excessive (water logging) and becomes likely to affect
plant performance. Water deficit is often referred to as drought stress. Drought, in nature, is an
event of decreased water availability, resulting from low rainfalls, and is usually associated with
high temperatures and irradiance. In climates characterized by hot dry summers, like the
Mediterranean, plants are subjected to high temperatures and irradiance and may or not have
enough water, but are generally dependent on irrigation. An efficient management of water
resources is imperative in order to optimise the use of water in these regions.
Studies in the greenhouse generally attempt to promote conditions as close as possible to
those that plants experience in the field, controlling the imposition of stress conditions in order to
understand plant responses to the different variables that may affect their growth and
productivity. Drought stress, here used as a synonym to water deficit, is one of the major factors
limiting plant photosynthetic performance.
Plants may adopt a number of strategies to resist drought stress, including the
development of mechanisms of drought avoidance or drought tolerance. In the first case, the
hydration of the tissues is maintained through processes that minimize water loss or increase
water uptake from the soil, whilst in the latter one the physiological and biochemical parameters
are maintained when tissue dehydration occurs (Chaves 1991; Bray et al. 2000; Larcher 2003;
Blum 2005). Drought avoidance strategies are much dependent on the early signals triggered at
the root level in response to the drying soil (Chaves & Oliveira 2004). These signals warn the
plant to the decreased water status in the soil and allow the development of a stress response in
order to better cope with the unfavourable environment. The regulation of plant growth and
development in response to drying soil often involves chemical signalling that may
concomitantly result in stomatal closure to avoid excessive water loss from the tissues (Davies et
al. 2005). In fact, both stomatal closure (Schulze 1986) and shoot growth inhibition (Kramer
1983) occur at the earliest stages of water deficit and are likely to be induced by stress sensors or
through signal transduction pathways triggered in response to stress (Bartels & Sunkar 2005).
The long-distance signalling, involving the transit of molecules (like the hormone ABA and
other chemical regulators) from the root to the shoot and vice-versa, provide the plant with a
23
Chapter I.
means to measure water availability in the soil (Zhang & Davies 1989; Davies et al. 2005) and
quickly adjust to the environment. Plants with plastic genotypes may alter their growth and
development, and thereby optimize their response to the environment (Trewavas 1986; Sage &
McKown 2006).
The improvement of plant performance and the achievement of high stable yields under
limited water availability depend on the ecophysiological understanding of the interaction
between the plant and the growing environment in combination with the genetic background
behind it (Chaves & Oliveira 2004; Parry et al. 2005). Most of the drought-responsive genes that
have been identified and manipulated in order to increase water use efficiency under drought
conditions are involved in the protection of the cellular structure and function and, thus, the
primary metabolism, through the scavenging of reactive oxygen species (to prevent oxidative
damage) and the accumulation of osmolytes (to maintain water status) (Umezawa et al. 2006).
Many of the studies with these transgenic plants retrieved no concluding results or showed
compromised plant productivity in associations with drought resistance (Parry et al. 2005).
Considerable further research is required with rigorous evaluation of plant physiological
performance under field-like drought conditions (Chaves & Oliveira 2004).
Severe water deficit is an unlikely event in agriculture and recreational areas; it is more
likely to occur in natural ecosystems of arid, rain-fed areas, where plant survival becomes a
priority (e.g. Utrillas & Alegre 1997; Volaire et al. 1998). In the urbanized areas of the modern
world it is more important to understand plant physiological and biochemical responses to
moderate drought stress promoted by gradually-decreased water availability, including the
identification of traits associated with increased resilience to drought in better adapted species
more likely to cope with the expected climate changes, in order to improve plant performance
and water use efficiency.
Plant growth and development under conditions of decreased water availability
Plants can sense and respond to decreased water availability by regulating their growth and
development in order to cope better with the environment. In many plant species, the decrease in
biomass production is one of the earliest responses to decreased soil water availability (Kramer
1983). Hsiao & Xu (2000) showed that under drought stress conditions root elongation continues
but shoot growth is limited. Decreased shoot growth in response to decreased soil water potential
may result from changes in leaf cellular elongation and/or in protein metabolism and can occur
before alterations in leaf gas-exchanges (Lambers et al. 1998) or in leaf water potential (Saab &
Sharp 1989). Leaf elongation can be limited by the loss of cell turgor or cell wall stiffening (van
24
General Introduction and Objectives
Volkenburgh & Boyer 1985) induced by chemical/hydraulic signals triggered in response to the
drying soil (Davies & Zhang 1991). However, despite initial suggestions of ABA accumulation
being the cause of growth inhibition, recent studies have shown that ABA restricts the
production of ethylene and functions to maintain rather than to inhibit plant growth (Sharp
2002). ABA seems to counteract completely the inhibition of root elongation by ethylene whilst
in the shoots the effect of ABA is not enough to avoid some growth limitation by ethylene and
therefore a reduction is observed in the aerial part of the plant but not in the root system. The
maintenance of root elongation at low soil water potentials is a strategy to improve water
absorption (Chaves et al. 2003) and allows plants to tolerate better, and in some cases survive,
water-limited conditions (Liu et al. 2005). On the other hand, the decreased shoot growth rate
and the diversion of the photoassimilates and energy into the synthesis of molecules involved in
plant defence can also be seen as an adaptive strategy essential for plant survival under severe
stress conditions (Chaves & Oliveira 2004).
Phenotypic plasticity or the ability of plants to adjust their development in response to the
environment may provide them with competitive advantage. For instance, plants native from
climates characterized by marked fluctuations have generally more plastic phenotypes and can
acclimate to the changing environment and cope better with those changes (Sage & McKown
2006). The leaf Kranz anatomy associated with C4 photosynthesis (Dengler et al. 1994) may
compromise the plasticity of C4 grasses, but both metabolic and/or structural adjustments have
been observed to occur in response to several environmental conditions and favour plant
performance (e.g. Cavaco et al. 2003; Soares et al. 2008). Moreover, the specialised leaf
structure of C4 grasses (discussed in further detail in Chapter III) is likely to be associated with
their increased drought resistance in comparison with C3 species.
Leaf water relations - diverse possibilities in response to water deficit
Water status can be defined in terms of the water chemical potential (adopting the Gibbs
concepts, see Kramer & Boyer 1995). The water potential, commonly used in ecophysiology,
depends on a number of components and, in the leaves, will essentially reflect pressure and
solute effects. Decreased leaf water potential (LWP) under water deficit conditions reflects
changes in the turgor pressure and in the osmotic potential. The turgor pressure depends on the
cell wall rigidity and has a determinant role in leaf expansion. The osmotic potential is a function
of the concentration of solutes in the cell: the accumulation of compatible compounds increases
the osmotic pressure and, as a consequence, the LWP becomes more negative, promoting the
water movement into the cell.
25
Chapter I.
Cell wall elasticity determines the extent to which LWP can decrease without
compromising leaf turgor. In general, species from drier climates have more elastic cell walls, a
drought-tolerance strategy that allows them to loose more water before reaching the turgor-loss
point (Lambers et al. 1998). On the other hand, the faster decrease of LWP in species with rigid
cell walls (see Kramer & Boyer 1995) increases the capacity of water absorption from the drying
soil, avoiding excessive leaf dehydration.
Osmotic adjustment is one of the most important mechanisms of adaptation to drought
stress (Bray 1993). Plants accumulate compatible solutes that reduce the osmotic potential of the
tissues, thereby minimizing water loss (Morgan 1984; Kramer & Boyer 1995). Examples of
substances with osmoregulation potential include amino acids, like proline; quaternary
ammonium compounds, like glycine-betaine and choline; carbohydrates, like sucrose and
trehalose; and other low molecular weight metabolites. As reviewed by Hare, Cress & van
Staden (1998), the advantage of increased contents in compatible solutes, or osmolytes, is
primarily, but not solely, related with turgor maintenance through increased osmotic pressure
and consequently decreased leaf water potential, which is a biophysical aspect, merely related
with cellular concentrations. The maintenance of the cellular water content maintains the
concentration of ions with regulatory properties unaffected, protecting the enzyme functioning
(Morgan 1984). As pointed by Hare et al. (1998), a number of other protective roles, possibly
with similar or even greater importance, have also been described for osmolytes and their
metabolism, including the scavenging of free radicals and the buffering of cellular redox
potential. Additionally, the same authors suggest that osmolyte accumulation might be a means
of storing energy and their degradation upon stress relief might provide the plant with reducing
power and sources of carbon and/or nitrogen for plant growth and recovery from stress injury.
Transgenic approaches to improve drought tolerance included the production of enzymes
involved in osmolyte biosynthesis and in the antioxidant defence system (e.g. Bajaj et al. 1999),
but most often giving no satisfactory results. Similarly, external application of proline to increase
plant resistance to drought stress has successfully increased crop yield in some, but not many,
plant species (Ashraf & Foolad 2007) and is likely to be of limited usefulness. Despite all the
putative benefits associated with osmolyte accumulation, which are likely to be of determinant
advantage for plant survival in natural ecosystems, osmoprotection of the above-ground plant
tissues seems to function mostly under severe dehydration levels. Osmolyte accumulation
usually occurs after gas-exchanges and leaf growth being affected and therefore is likely to have
little relevance for crop yield and production (Serraj & Sinclair 2002). However, osmolyte
accumulation at the below-ground level might give a positive and valuable contribution in the
26
General Introduction and Objectives
maintenance of root growth as soil water availability decreases (Voetberg & Sharp 1991; Hsiao
& Xu 2000).
Protective roles associated with the accumulation of proline and other amino acids
In most plant species, including C4 grasses (Jones 1985), proline accumulates in response to
drought and other environmental stress conditions and it is likely that the unique characteristics
of its metabolism and the interaction with other intermediary pathways (see Hare & Cress 1997)
are in the basis of this widely distributed phenomenon. The protective roles of proline
accumulation under drought stress were recently reviewed (Ashraf & Foolad 2007) and include
the maintenance of cell turgor through osmotic adjustment, maintenance of protein function and
membrane integrity, scavenging of reactive oxygen species and buffer cellular redox potential.
As reviewed by Hare & Cress (1997) the involvement of proline metabolism in the
regulation of the cellular redox potential (and pH) results from the contribution of the
biosynthetic pathway to the oxidation of NADPH (and consumption of H+). The accumulation of
proline under stress is thought to result mostly from increased synthesis through the glutamate
pathway (Delauney & Verma 1993), which contributes twice as much to these processes than the
ornithine pathway. In fact, the shift in nitrogen metabolism associated with proline accumulation
is likely to be more associated with the reactions involved than with the product concentration
itself, most times insufficient to account for a significant contribution to the osmotic adjustment
and benefits associated with this biophysical aspect. This is supported by the observation that, in
most cases, external application of proline results in no benefit for plant drought resistance
(Ashraf & Foolad 2007). The increased activity of Δ1-pyrroline-5-carboxylate reductase,
involved in the final step of proline synthesis, is likely to be itself associated with the regulation
of intermediary metabolism through the maintenance of compatible redox potentials (Hare &
Cress 1997). Proline accumulation occurs especially under severe drought conditions (Jones
1985) and it is likely to have a particularly important role in plant survival where intensive
summer-droughts predominate (Utrillas & Alegre 1997; Volaire et al. 1998). As proposed by
Hare & Cress (1997), increased proline synthesis during the stress event results in increased
NADP+/NADPH ratios which promote increased activity of the oxidative pentose phosphate
pathway. This pathway provides precursors for the synthesis of secondary metabolites with
important roles in stress defence, and may protect the cells from photoinhibition when the use of
reducing power by the Calvin cycle is decreased by water deficit. The subsequent proline
degradation upon stress relief results in the formation of NADPH and increased ATP synthesis,
27
Chapter I.
which will benefit plant growth and recovery from stress. From this point of view, proline
accumulation can be seen as a means of energy storage (Hare & Cress 1997).
Hare & Cress (1997) discussed the tight link between proline biosynthesis and the
oxidative pentose phosphate pathway, which is regulated by the ratio NADP+/NADPH and will
be activated by NADPH oxidation. Both the oxidative and reductive pentose phosphate pathways
can provide carbon in the form of E4P, one of the precursors to the biosynthesis of phenylalanine
(together with PEP) by the shikimate pathway (Ireland 1997). Because photosynthesis (and the
Calvin cycle) becomes limited under stress conditions, E4P for the production of
phenylpropanoids and other aromatic secondary compounds potentially involved in stress
defence mechanisms, is likely to result essentially from the oxidative pentose phosphate pathway
(Dennis et al. 1997). The activity of this pathway is stimulated by the increased NADP+
production resulting from increased proline biosynthesis. Additionally, Hare & Cress (1997)
suggested the potential involvement of proline in stress metabolic signalling, as a secondary
messenger. This was supported by the observation of a sixty-fold increase in proline content in
the phloem sap of Medicago sativa possibly associated with the transport of the imino acid to
leaf meristematic tissues where it would promote growth maintenance (Girousse et al. 1996).
Methionine is a sulphur-containing amino acid with pivotal roles in the structure and
catalytic function of proteins. The methionine cycle leads to the production of S-adenosyl-Lmethionine (SAM), which can be used to the synthesis of ethylene, involving the intermediate 1aminocyclopropane-1-carboxylic acid (ACC), or to the synthesis of polyamines. Under stressful
conditions, including decreased water availability, methionine metabolism can be directed to an
enhanced production of ethylene (Wang et al. 2002) and polyamines (Groppa & Benavides
2008). Valine, leucine and isoleucine are precursors of secondary metabolites including
cyanogenic glycosides, glucosinolates and acyl-sugars (Coruzzi & Last 2000). Phenylalanine is
involved in the production of numerous secondary metabolites, including defensive phytoalexins,
bioactive alkaloids and structural lignin (Herrmann 1995). Lignin deposition reinforces vascular
tissues in grasses and its production involves the participation of free radicals. This structural
polymer and other phenolic compounds are often induced under stress conditions and can act in
the scavenging of reactive oxygen species (ROS), rendering the plant with increased antioxidant
capacity (Grace & Logan 2000).
The interactions between nitrogen and carbon metabolism and the metabolic shifts,
involving for instance proline metabolism and cycling with carbohydrates, SAM-dependent
methylation reactions and photorespiration, are likely to provide the plant with some flexibility
that enhances the capacity to cope with environmental stress conditions (Hare et al. 1998).
28
General Introduction and Objectives
Stomatal closure in response to soil water deficit
The decrease of stomatal conductance is one of the first responses to water deficit (e.g. Flexas &
Medrano 2002; Chaves et al. 2003) and can occur due to leaf dehydration or due to signals
synthesized by the drying roots to warn the plant of water shortage and promote stomatal closure
before the leaf water status changes (Majumdar et al. 1991; Zhang et al. 2001). Chemical signals,
namely ABA synthesized in the roots when water availability is reduced (Davies & Zhang 1991;
Davies et al. 2005), control stomatal behaviour at moderate water deficits (Borel et al. 2001;
Sauter et al. 2001; Wilkinson & Davies 2002). Drought stress increased ABA concentrations and
decreased stomatal conductance in C3 (Zhang et al. 2001; Liu et al. 2003) and in C4 plants
(Stikic & Davies 2000). Hydraulic (Chazen & Neumann 1994; Comstock 2002) and electrical
(Wilkinson & Davies 1997; Fromm & Fei 1998) signals were also involved in stomatal closure.
As reviewed by Wilkinson & Davies (2002) chemical, hydraulic or electrical signals as well as
the interaction between them can mediate the regulation of stomata under drying soil conditions.
In the field, the control of stomatal conductance may be used in order to improve the water use
efficiency, which reflects the amount of carbon assimilated per amount of water lost (Chaves et
al. 2002). In some, but not other, grass species, stomata respond directly to changes in
evaporative demand, decreasing stomatal conductance in response to increased vapour pressure
deficit and thereby increasing water use efficiency and improving the capacity to resist drought
stress (Maroco et al. 1997).
Because decreased stomatal conductance is a widely generalised response of plants to
decreased water availability it has been suggested as a reference parameter to access the relative
importance and timing of different metabolic limitations to photosynthesis (Medrano et al. 2002;
Flexas et al. 2004).
Limitations to photosynthesis under drought stress
Water deficit may limit photosynthesis through stomatal closure and/or metabolic impairment
(see review by Chaves et al. 2003) and the timing and relative importance of these factors have
been much debated in the past (Flexas & Medrano 2002; Lawlor 2002; Lawlor & Cornic 2002).
The CO2 diffusion inside the leaf can also be limited and contribute to decreased assimilation
under drought conditions (see review by Flexas et al. 2008).
Both stomatal and non-stomatal factors were found to limit photosynthesis in droughtstressed C4 plants and the relative importance of these factors may be different depending on the
rate of stress imposition (Saccardy et al. 1996; Marques da Silva & Arrabaça 2004a). In general,
29
Chapter I.
when water deficit is induced gradually, stomatal closure seems to be the major limitation to
photosynthesis under moderate stress with metabolic limitations becoming more relevant as
drought severity increases (e.g. Du et al. 1996). This may reflect a down-regulation of the
photosynthetic metabolism in order to match decreased availability of CO2 and growth rates
under drought (Chaves et al. 2003; Flexas et al. 2004).
The major metabolic processes limiting photosynthesis in conditions of decreased water
availability include photophosphorylation (Tezara et al. 1999), the capacity for regeneration of
RuBP (Giménez et al. 1992; Gunasekera & Berkowitz 1993) and the activity of Rubisco (Parry
et al. 2002). The importance of these and other metabolic factors will depend on the genotype
and plant developmental stage as well as on the rate, severity and duration of stress (Chaves et al.
2003). In C3 plants, decreased synthesis of ATP and RuBP are generally early events compared
to impairment of photochemical reaction and decreased Rubisco activity, which are mostly
observed only under severe stress conditions (Flexas & Medrano 2002).
In the field, plants are generally exposed to other stresses in addition to drought,
including high temperatures and irradiances, which ultimately result in oxidative stress. Powerful
plant antioxidant defence systems determine the resistance to stresses likely to cause oxidative
damage (Smirnoff 1998) by avoiding the irreversible impairment of the photosynthetic
machinery. Under conditions leading to decreased CO2 assimilation (e.g. water deficit) the
excess light may cause photoinhibition or promote the down-regulation of photosynthesis.
In plants with the C4 photosynthetic pathway, water deficit may affect the activities
and/or regulation of the enzymes involved in the C3 and C4 cycles and the regeneration of
substrates, thereby limiting or adjusting photosynthetic efficiency. A number of studies revealed
different possibilities of response to water deficit by the enzymes and substrates involved in the
mesophyll and bundle sheath reactions of C4 photosynthesis (Du et al. 1996; Lal & Edwards
1996; Saccardy et al. 1996; Foyer et al. 1998; Jagtap et al. 1998; Carmo-Silva et al. 2004;
Marques da Silva & Arrabaça 2004b; Carmo-Silva et al. 2007). Further detail on the activities
and regulation of the carboxylating and decarboxylating enzymes under drought conditions is
given in Chapters IV and V.
The understanding of the processes involved in the drought-induced limitation of
photosynthesis is fundamental to optimize plant performance and water use efficiency in the
present climate change scenario.
30
General Introduction and Objectives
OBJECTIVES OF THE THESIS
The main objective of this work was the study and comparison of drought-stress physiology and
biochemistry of three C4 grasses from different subtypes. The grasses Paspalum dilatatum
(NADP-ME), Cynodon dactylon (NAD-ME) and Zoysia japonica (PEPCK) were chosen due to
their high photosynthetic performance and water use efficiency, which are duly associated with
their economic and agronomic importance. The target was to understand how these different
species respond to gradually-induced drought stress and what characteristics determine their
resistance to conditions of decreased water availability.
To achieve this goal, the water relations, relative growth and leaf anatomy of wellwatered and drought-stressed plants of the three species were first studied (Chapter II) in an
attempt to understand the factors determining the water loss in their leaves and their possible
association with plant resilience to water deficits.
The effects of water deficit on leaf gas-exchanges and variations in photorespiratory rates
were subsequently assessed (Chapter III) through the analysis of CO2 assimilation at several
concentrations of CO2 and O2, carbon and oxygen isotope compositions and amino acids content
in fully illuminated leaves and after a period of 30 s in darkness.
The activities of the enzymes involved in the primary carboxylation and subsequent
decarboxylation steps of C4 photosynthetic pathway were studied in leaves from well-watered
and drought-stressed plants of the three grasses (Chapter IV) including an assessment of C4
subtype biochemistry.
The amount, activities and regulation of Rubisco from the three C4 species, including the
analysis of RuBP and CA1P contents, were investigated under water deficit (Chapter V).
Additionally, the kinetic properties of Rubisco were determined in order to allow future studies
with a more realistic modelling of C4 photosynthesis and photorespiration under drought
conditions promoting stomatal closure and decreased CO2 availability.
All these approaches were conducted in order to attain the global target of improving the
understanding of photosynthesis and photorespiration in C4 grasses under water deficit.
31
Chapter I.
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45
Chapter II.
WATER RELATIONS AND LEAF ANATOMY OF C4
GRASSES
Chapter II.
A shorter version of this chapter will be shortly submitted to Annals of Botany:
Carmo-Silva A.E., Francisco A., Powers S.J., Keys A.J., Ascensão L., Parry M.A.J. & Arrabaça M.C. (To
Submit) Drought resistance mechanisms are more effective in the C4 grasses Cynodon dactylon and
Zoysia japonica than in Paspalum dilatatum.
Alfred J. Keys (Department of Plant Sciences, Rothamsted Research) was involved in the measurements
of amino acids content and in the identification of HNV.
Ana Francisco and Lia Ascensão (Centro de Biotecnologia Vegetal and Departamento de Biologia
Vegetal, Faculdade de Ciências, Universidade de Lisboa) were involved in the studies of leaf anatomy,
including the sample preparation, analysis and interpretation of the results obtained.
Stephen J. Powers (Department of Biomathematics and Bioinformatics, Rothamsted Research) was
involved in the experimental design and in the statistical analysis of the data, especially the non-linear
modelling and the analyses of variance.
48
Water Relations and Leaf Anatomy of C4 Grasses
DROUGHT RESISTANCE MECHANISMS ARE MORE EFFECTIVE
IN THE C4 GRASSES Cynodon dactylon AND Zoysia japonica THAN IN
Paspalum dilatatum
ABSTRACT
Plants respond to drought stress with a number of changes that may confer resistance to the
harmful environmental condition. The effects of gradually-induced water deficit were studied in
three grasses with different C4 photosynthetic mechanisms, characterized by different
biochemical and anatomical features: Paspalum dilatatum Poiret (NADP-ME), Cynodon
dactylon (L.) Pers (NAD-ME) and Zoysia japonica Steudel (PEPCK). The leaf relative water
content decreased with drought in the three species but faster in P. dilatatum. The leaf water
potential decreased to a much greater extent in Z. japonica, the species that presented a higher
capacity to accumulate compatible solutes and has more rigid cell walls. All plants produced
fewer leaves and tillers in response to the drying soil. Increased leaf dry matter was observed in
all three species but leaf structure was not much altered by moderate drought stress. The
presence of cells with a specific role in leaf morphological responses that minimize water loss
was evident. Especially in C. dactylon and Z. japonica, drought stress caused increased contents
of amino acids in the soluble fraction of leaves. An unusual hydroxylated amino acid, not
previously reported in leaves, was present in Z. japonica and its synthesis was induced by water
deficit in C. dactylon.
KEYWORDS:
Water relations, shoot growth, leaf anatomy, amino acids, water deficit
49
Chapter II.
INTRODUCTION
Decreased water availability and the need to improve plant performance with increased water use
efficiency constitute one of the major challenges of the modern humanity, as recognised by the
Global Environment Outlook: Environment for Development (Geo-4), issued by the United
Nations Environment Programme in 2007. The use of species better adapted to conditions of
mild to moderate water deficit and the production of new varieties with increased drought
resistance, through plant breeding programmes and/or genetic engineering, are promising
approaches that depend on the understanding of plant responses under gradually-induced, fieldlike drought conditions. A number of morphological, physiological, biochemical and molecular
changes might result in increased capacity of plants to resist unfavourable environments (Bray
1997; Chaves et al. 2003). Drought resistance strategies include mechanisms that minimize
tissue dehydration and mechanisms of dehydration tolerance, the latter allowing the maintenance
of active physiological and biochemical processes. Internal regulations including root-shoot
signals (Davies & Zhang 1991) and structural components in the leaves are likely to play a
determinant role in plant responses to drought stress (Lösch & Schulze 1995).
Some plants minimize water loss from the leaves through mechanisms that decrease
evapotranspiration, including stomatal closure and leaf rolling or folding, whilst others adopt
mechanisms that increase water absorption from the drying soil, for instance decreasing the leaf
water potential. The latter may involve changes in turgor pressure, which depends on cell wall
plasticity, or in the osmotic potential, which is a function of the concentration of solutes in the
cell (Kramer & Boyer 1995; Lambers et al. 1998). Osmotic adjustment, the ability of the plant to
decrease the osmotic potential through the accumulation of compatible, non-toxic solutes, and
hence decrease the water potential of the tissues, minimizing water loss (Morgan 1984; Kramer
& Boyer 1995), has long been recognized as one of the most important mechanisms of plant
adaptation to drought stress (Bray 1993). The capacity to adjust osmotically might be
determinant for survival under conditions of severe water deficit in regions where long summerdroughts dominate (e.g. Utrillas & Alegre 1997; Volaire et al. 1998). A number of protective
roles have been ascribed to the presence of some osmolytes and their metabolism in the cells,
including the protection of membranes and proteins, the scavenging of free radicals and the
buffering of cellular redox potential (Hare et al. 1998). The beneficial roles of drought-induced
osmolyte accumulation for plant productivity and crop yield are still a matter of controversy
(Serraj & Sinclair 2002) and their correlation with plant drought tolerance is not always observed.
Nevertheless, the capacity for osmotic adjustment, involving either roles in osmoprotection or
50
Water Relations and Leaf Anatomy of C4 Grasses
the antioxidant defence system, has been associated with drought tolerance in maize (Nayyar
2003; Nayyar & Gupta 2006), sugarcane (Molinari et al. 2007) and pearl millet (Kusaka et al.
2005). Amino acids, and especially proline, can increase markedly in C4 grasses under water
deficit and are likely to play a role in their drought resistance capacity (Jones 1985). In addition
to the possible contribution to osmoregulation, increased contents of some amino acids such as
methionine and phenylalanine may be also associated with increased synthesis of compounds
involved in plant stress responses.
Decreased rates of shoot growth, mediated by ethylene (Hussain et al. 1999) are well
established effects of drought stress on C4 grasses and may involve decreased leaf expansion
(fewer or smaller cells), decreased rate of leaf appearance and/or leaf development (Jones 1985).
Inhibition of maize shoot growth in response to decreased water availability can occur before the
leaf water potential is decreased (Saab & Sharp 1989). Increased ABA production in response to
drying soil restricts ethylene production (Sharp 2002), thereby counteracting the inhibition of
root elongation and attenuating the inhibition of shoot growth. Root growth maintenance under
water deficit conditions is also an adaptive mechanism that allows water to be taken up from
deeper soil layers (Bartels & Sunkar 2005).
Water loss from the cells may cause leaf shrinkage. Severe dehydration often results in
impaired membranes and distorted organelles (Kramer & Boyer 1995) and, ultimately, folding of
the cell walls can be observed (e.g. Utrillas & Alegre 1997). Some plants may develop adaptive
strategies to resist cell shrinkage, for instance, through enhanced deposition of rigid cell wall
constituents (e.g. lignin). Controlled changes in leaf anatomy developed under gradually-induced
drought conditions may increase plant resistance by favouring the stability of macromolecules
and the maintenance of biochemical reactions. The leaf Kranz anatomy associated with the C4
photosynthetic pathway present in many grasses (see Dengler et al. 1994) may diminish their
potential for phenotypic plasticity and thus minimize their capacity for acclimation to the
environmental changes (Sage & McKown 2006).
Three classical biochemical subtypes of C4 photosynthesis have been identified based on
the major enzyme responsible for the decarboxylation of C4 acids, which are produced in the
mesophyll (M) cells, where primary assimilation of CO2 takes place, and then transported to the
bundle sheath (BS) cells, where carbon is assimilated through the C3 cycle. The decarboxylating
enzymes that give name to these subtypes are NADP-malic enzyme (NADP-ME subtype), NADmalic enzyme (NAD-ME subtype) and phosphoenolpyruvate carboxykinase (PEPCK subtype).
In addition to the biochemistry, there are differential anatomical features between these subtypes
(see Dengler & Nelson 1999 for review) and most C4 grasses fit into one of the three classical
51
Chapter II.
anatomical-biochemical subtypes (Hattersley & Watson 1992). Differences in leaf structure may
be related with differential water loss by the tissues and therefore it becomes important to
understand the possible contribution of leaf anatomy to plant responses under limited water
availability.
C4 plants are generally assumed to be better adapted to conditions of decreased water
availability than their C3 counterparts, mostly due to their greater water use efficiency (Long
1999). The use of warm-season (C4) turfgrasses in recreational areas might be a means to save
irrigation water. Several studies suggest an association between rainfall gradients and the
distribution of grass species from the different biochemical subtypes of C4 photosynthesis
(Henderson et al. 1995; Cabido et al. 2008), possibly reflecting different strategies or features
that allow some species to cope better with drought. In a previous study, different responses to
rapidly-induced water deficit conditions were observed in three warm-season turfgrasses of the
different C4 metabolic subtypes: Paspalum dilatatum Poir. (NADP-ME), Cynodon dactylon (L.)
Pers (NAD-ME) and Zoysia japonica Steudel (PEPCK) (Carmo-Silva et al. 2007). However, it
was not possible to define whether those responses would reflect a better capacity of a certain
species to cope with conditions of decreased water supply in the field.
The aim of this work was to understand the primary responses to gradually-imposed
drought conditions in these three different C4 grasses. Water availability in the soil was
characterised and the corresponding water deficit in the plants measured, through the leaf
relative water content and water potential. The effects of drought stress on leaf and tiller
production, leaf dry matter and specific leaf area, leaf anatomy and soluble amino acids were
analysed in well-watered and non-watered plants of P. dilatatum (dallisgrass), C. dactylon
(bermudagrass) and Z. japonica (zoysiagrass).
52
Water Relations and Leaf Anatomy of C4 Grasses
MATERIAL AND METHODS
Plant material and drought stress induction
The C4 grasses Paspalum dilatatum Poir. cv. Raki (NADP-ME), Cynodon dactylon (L.) Pers var.
Shangri-Lá (NAD-ME) and Zoysia japonica Steudel ‘Jacklin Sunrise Brand’ (produced by
Jacklin Seed Company, USA) (PEPCK) were grown from seeds in pots with peat-free compost
(Petersfield Products, Leicester, UK) supplemented with a slow-release fertiliser (Hydro Agri
Ltd, Lincs, UK) in a glasshouse. Artificial light was provided whenever the natural light was
below a photosynthetic photon flux density (PPFD) of 500 μmol m-2 s-1 during a 16 h
photoperiod. Temperature was maintained at a minimum of 25ºC during the day and at 18ºC
during the night.
Seeds of each species were washed with 10% hypochlorite and soaked in water for 1 hour
before sowing. Water was supplied whenever needed during two weeks for P. dilatatum and C.
dactylon and four to five weeks for the slower growing Z. japonica. The seedlings were
transplanted to 1-L cylindrical pots containing equal amounts of soil. Five seedlings were used
per pot. All plants were well-watered until the beginning of the drought stress treatment. Pots
were placed according to a split-plot design, where each column of pots was a main plot of a
particular species and the sampling-days and the treatments (control vs. drought stress) were
randomised in the split-plots. Each pot corresponded to an independent sample. Water deficit
was imposed consecutively on the three species, separated by one day, with watering of the
‘stress’ pots being stopped first in C. dactylon, then in Z. japonica and at last in P. dilatatum in
order to get more similar degrees of leaf dehydration for the three species because the leaves
from P. dilatatum tended to dehydrate faster than those from Z. japonica and C. dactylon. The
‘control’ pots were watered once per day.
All three species were analysed simultaneously, for four consecutive days during the
drought period, starting when the weight of non-watered pots had been suitably decreased and
ending after a maximum of twelve days without watering. Five-week old plants of P. dilatatum
and C. dactylon and nine-week old plants of Z. japonica were analysed. The youngest fully
expanded leaf of each P. dilatatum plant and two young fully expanded leaves of each plant of C.
dactylon or Z. japonica were always used. Samples were collected in the growth environment
four or five hours after the beginning of the photoperiod. It was assumed that, within each pot,
all the leaves used were identical in terms of developmental stage, physiological and biochemical
properties, and would have experienced the same drought condition.
53
Chapter II.
Water relations and leaf parameters
Four control and eight non-watered pots were used per species per day during four consecutive
days making a total of 40 samples per species (16 control and 24 non-watered). A leaf sample
was collected from each pot, four hours after the beginning of the photoperiod, for determination
of the leaf relative water content (RWC). Taking into account the different leaf sizes, each
sample of P. dilatatum consisted of two leaves from two plants whereas each sample of C.
dactylon or Z. japonica consisted of five leaves from two plants. The fresh (FW), turgid (TW)
and dry (DW) weights were determined and used to calculate RWC by the equation RWC =
100((FW-DW)/(TW-DW)) (Catsky 1960). Leaf area was determined by scanning the turgid
leaves and analysing the image using the software Paint Shop Pro 9 (Jasc Software, Inc., USA)
and Image J 1.33u (National Institutes of Health, USA). The ratio between the dry and the turgid
leaf weight (DW/TW) was calculated as a percentage and the specific leaf area (SLA) was
calculated as the ratio between the leaf area and DW.
The leaf water potential (LWP) was measured on the middle part of young fully
expanded leaves taken from each pot five hours after the beginning of the photoperiod using a
pressure-chamber (Ritchie & Hinckley 1975).
The water weight in each pot (WWP) was determined as the weight of the pot at each
sampling time less 400 g (the mean weight of the pots with plants and totally dried soil was 404
± 16 g). The soil water content (SWC) was determined in three opposite locations in each pot
using an HH2 moisture meter with a Theta probe (type ML2x, AT Delta-t Devices Ltd.,
Cambridge, UK).
Growth parameters
Three control and three non-watered pots were used per species and all samples were analysed
on the same day. The mean numbers of leaves and tillers formed by the control and droughtstressed plants in twelve days were determined by analysing all the five plants in each pot (15
plants per species per treatment, with a total of 90 plants) at the beginning of the experiment, the
last watering day for the first species, and at the end of the drought treatment, corresponding to
10, 11 and 12 days without watering the plants of P. dilatatum, Z. japonica and C. dactylon,
respectively.
54
Water Relations and Leaf Anatomy of C4 Grasses
Leaf anatomy
Leaf samples of three control and three non-watered pots of each species were taken for
anatomical observations at the end of the drought treatment (10-12 days without watering),
simultaneously for the three species. Because anatomical variation occurs along the leaf length
(e.g. Miranda et al. 1981), samples were taken from the middle portion of leaf blades, equidistant
from both ends. The leaf segments were fixed with 2.5% glutaraldehyde in 0.1 M sodium
phosphate buffer (pH 7.2), dehydrated in a graded ethanol series and then infiltrated with and
embedded in Leica Historesin (Leica Microsystems, Germany). Transverse sections (2 μm) were
cut using a rotatory microtome (RM2155, Leica Microsystems) and subsequently stained with
0.05% Toluidine Blue O (Gutmann 1995). Observations were carried out on a BX60F5 Olympus
light microscope (Olympus Optical Co., Ltd., Japan) and images were recorded digitally using a
DP50 Olympus camera and the Viewfinder Lite (Pixera Corporation, USA) software.
Quantitative anatomical data from the images of leaf transverse sections was gathered
using the Image-Pro Express 6.0 (Media Cybernetics Inc., USA) software. Three analytical
replicates (micro slides) were analysed for each biological sample (three controls and three
drought-stressed per species). The interveinal distance was measured for each pair of
longitudinal veins present on a half-leaf section. The leaf thickness was measured on the thickest
part of each vein of the same half-leaf section. The transverse half-leaf sections were divided
into four different zones, from the centre to the margin of the leaf, in order to have a
representative sampling of the leaf anatomy. These four zones corresponded to: (1) the central
longitudinal vein, (2) a large longitudinal vein or (3) two small longitudinal veins in between the
centre and the margin of the leaf, and (4) the two small longitudinal veins closest to the margin.
The cross-sectional areas of the bundle sheath cells, mesophyll cells, vascular tissues (including
mestome sheath if present), bulliform and colourless cells, sclerenchyma cells, intercellular
spaces, lower and upper epidermis (see diagram from Dengler et al. 1994 Figure 1), were
measured considering the four zones above.
Analysis of amino compounds by HPLC
Amino acids and other amino compounds contained in the leaves were determined by HighPerformance Liquid Chromatography (HPLC) of o-pthaldialdehyde (OPA) derivatives (Noctor
& Foyer 1998). Three control and five non-watered pots were used per species per day during
three consecutive days, making a total of 24 samples per species (nine control and fifteen nonwatered pots). The plants within each pot were used for the collection of a leaf sample that was
55
Chapter II.
immediately frozen in liquid N2 (LN2) in fully illuminated conditions (four hours after the
beginning of the photoperiod) and another leaf sample for the determination of RWC. Taking
into account the different leaf sizes, each frozen sample consisted of one leaf for P. dilatatum
and three leaves for C. dactylon and Z. japonica. Similarly, the corresponding RWC samples
consisted of two leaves for P. dilatatum and four leaves for C. dactylon and Z. japonica.
Reversed-phase HPLC was performed using a Waters Alliance 2695 Separation Module and a
474 Scanning Fluorescence Detector operated by the Millenium32 software (Waters, Milford,
USA) with a Waters Symetry C18 4.6×150 mm column (Part No. WAT 054278) protected with a
4×3 mm guard cartridge (Phenomenex, Torrance, USA). Since the fluorescent adducts formed by
reaction with OPA in the presence of 2-mercaptoethanol are unstable, the autosampler was set to
mix and pre-incubate 10 μL of each sample with 15 μL of OPA reagent for 2 min before
injecting the mixture onto the column. The eluent used for the amino compounds separation was
obtained by mixing solvents containing different proportions of methanol, sodium acetate pH 5.9
and tetrahydrofuran.
Amino compounds were extracted from the frozen leaf samples stored at -80ºC. Each
sample was ground in LN2 and then 1.4 mL of 0.1 M HCl was added to the fine powder. The
mixture was ground further during thawing and the homogenate was centrifuged for 10 min at
16000 ×g and 4ºC. Samples for HPLC were prepared by adding a sub-sample of each
supernatant to the internal standard and pure water, and these mixtures were stored at -20ºC. On
the following day, the mixtures were centrifuged for 40 min at 16000 ×g and 4ºC and then
filtered with syringe filters (0.2 μm) into HPLC autosampler vials. Standard solutions of αamino-n-butyric acid (internal standard), valine, methionine, phenylalanine, isoleucine, leucine,
ethanolamine and 5-hydroxy-norvaline were prepared in 0.1 M HCl and diluted to have
increasing concentrations for the calibration curves (0, 5, 10, 15, 20 and 25 μM).
Proline estimation
The proline content in the acid leaf extracts above was determined after reaction with ninhydrin
(Bates et al. 1973). Standard solutions of proline with concentrations rising from 0 to 100 μg /
mL in 0.1 M HCl were used for the calibration curve. According to Chinard (1952) a linear
relationship is obtained between the concentration of proline and the optical density in the range
0.02 mM and 0.1 mM. A volume of 0.25 mL was taken from each sample and standard solution
into a 15 mL centriuge tube and mixed with 0.75 mL of 3% sulfosalicylic acid, 1 mL of glacial
acetic acid and 1 mL of acid-ninhydrin reagent (140 mM ninhydrin in a solution 2:3 of glacial
56
Water Relations and Leaf Anatomy of C4 Grasses
acetic acid and 6 M orthophosphoric acid), consecutively. The tubes were thoroughly vortexed
and incubated at 100ºC for 1 hour. At a very low pH (~1.0) a red water-insoluble product is
formed in the reaction between proline and ninhydrin and no significant amounts of coloured
products seem to be formed by most of the other amino acids under these extremely acidic
conditions. The reaction was stopped by placing the tubes on ice for 5 min. Toluene (2 mL) was
added to each tube, and after mixing thoroughly, the phase separation occurred and the
absorbance of the upper phase at 520 nm was measured (CARY 300 Bio UV-Visible
Spectrophotometer, Varian Analytical Instruments, Varian Inc., Surrey, UK) using toluene as
background.
Statistical analysis
All the statistical analyses were made using GenStat® 9.2, 2005 (Lawes Agricultural Trust,
Rothamsted Research, UK). Regression analysis was applied to model the relationship between
WWP or SWC with days without watering, between SWC and WWP and between LWP or
DW/TW or SLA with SWC, taking into account the control vs. non-watered condition of the pots.
Regression analysis was also applied to model the variation of the amino acids content with
RWC, this time considering a squared term in this variable to check for non-linearity. The
variation of RWC with SWC and the relationship between LWP and RWC were assessed
through the use of non-linear regression (by Stephen J. Powers), fitting an asymptotic
exponential and a simple exponential model, respectively, and accounting for differences
between species by way of model parameters. Nested models were compared using F-tests and
then the non-significantly different (P > 0.05) parameters (t-tests) in the significant terms (P <
0.05) of each model were amalgamated in order to attain parsimony. The resulting best models
were plotted and the parameter estimates with their respective standard errors (s.e.), the
percentage of variance accounted for by the model (R2), the residual mean square (s2) and the
degrees of freedom (d.f.) are given with the plots. The residuals were checked and found to
generally conform to the assumptions of the analysis. All the absolute values and percentages
presented in the text were calculated in accordance with the regression analysis performed.
Analysis of variance (ANOVA) was applied (by Stephen J. Powers) to the growth
parameters (numbers of leaves and tillers) and leaf anatomy measurements (number of veins,
interveinal distance, leaf thickness, and the percentages of cellular areas) to check for statistically
significant (P < 0.05) differences between species, treatments (control vs. drought stress) and for
the interaction between these two factors. Prior to ANOVA, a square-root transformation was
57
Chapter II.
applied to the number of leaves and the number of tillers produced per plant. This transformation
ensured the assumptions of the analysis were not violated. For the analysis, the sets of five
individual plants in each pot were taken as analytical replicates, the variability within these sets
being output in the ANOVA. The numbers of leaves and tillers in the control and non-watered
plants at the beginning of the drought period were checked to be not significantly different
between the two treatments for each species (P > 0.05). The number of veins in each half-leaf
was analysed using a generalised linear mixed model (GLMM) by the fitting method of Schall
(1991) and assuming a Poisson distribution with a log link function for the model, which takes
account of the design structure of micro slides (analytical replicates) within samples. The
statistical significance of the different effects was assessed through an F-test (Welham &
Thompson 1997) on the appropriate degrees of freedom. The mean values of interveinal
distances, measured between each pair of consecutive veins, and the leaf thickness, measured at
the thickest part of each vein, per half-leaf were analysed using ANOVA, with a log
transformation of the data to ensure homogeneity of variance, and taking into account the design
structure of laminas within samples. For the different tissue areas, the grand total of area
measured per half-leaf was calculated as the sum of the areas of all four zones, and the
percentage of each type of tissue in relation to the grand total was determined. The values
obtained for each half-leaf of total area, tissue percentages and ratio M/BS were analysed using
ANOVA, again with a log transformation required. After all ANOVAs, following assessment of
the statistical significance of factors (species, treatments and their interaction), the Least
Significant Difference (LSD) at the 5% level of significance was used to test between relevant
means.
58
Water Relations and Leaf Anatomy of C4 Grasses
RESULTS
Amount of water in the soil
The amount of water in the soil of all pots was measured both as the water weight in pot (WWP)
and as the soil water content (SWC). The WWP and the SWC decreased linearly along the
sampling period, whilst the corresponding control pots used in each day showed no significant
variation in water availability (Figure II.1A-D). Parallel lines fitted the data from the three
species and hence the plants that were deprived of water in separate, consecutive days, had
reasonably the same amount of water in the soil on each sampling-day.
There was a very strong correlation (r = 0.996) between the SWC and the WWP (Figure
II.1E-F). The variation of SWC with WWP in the pots of C. dactylon and Z. japonica was not
significantly different (P > 0.05) and the two species were fitted with the same lines, whereas for
P. dilatatum separate, but parallel, lines were fitted, which is probably related with the plants of
this species being heavier than the former two species.
Leaf water status
The relative water content (RWC) in the leaves of P. dilatatum, C. dactylon and Z. japonica
started to decrease only when the amount of water in the soil decreased below a certain threshold,
so that the variation of RWC with the soil water content (SWC) was described by an asymptotic
exponential model (Figure II.2A). There was no significant difference (P > 0.05) between the
three species.
The leaf water potential (LWP) was lower in the non-watered plants compared to their
controls (Figure II.2B), decreasing linearly with SWC in the non-watered pots. In fully hydrated
leaves of P. dilatatum and C. dactylon the LWP was higher than in Z. japonica and droughtstressed plants of the latter species showed a much steeper decrease of LWP with SWC.
As could be expected from the above, the relationship between the two variables related
with the leaf water status, RWC and LWP, was clearly different for Z. japonica than for the other
two species (Figure II.2C). An exponential model was used for the variation of LWP with the
RWC in P. dilatatum and C. dactylon. The LWP decreased from -0.6 MPa to a minimum of -1.3
MPa, as the RWC decreased from 99 to 93%, and then remained constant for further decreased
RWC. On the contrary, in Z. japonica the LWP decreased linearly with the RWC, from a value
59
Chapter II.
Control pots
Non-watered pots
500 A
B
200
WWP (g)
400
150
300
100
200
50
100
y = 346 - 26.4 x (s.e. 24; 2.7)
y = 382 - 26.4 x (s.e. 29; 2.7)
y = 367 - 26.4 x (s.e. 26; 2.7)
0
7
SWC (%)
40
8
9
10
11
12
C
7
8
9
10
11
0
12
D
12
10
30
8
20
6
4
10
y = 20.8 - 1.62 x (s.e. 1.5; 0.17)
y = 23.6 - 1.62 x (s.e. 1.9; 0.17)
y = 22.7 - 1.62 x (s.e. 1.7; 0.17)
0
7
8
9
10
11
12
7
Days without watering
8
9
10
11
2
0
12
Days without watering
Strong correlation between SWC and WWP (r = 0.996)
E
F
12
40
SWC (%)
Control pots
10
Non-watered pots
8
35
6
4
30
2
25
y = 7.69 + 0.0585 x (s.e. 0.73; 0.0011)
y = 8.82 + 0.0585 x (s.e. 8.82; 0.0011)
300 350 400 450 500 550
WWP (g)
y = 0.0585 x (s.e. 0.0011)
y = 0.54 + 0.0585 x (s.e. 0.17; 0.0011)
0
50
100
150
0
200
WWP (g)
Figure II.1. Amount of water in the soil in the control and non-watered pots with plants of P. dilatatum
(black diamonds), C. dactylon (grey squares) and Z. japonica (white triangles). (A-D) Variation of the
water weight in pot (WWP, g) and soil water content (SWC, %) during the sampling days, from the
beginning of treatments when water was withheld from the stress pots of each species. WWP was
calculated considering that pots with dry soil weighed 400g. (E-F) Relationship between SWC and WWP.
Overall correlation coefficient, r = 0.996. In all cases, each data point corresponds to one sample (with 40
samples per species). The regression lines applied correspond to the best models statistically significant
(P. dilatatum, solid lines; C. dactylon, dashed lines; Z. japonica, dotted lines; A-B, R2 = 97.7%, s2 =
665.9, d.f. = 113; C-D, R2 = 98.6%, s2 = 2.72, d.f. = 113; E-F, R2 = 99.7%, s2 = 0.469, d.f. = 116).
60
Water Relations and Leaf Anatomy of C4 Grasses
of -1.2 MPa, obtained in fully hydrated leaves, to -3.5 MPa in the most dehydrated leaves, with
ca. 77% RWC.
100
A
RWC (%)
95
90
85
80
75
- 0.4498 x
y = 98.30 (1 - e
)
(s.e. 0.31; 0.0100)
70
0
0
5
10
30 40
B
LWP (MPa)
-1
-2
-3
y = - 1.52 + 0.042 x (s.e. 0.10; 0.014)
-4
Figure II.2. Water relations in the leaves of
control and non-watered plants of P. dilatatum
(black diamonds), C. dactylon (grey squares)
and Z. japonica (white triangles). (A-B)
Variation of the leaf relative water content
(RWC, %) and leaf water potential (LWP,
MPa) with the soil water content (SWC, %).
(C) Relationship between LWP and RWC.
Each data point corresponds to one sample
(with 40 samples per species). The best models
statistically significant were applied (P.
dilatatum, solid lines; C. dactylon, dashed
lines; Z. japonica, dotted lines; A, R2 = 81.5%,
s2 = 6.387, d.f. = 118; B, R2 = 84.4%, s2 =
0.04003, d.f. = 114; C, R2 = 84.1%, s2 =
0.0408, d.f. = 115).
y = - 3.21 + 0.167 x (s.e. 0.17; 0.022)
0
5
10
30 40
SWC (%)
0
C
LWP (MPa)
-1
-2
-3
-17
0.3795 x
y = - 1.347 + 3.47x10 e
-16
(s.e. 0.052; 2.54x10 ; 0.0692)
-4
y = - 11.315 + 0.1021 x (s.e. 0.309; 0.0006)
70
75
80
85
90
95
100
RWC (%)
61
Chapter II.
Leaf parameters
The ratio between the dry and the turgid leaf weight (DW/TW) was much higher in Z. japonica
than in C. dactylon and was the smallest in P. dilatatum (Figure II.3A), revealing that the
percentage of dry matter in the leaves of the former species is greatest. An increase of the ratio
DW/TW with SWC was observed for the non-watered plants of the three species. The specific
leaf area (SLA) was higher in the control plants of P. dilatatum and C. dactylon than in Z.
japonica and decreased with SWC for non-watered plants of the first two grasses, but did not
change significantly (P > 0.05) in the drought-stressed leaves of Z. japonica (Figure II.3B).
40
A
DW/TW (%)
30
20
10
y = 17.96 - 0.3755 x (s.e. 0.74; 0.0690)
y = 22.37 - 0.4109 x (s.e. 0.74; 0.0760)
y = 33.93 - 0.5250 x (s.e. 0.74; 0.0713)
0
0
60
5
10
30 40
B
-1
40
2
SLA (m Kg )
50
30
Figure II.3. Leaf parameters of control and
non-watered plants of P. dilatatum (black
diamonds), C. dactylon (grey squares) and Z.
japonica (white triangles). Variation of the
ratio between the leaf dry weight and turgid
weight (DW/TW, %) and the specific leaf area
(SLA, m2 Kg-1) with the soil water content
(SWC, %). Each data point corresponds to one
sample (with 40 samples per species). The best
models statistically significant were applied (P.
dilatatum, solid lines; C. dactylon, dashed
lines; Z. japonica, dotted lines; A, R2 = 97.4%,
s2 = 1.131, d.f. = 113; B, R2 = 95.1%, s2 =
4.363, d.f. = 114).
20
10
y = 41.09 + 1.046 x (s.e. 1.40; 0.188)
y = 38.92 + 1.473 x (s.e. 1.53; 0.221)
0
0
5
10
30 40
SWC (%)
Number of leaves and tillers
Although the plants were still not looking very stressed and no substantial decrease was observed
in their RWC by the end of the drought period, which last for a maximum of 12 days in this
experiment, there was a visible decrease in the number of leaves and tillers produced during the
drought period by the plants of all three species. Statistical analysis was applied to the square
62
Water Relations and Leaf Anatomy of C4 Grasses
root of the numbers measured. The graphs in Figure II.4 are presented in order to give a visual
impression of the variation of the raw data. There was a highly significant (P < 0.001) difference
between species and a strong effect (P ≤ 0.01) of the drought stress treatment on the square root
of the number of leaves and the square root of the number of tillers formed per plant during the
period of stress. However, there was no significant interaction between the species and treatment,
A
8
B
-1
20
Number of Tillers (plant )
-1
Number of Leaves (plant )
the stress effect being the same for the three species.
15
10
5
0
6
4
2
0
P. dilatatum C. dactylon Z. japonica
Square root of
N. of Leaves
P. dilatatum C. dactylon Z. japonica
P. dilatatum
C. dactylon
Z. japonica
Control
Non-watered
2.54
3.95
3.95
3.64
3.32
SED
0.14
d.f.
12
12
LSD(5%)
0.29
0.24
Square root of
N. of Tillers
SED
1.37
1.76
0.09
0.11
2.35
1.94
1.71
0.07
d.f.
12
12
LSD(5%)
0.19
0.15
Figure II.4. Number of leaves and tillers produced (per plant) by the control (black bars) and nonwatered (white bars) plants of P. dilatatum, C. dactylon and Z. japonica during the drought period (twelve
days). Each value corresponds to the mean of fifteen plants (three control and three non-watered pots with
five plants each were used per species) ± standard errors. Also presented are the means of the square root
of the numbers of leaves and tillers produced by the plants of each species and by the control and nonwatered plants, as given by the ANOVA, the standard error of the difference between means (SED), the
degrees of freedom (d.f.) and least significant differences (LSD) at the 5% level for comparison between
species and for comparison between tretaments.
Leaf anatomy
Leaf anatomical features of the C4 grasses, P. dilatatum, C. dactylon and Z. japonica, are shown
in Figures II.5 and II.6. Paspalum dilatatum is characterized by a proeminent midrib with
63
Chapter II.
M
BS
M
BS
Scl
A
D
Bul
M
BS
MS
M
BS
Scl
B
E
Bul
M
BS
MS
M
BS
Scl
C
F
Figure II.5. Transverse sections of young fully-expanded leaves of P. dilatatum (A, D), C. dactylon (B,
E) and Z. japonica (C, F) stained with Toluidine Blue O. The central longitudinal vein (A-C) and one
large longitudinal vein (D-F) of a half-leaf considered for the measurement of cellular areas are shown.
Abbreviations used: BS, bundle sheath; Bul, bulliform cells; M, mesophyll; MS, mestome sheath; Scl,
sclerenchyma. Scale bars = 30 μm.
64
Water Relations and Leaf Anatomy of C4 Grasses
Scl
ICS
A
B
D
E
Scl
C
F
Figure II.6. Transverse sections of young fully-expanded leaves of P. dilatatum (A, D), C. dactylon (B,
E) and Z. japonica (C, F) stained with Toluidine Blue O. Two small longitudinal veins between the centre
and the margin (A-C) or at the margin (D-F) of one half-leaf considered for the measurement of cellular
areas are shown. Abbreviations used: ICS, intercellular space; Scl, sclerenchyma. Scale bars = 60 μm.
numerous colourless cells and containing several veins, whilst in the other two species the midrib
is formed by one single vein. Bulliform cells are present in between consecutive veins in C.
dactylon and Z. japonica, but are absent in P. dilatatum (Figures II.5 and II.6). Sclerenchyma is
present in greatest proportion in Z. japonica and occurs at the leaf margins of both P. dilatatum
and Z. japonica, but not in C. dactylon.
65
Chapter II.
In the central longitudinal veins, the BS cells, filled with abundant chloroplasts, surround
the vascular tissues and are surrounded by the M cells, with less abundant and smaller
chloroplasts. In C. dactylon and Z. japonica, but not in P. dilatatum, a mestome sheath is present
between the BS cells and the vascular bundles. The chloroplasts of BS cells are oval and
distributed centrifugally or scattered around the cells in P. dilatatum and Z. japonica whilst in C.
dactylon the BS chloroplasts are elongated and have a centripetal position in the cells.
Quantitative data on leaf anatomy was obtained by measuring lengths and cellular areas
in the leaf transverse sections. A log transformation of each set of data presented in Figures II.7,
II.8 and II.9 was required in order to stabilise the variance across treatments and allowed the
ANOVA to be performed. Therefore, the graphs with the raw data are presented but all the
comparisons referred to were analysed on the log scale.
The leaves of P. dilatatum are wider and longer than those of the other two species,
which is associated with the greater number of veins in P. dilatatum than in Z. japonica and
lowest in C. dactylon (Figure II.7A). The analysis performed revealed no significant effect (P >
0.05) of drought stress on the vein number. Similarly, no significant differences (P > 0.05)
between control and drought-stressed plants were output by the analysis of the interveinal
distances (IVD) and leaf thickness (Figure II.7B,C). The IVD was much higher in C. dactylon
than in Z. japonica and was the lowest in P. dilatatum. The leaf thickness was lower in Z.
japonica than in the other two species.
A
B
20
10
0.15
Thickness (μm)
0.15
IVD (μm)
Number of veins
30
C
0.10
0.05
0
0.00
P. dilatatum
C. dactylon
Z. japonica
0.10
0.05
0.00
P. dilatatum
C. dactylon
Z. japonica
P. dilatatum
C. dactylon
Z. japonica
Figure II.7. Number of veins per half-leaf (A), interveinal distance (B, IVD) and leaf thickness (C) of
control (black bars) and non-watered (white bars) plants of P. dilatatum, C. dactylon and Z. japonica at
the end of the drought period (after twelve days without watering). Values are means ± standard errors of
three leaf sections (analytical replicates) from each of three plant samples (biological replicates).
66
Water Relations and Leaf Anatomy of C4 Grasses
B
Percentage tissue (%)
A
Mesophyll
Bundle Sheath
C
D
30
Intercellular
spaces
20
Bulliform cells
10
0
P. dilatatum C. dactylon
Z. japonica
Z. japonica
F
E
Percentage tissue (%)
P. dilatatum C. dactylon
P. dilatatum C. dactylon
Z. japonica
G
P. dilatatum C. dactylon
Z. japonica
H
30
Lower epidermis
Sclerechyma
Vascular tissues
Upper epidermis
20
10
0
P. dilatatum C. dactylon
Z. japonica
P. dilatatum C. dactylon
Z. japonica
P. dilatatum C. dactylon
Z. japonica
P. dilatatum C. dactylon
Z. japonica
Mesophyll / Bundle Sheath
Figure II.8. Percentage of cellular area occupied by different tissues in the leaf transverse sections of
control (black bars) and non-watered (white bars) plants of P. dilatatum, C. dactylon and Z. japonica at
the end of the drought period (after twelve days without watering). Values correspond to the overall
percentage of each tissue in relation to the total area of leaf measured considering the sums of all four
zones (see Figures II.5 and II.6) and are means ± standard errors of three leaf sections (analytical
replicates) from each of three plant samples (biological replicates).
3
M/BS
2
1
0
P. dilatatum
C. dactylon
Z. japonica
Figure II.9. Proportion of leaf transversesectional area occupied by mesophyll or bundle
sheath cells in the control (black bars) and nonwatered (white bars) plants of P. dilatatum, C.
dactylon and Z. japonica at the end of the
drought period (after twelve days without
watering). Values correspond to the ratio
between overall M or BS areas in a half-leaf
considering the sums of all four zones measured
(see Figures II.5 and II.6) and are means ±
standard errors of three leaf sections from each
of three plant samples.
67
Chapter II.
The cellular areas of half-leaf transverse sections were measured in four representative
zones, with two zones (Figure II.5) corresponding to one large (central or not) longitudinal vein
and two other (Figure II.6) corresponding to two small longitudinal veins each. Even though the
leaves of C. dactylon are smaller than those of Z. japonica and P. dilatatum, the total area
measured was higher in the first species (data not shown), due to its higher IVD (Figure II.7B).
The three species were characterised by different percentages of the total area measured being
occupied by the different types of tissues (Figure II.8). Hence, the percentage of BS was higher
in C. dactylon and Z. japonica than in P. dilatatum whilst M cells were proportionally less
abundant in Z. japonica than in the other two species. As a result, the ratio M/BS was much
greater in P. dilatatum than in C. dactylon and was lowest in Z. japonica (Figure II.9).
The percentage of ICS was substantially larger in P. dilatatum than in C. dactylon and
very low in Z. japonica (Figure II.8), whereas the percentage of leaf area occupied by bulliform
cells was considerable in C. dactylon and Z. japonica, but larger in the latter. No bulliform cells
were observed in P. dilatatum. The presence of sclerenchyma was more abundant in Z. japonica
than in the other two species (Figures II.5 and II.6) and the percentage of leaf area occupied by
sclerenchyma was highest in Z. japonica, lowest in C. dactylon and intermediate in P. dilatatum
(Figure II.8). The percentage of area occupied by vascular tissues was also highest in Z. japonica.
The epidermal cells were larger in P. dilatatum than in C. dactylon and Z. japonica (Figures II.5
and II.6), resulting in higher relative areas being occupied by the epidermis in P. dilatatum than
in the other two species (Figure II.8).
Gradually-imposed decrease in water availability did not cause many changes in the leaf
quantitative anatomy. Nevertheless, there was an overall marginal effect (P ≤ 0.1) of drought
stress on the percentage of area occupied by the M cells and on the ratio M/BS, with an increase
of the ratio M/BS in the leaves of the non-watered plants of P. dilatatum and Z. japonica, but not
of C. dactylon, compared to the controls (Figure II.9).
Amino acids
The proline content was much increased with drought stress in the leaves of three grasses under
study (Figure II.10A). A quadratic variation with RWC was found for this amino acid, the
increase being steeper as dehydration became more severe. Proline increased by about 2.5-fold in
the three species when the RWC decreased to 80% and the values rose abruptly below a certain
threshold of RWC (70%), reaching values of 8 mmol m-2 in the most dehydrated samples of Z.
japonica (40% RWC).
68
Water Relations and Leaf Anatomy of C4 Grasses
The content in methionine was higher in Z. japonica and C. dactylon than in P. dilatatum
and increased with the decrease of RWC in a non-linear way for the three species, more
accentuated as the level of dehydration increased (Figure II.10B). The amino acids phenylalanine,
valine, isoleucine and leucine were also much increased in the dehydrated leaves of Z. japonica
(Figure II.10C-F). In P. dilatatum and C. dactylon the content in leucine was negligible and the
other three amino acids increased with drought stress to a much lower extent than in Z. japonica.
Nevertheless, in C. dactylon valine increased three-fold when RWC decreased from 98 to 60%.
10
A
2
y = 2106 - 42.22 x + 0.2124 x
(s.e. 134; 3.72; 0.0248)
0.015
0.25
y = 62.50 - 1.159 x + 0.00543 x
(s.e. 4.35; 0.128; 0.00089)
0.20
Met
0.009
0.10
2
0.003
0.05
0.000
0.5
D
0.4
60
80
100
Val
0.3
0.00
40
2
y = 48.19 - 0.993 x + 0.00521 x
(s.e. 7.90; 0.178; 0.00104)
2
y = 49.73 - 0.993 x + 0.00521 x
(s.e. 7.83; 0.178; 0.00104)
2
y = 76.68 - 1.287 x + 0.00521 x
(s.e. 5.10; 0.150; 0.00104)
0.15
E
60
80
100
40
2
y = 11.35 - 0.2290 x + 0.00118 x
(s.e. 3.47; 0.0791; 0.00046)
2
0.15
0.12
Ile
0.06
0.1
0.03
0.03
0.00
80
RWC (%)
100
100
2
y = 19.14 - 0.1983 x + 0.000030 x
(s.e. 0.72; 0.0093; 0.000015)
Leu
0.09
0.06
60
80
0.12
0.2
40
F
60
y = 27.17 - 0.3938 x + 0.00118 x
(s.e. 2.27; 0.0667; 0.00046)
0.09
0.0
Phe
0.15
0.006
0
2
y = 40.14 - 0.937 x + 0.00543 x
(s.e. 6.65; 0.152; 0.00089)
2
4
40
C
2
Pro
6
2
y = 3.81 - 0.0834 x + 0.000456 x
(s.e. 0.24; 0.0066; 0.000044)
y = 3.90 - 0.0834 x + 0.000456 x
(s.e. 0.24; 0.0066; 0.000044)
0.012
8
-2
Amino Acids Content (mmol m )
B
0.00
40
60
80
RWC (%)
100
40
60
80
100
RWC (%)
Figure II.10. Variation of the amino acids content (mmol m-2) with the relative water content (RWC, %)
in the leaves of P. dilatatum (black diamonds), C. dactylon (grey squares) and Z. japonica (open
triangles): proline (Pro), valine (Val), methionine (Met), phenylalanine (Phe), isoleucine (Ile), leucine
(Leu) (A-F, respectively). Each data point corresponds to one sample, with 24 samples per species. The
regression lines and curves, when applied, correspond to the best models statistically significant (P.
dilatatum, solid lines; C. dactylon, dashed lines; and Z. japonica, dotted lines; A, R2 = 90.5%, s2 = 3255,
d.f. = 69; B, R2 = 94.3%, s2 = 3.792, d.f. = 66; C, R2 = 89.1%, s2 = 0.0102, d.f. = 68; D, R2 = 92.7%, s2 =
2.763, d.f. = 67; E, R2 = 93.7%, s2 = 0.7513, d.f. = 67; F, R2 = 91.2%, s2 = 0.8410, d.f. = 69).
69
Chapter II.
0.10
A
2
y = - 6.93 + 0.2503 x - 0.00167 x
(s.e. 2.40; 0.0655; 0.00043)
0.25
B
2
y = 0.006967 x - 0.0000701 x
(s.e. 0.000507; 0.0000055)
y = - 5.39 + 0.2503 x - 0.00167 x
(s.e. 2.34; 0.0667; 0.00046)
0.08
0.06
0.04
0.02
0.00
MEA
40
2
HNV (mmol m- )
2
MEA (mmol m- )
2
y = 0.2330 - 0.001206 x
(s.e. 0.0169; 0.000213)
0.20
0.15
0.10
0.05
HNV
0.00
60
80
RWC (%)
100
40
60
80
100
RWC (%)
Figure II.11. Variation of the content (mmol m-2) in mono-ethanolamine (A, MEA) and 5-hydroxy-Lnorvaline (B, HNV) with the relative water content (RWC, %) in the leaves of P. dilatatum (black
diamonds), C. dactylon (grey squares) and Z. japonica (open triangles). Each data point corresponds to
one sample, with 24 samples per species. The regression lines and curves, when applied, correspond to
the best models statistically significant (P. dilatatum, solid lines; C. dactylon, dashed lines; and Z.
japonica, dotted lines; A, R2 = 38.7%, s2 = 0.9491, d.f. = 67; B, R2 = 89.8%, s2 = 0.03151, d.f. = 68).
Higher values of mono-ethanolamine (MEA) were generally observed for Z. japonica
than for P. dilatatum or C. dactylon (Figure II.11A). Despite the high variability in the values
obtained, namely for Z. japonica, there was a slight increase of MEA in the earlier stages of
dehydration in all species, followed by a decrease in the most dehydrated samples. The nonprotein amino acid, 2-amino-5-hydroxypentanoic acid (or 5-hydroxy-L-norvaline, HNV) was
present in all leaves of Z. japonica and in leaves from drought-stressed plants of C. dactylon,
whereas no significant (P > 0.05) amounts were found to be present in P. dilatatum (Figure
II.11B). The content of HNV increased with leaf dehydration in both C. dactylon and Z. japonica,
but the increase was much steeper in the former species, attaining values similar to those found
in Z. japonica at ca. 65% RWC.
70
Water Relations and Leaf Anatomy of C4 Grasses
DISCUSSION
Soil and leaf water relations
Similar extents of decreased water availability in the soil of non-watered pots of each of the three
species were obtained with water withheld from the pots of C. dactylon first, Z. japonica next
and P. dilatatum last, in consecutive days, as given by the parallel lines of decreased water
availability through the experiment (Figure II.1), suggesting faster soil water absorption by P.
dilatatum. Withholding water from the pots of each species on consecutive, separate days was a
strategy adopted in order to obtain more similar levels of drought stress in the three species and
allow for a better comparison of their responses. Different rates of soil drying have been reported
for a C3 in comparison to a C4 monocot due to lower water consumption and increased water use
efficiency by the latter (Kalapos et al. 1996). The WWP and the SWC were highly correlated
(Figure II.1). The WWP corresponding to a certain SWC was greater in the pots of P. dilatatum
than in the pots of C. dactylon or Z. japonica, due to the greater biomass of the former species.
Accordingly, the drought-induced decrease of shoot growth in the three species (Figure II.4)
resulted in different, parallel lines applied to the control and non-watered pots (Figure II.1). The
soil water content, measured as the amount of water in relation to the amount of soil, might not
be a very useful tool because the soil water potential does not decrease linearly with the soil
water content (Kramer & Boyer 1995) and the water in the soil might not be all available to the
plant. The use of an appropriate pot to the plant size, allowing the roots to be homogeneously
distributed and occupying most of, but not exhaustively, the soil volume (results not shown), is
likely to have favoured the imposition of very similar levels of water deficit for the three species.
The leaf relative water content (RWC) decreased in the non-watered plants of the three
species (Figure II.2A), but only a few severely stressed samples were obtained. The sequence in
which watering was stopped for the plants of C. dactylon, Z. japonica and P. dilatatum in
consecutive days seemed adequate since similar RWC values were obtained for the samples of
the three species in this experiment. Occasionally, if the drying conditions were high, namely
through higher evaporative demand promoted by slightly increased temperatures in the first days
of the drought treatment (when water was being withheld from the pots of one species but not yet
from another), different levels of dehydration were attained in the leaves of each species, as
observed in the amino acid experiment. The faster absorption of water from the drying soil and
the faster water loss from the leaves of P. dilatatum than from the leaves of C. dactylon and Z.
japonica contrast with results previously obtained under rapidly-induced water deficit (Carmo71
Chapter II.
Silva et al. 2007) and are likely to be due to a dramatic response of C. dactylon to the artificial
PEG-induced decrease of water availability used in that study, promoting a fast dehydration in
the leaves of this species.
In P. dilatatum and C. dactylon, the LWP showed an initial decrease as the RWC started
to decrease but remained constant after reaching a minimum value of -1.4 MPa (Figure II.2).
This suggests a possible drought adaptation by these two species in the direction of producing
cells with increased plasticity, decreasing the turgor pressure and hence keeping the LWP
constant as RWC decreases (Lambers et al. 1998). LWP decreased to ca. -2.4 MPa in droughtstressed NADP-ME and NAD-ME grasses (Ghannoum et al. 2002) and to a minimum of ca. -2.0
MPa in C. dactylon during a Mediterranean summer-drought (Utrillas & Alegre 1997).
Conversely, in Z. japonica the linear decrease of LWP with decreasing RWC suggests the
presence of rigid cell walls, associated with the hard, tough and stiff leaves in this species, and
reflected in the higher proportion of dry weight than in the other two species (Figure II.3). In fact,
Z. japonica is characterised by rigid cell walls that may become even more rigid upon exposure
to drought stress (White et al. 2001). The accumulation of compatible solutes, namely amino
acids like proline (Figure II.9), can also contribute to the osmotic adjustment of the tissues and to
the decreased leaf water potential, but the changes in turgor pressure are likely to have a
prevailing role in the drought-induced lowering of LWP (Kramer & Boyer 1995). The LWP at
zero turgor, reflecting changes in osmotic potential, decreased only slightly (less than 0.2 MPa)
in several Z. japonica genotypes (White et al. 2001), suggesting little contribution of compatible
solute accumulation to the decrease in LWP in this species (Figure II.2). The presence of rigid
cell walls promoting the decrease of LWP in response to the drying soil conditions can be seen
as a drought tolerance mechanism, by which water absorption from the soil is increased avoiding
excessive leaf dehydration. Both the capacity for osmotic adjustment and the cell wall rigidity
tend to be greater in C4 than in C3 grasses (Barker et al. 1993). However, these processes seem to
function essentially under severe drought conditions and might therefore have a pivotal role for
plant survival when severe summer-droughts are observed (Utrillas & Alegre 1997; Volaire et al.
1998) but may be of little significance when plant productivity is the target (Serraj & Sinclair
2002). Increased root depth, total root weight and/or root number might also result in increased
capacity of Z. japonica to keep absorbing water from the dry soil (Marcum et al. 1995). Carrow
(1996) associated increased drought resistance in bermudagrasses (C. dactylon) compared to
zoysiagrasses (Z. japonica) with deeper root systems of the former. Moreover, the presence of
salt glands in these two species (Oross & Thomson 1982; Marcum & Murdoch 1990) and the
72
Water Relations and Leaf Anatomy of C4 Grasses
active salt secretion from their leaves are associated with salt tolerance (Marcum 1999) and may
also benefit plant performance under drought conditions.
Plant growth and leaf structure
Plants of the three grass species produced fewer leaves and tillers in response to the drying soil
(Figure II.4). Decreased shoot growth is a well established effect of water deficit (Jones 1985;
Hsiao & Xu 2000) and seems to be an early event associated with signals produced in the roots
(Davies & Zhang 1991). Decreased shoot elongation rates were observed in several C4 grasses in
response to decreased water availability (Ghannoum et al. 2002) and decreased leaf production
during a Mediterranean summer was reported for two legume species under low rainfall field
conditions compared to irrigated conditions (Lefi et al. 2004). Decreased shoot growth may be
part of an adaptive response to drought stress with photoassimilates and energy being diverted to
the synthesis of molecules involved in plant defence (Chaves & Oliveira 2004).
The relative increase of leaf dry matter with water deficit (Figure II.3) might be
associated with the accumulation of starch or soluble compounds, including osmolytes with a
potentially protectant role against drought effects; or may reflect increased xeromorphy. For
instance, increased cell wall rigidity may result in increased dry matter due to changes in cell
wall thickness and constituents (Wilson et al. 1980). Increased proportion of leaf dry matter and
cell wall rigidity were observed in several Z. japonica genotypes exposed to drought conditions
(White et al. 2001). The increase in dry matter was not associated with increased leaf thickness
(Figure II.7), in agreement with previous observations for C. dactylon under field induced
drought conditions during a Mediterranean summer (Utrillas & Alegre 1997). The lower dry
matter and thicker leaves of P. dilatatum contrasted with the higher dry matter and thinner leaves
of Z. japonica. The decreased SLA in the non-watered plants of P. dilatatum and C. dactylon
may have resulted directly from the increased DW/TW (Figure II.3). On the other hand, the
unchanged SLA in drought-stressed compared to control plants of Z. japonica suggests increased
leaf area. No effects were observed on the number of veins or on IVD (Figure II.7), indicating
that leaf length rather than width might have changed.
Morphological changes including leaf ‘rolling’ and ‘folding’ are related with decreased
water content and leaf water potential and can play an important role in the response of grasses
to drought (O'Toole & Cruz 1980). These leaf movements can be delayed by osmotic adjustment
(Hsiao et al. 1984). The reduction of evaporative surface area contributes to minimize water loss
and the decreased light absorption minimizes photoinhibitory injury (Kramer 1983). In P.
dilatatum, C. dactylon and Z. japonica these morphological alterations were observed only at
73
Chapter II.
RWC values lower than ca. 80-75% (results not shown), which reflects the increasing
importance of these drought tolerance mechanisms as stress severity increases. Variations in leaf
morphological responses occur among C4 grasses under drought conditions (see Sanderson et al.
1997 for review). In the present work, the colourless and bulliform cells present in the leaves of
the three grasses caused different morphological responses. The leaves of P. dilatatum are
characterized by a large number of colourless cells in the midrib (Figure II.5) and the loss of
water from these cells results in the leaf folding. Conversely, in C. dactylon and Z. japonica the
midrib is formed solely by the central vascular bundle and bulliform cells are spread through the
leaf, in between each pair of consecutive vascular bundles. The differential arrangement of these
water-storing cells results in leaf shrinkage in C. dactylon and leaf curling in Z. japonica when
considerable dehydration occurs.
The anatomical characteristics of P. dilatatum, C. dactylon and Z. japonica (Figure II.5)
agree with the leaf anatomical features associated with each of the classical subtypes of C4
grasses (Dengler et al. 1994) and with previous descriptions for the three species (Watson &
Dallwitz 1992). Drought stress did not cause many changes in the leaf structure of the three
species but species-specific characteristics are likely to be associated with the differential
capacity to withstand low water availability. For instance, the higher percentage of leaf area
occupied by sclerenchyma in Z. japonica (Figure II.8) is likely to be associated with the leaf
stiffness and stronger xeromorphic characteristics of the species compared with the other two.
On the other hand, the higher percentage of area occupied by the vascular tissues in this species
may reflect improved water transport system. In several maize lines, drought resistance was
associated with larger leaf transverse sectional areas occupied by vascular tissues (Ristic & Cass
1991), possibly reflecting increased capacity for water uptake.
The ratio between the relative leaf areas occupied by mesophyll or bundle sheath cells
(M/BS) was greater in P. dilatatum than in the other two species (Figure II.9), which is in
agreement with previous descriptions for NADP-ME C4 grasses (Hattersley 1984; Dengler et al.
1994) and results essentially from the lower percentage of area occupied by BS cells (Figures
II.5, II.6 and II.8). The ratio M/BS increased in leaves of drought-stressed plants of P. dilatatum
and Z. japonica compared to well-watered plants, but was not changed in C. dactylon. The
ability of plants to alter their development and adjust, or acclimate, to environmental variations
may be of great importance to maintain photosynthetic performance. However, the structural
changes in leaves of C4 grasses must not disturb the functionality of the C3 and C4 cycles (Sage
& McKown 2006). Increased M area relative to BS area may result in increased CO2 being
delivered in the BS to be assimilated by the C3 cycle, which can be a strategy to overcome the
74
Water Relations and Leaf Anatomy of C4 Grasses
decreased CO2 concentrations resulting from stomatal closure. The high photosynthetic
performance of C. dactylon (Carmo-Silva et al. 2007; 2008) suggests no need for leaf structure
adjustments in moderate drought conditions, even though impairment of structural functionality
may be observed under severe drought stress (Utrillas & Alegre 1997).
Accumulation of soluble amino acids
The increased amino acids content in the drought-stressed leaves of the three species (Figure
II.10) is in accordance with previously reported data obtained for C4 grasses and other monocot
species (Barnett & Naylor 1966; Jones et al. 1980; Ford & Wilson 1981; Thakur & Rai 1982;
Kusaka et al. 2005; Simon-Sarkadi et al. 2006). Impaired protein metabolism, including both
decreased protein synthesis and increased hydrolysis, has been suggested to cause free amino
acids content to rise by several-fold (Barnett & Naylor 1966; Jones et al. 1980). However,
accumulation of amino acids is more likely to result from increased synthesis, possibly reflecting
the ability of plants to adjust osmotically in response to the drying soil or their enhanced
metabolism in the direction of synthesis of secondary metabolites with different roles in stress
defence. In Pennisetum glaucum (pearl millet, C4 NAD-ME), greater amino acids content were
observed in drought-stressed leaves of tolerant than sensitive accessions (Kusaka et al. 2005).
Amino acids accumulation is likely to result in increased osmotic potential and contribute
therefore to minimize water loss from the leaves and to stabilize membranes and proteins in the
cells. Moreover, their active biosynthesis of some amino acids is likely to be involved in the use
of excessive reducing power (in the event of drought-induced decreased photosynthetic
metabolism) as well as in the protection against reactive oxygen species.
Osmotic adjustment in response to drying soil has been shown to occur in C4 grasses to a
greater extent than in some of their C3 counterparts (Barker et al. 1993). Accumulation of
inorganic ions can play an important role in the osmotic adjustment of C4 grass species under
drought conditions (Ford & Wilson 1981; Utrillas et al. 1995) but soluble sugars seem to have a
relatively little contribution (e.g. Marques da Silva & Arrabaça 2004). However, in Sorghum
bicolor both increased sugars and inorganic ions accounted for most of the decrease in osmotic
potential under water deficit (Jones et al. 1980). Proline is one of the compatible solutes most
widely accumulated in response to hyperosmotic stresses (Delauney & Verma 1993) and, even
though its role in osmotic adjustment is a matter of controversy (see Hare & Cress 1997), the
capacity to accumulate this imino acid under drought conditions was related to the drought
resistance of different Sorghum bicolor varieties (Blum & Ebercon 1976). Moreover, increased
contents of proline (especially) and other amino acids, like valine, isoleucine and leucine, were
75
Chapter II.
observed in the phloem sap of Medicago sativa (drought-resistant C3 plant) in response to
progressively-induced drought stress (Girousse et al. 1996).
The proline content doubled when RWC decreased to nearly 80% in the leaves of the
three species studied (Figure II.10) but accumulation occurred only under severe leaf
dehydration, as described for other C4 grasses (Jones 1985). As recently reviewed by Ashraf &
Foolad (2007), the protective role of proline in the adaptation of plants to drought conditions is
likely to involve a number of functions other than osmoregulation. In Saccharum spp.
(sugarcane), for instance, the advantage of proline accumulation under water deficit was
associated with plant protection against oxidative stress, rather than osmoprotection (Molinari et
al. 2007). In several other C4 grass species, the contribution of proline accumulation to osmotic
adjustment was negligible, i.e. both occurred concomitantly in response to drought without one
being the cause of the other and the beneficial role of proline was more likely related with other
aspects of plant metabolism (Barker et al. 1993).
Increased content of methionine, especially in C. dactylon and Z. japonica, with leaf
dehydration (Figure II.9), is likely to reflect enhanced production of ethylene and/or polyamines
with drought. Ethylene is known to be involved in shoot growth restriction in response to the
drying soil (Hussain et al. 1999) whilst polyamines are thought to provide the plant with
increased drought tolerance, namely through antioxidant activity, although their mechanism of
action remains unclear (Groppa & Benavides 2008). Curiously, the content in methionine was
higher in C. dactylon and Z. japonica, the two C4 species having aspartate as the stable
photosynthetic product resulting from primary fixation of CO2 in the mesophyll cells, than in the
malate-producing species, P. dilatatum. However, the same pattern was not observed for
phenylalanine, also derived from aspartate. Greater increases in the contents of the amino acids
phenylalanine, valine, isoleucine and leucine were observed in Z. japonica than in the other two
species (Figure II.10), possibly contributing to the lower values of LWP (Figure II.2) and
eventually reflecting enhanced involvement of secondary metabolism. If the production of stressinduced secondary metabolites is triggered, in order to allow the plant to cope with, and survive,
drought conditions, the metabolic fluxes will be diverted in that direction and increased contents
of the amino acids involved can be observed. For instance, induction of a protein involved in
lignin biosynthesis was observed in the roots of rice plants exposed to salt stress (Salekdeh et al.
2002). Increased lignin deposition might contribute to the cell wall strengthening under
conditions that favour water loss. Phenylalanine is involved in the synthesis of this polymer and
other aromatic secondary compounds that may play an important role in cellular protection
against reactive oxygen species (Grace & Logan 2000).
76
Water Relations and Leaf Anatomy of C4 Grasses
Mono-ethanolamine (MEA) was present in greater amount in Z. japonica than in C.
dactylon and P. dilatatum and increased slightly in the first stages of leaf dehydration (Figure
II.11). This amino alcohol is a precursor of phosphatidylethanolamine, one of the most abundant
phospholipids constituting cell membranes, and an intermediate in the biosynthetic pathway of
choline, a precursor of quaternary ammonium compounds known to confer protection under
stress conditions (Rhodes & Hanson 1993). Ethanolamine is the direct product of serine
decarboxylation and its increase in a C4 grass under moderate drought conditions, concomitant
with increased serine, suggested transiently enhanced photorespiration (Martinelli et al. 2007). In
the three C4 grasses here studied, a concomitant increase of both serine and ethanolamine was
observed when RWC decreased to ca. 80%, suggesting slightly increased photorespiratory
metabolic fluxes (see Chapter III, Carmo-Silva et al. 2008).
An unusual amino acid was also stress-responsive in the leaves of C. dactylon and Z.
japonica, but not of P. dilatatum (Figure II.11). To the best of our knowledge, 5-hydroxy-Lnorvaline has not been reported before to be present in plant leaves, although it is a well known
constituent of legume seeds (Thompson et al. 1964). The presence and drought-induced increase
of 5-hydroxy-L-norvaline in Z. japonica and it appearance in increasing concentrations in
dehydrated leaves of C. dactylon suggest that this non-protein amino acid might be involved in
the stress response and, possibly, playing a role in plant defence mechanisms.
CONCLUSIONS
Drought resistance mechanisms were present in the three grasses but were more effective in C.
dactylon (NAD-ME), and in Z. japonica (PEPCK) than in P. dilatatum (NADP-ME), with
greater water loss in the latter. The use of bermudagrass (C. dactylon) for recreational purposes,
including golf courses, seems to be a promising strategy to optimize water use. Zoysiagrass (Z.
japonica) can be used as an alternative and especially for zones where leaf hardness, toughness
and stiffness are not a concern. Dallisgrass (P. dilatatum) has a greater requirement for water and
is therefore less promising for the use as turfgrass. Nonetheless, its high productivity might be of
great advantage in its use as forage grass.
The different relationships between LWP and RWC in Z. japonica suggest increased cell
wall rigidity which might be involved in a strategy to improve water uptake from the soil. The
lack of considerable alterations in leaf anatomy under moderate drought stress in all three species
is probably associated with a low phenotypic plasticity and with the high leaf structure
specialization in these C4 grasses conferring considerable drought resistance. The increased
amino acids content are likely to be associated with enhanced synthesis of compounds involved
77
Chapter II.
in plant stress response and may additionally contribute to osmotic adjustment and, thus, to
minimize water loss. The presence and drought-induced increase of HNV on the leaves of the C4
grasses with greater drought resistance must be further exploited in order to understand the role
of this unusual amino acid and its possible association with stress defensive mechanisms.
78
Water Relations and Leaf Anatomy of C4 Grasses
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84
Chapter III.
PHOTORESPIRATION AND C4
PHOTOSYNTHESIS UNDER DROUGHT STRESS
Chapter III.
An integral copy of this chapter is In Press in Plant, Cell and Environment:
Carmo-Silva A.E., Powers S.J., Keys A.J., Arrabaça M.C. & Parry M.A.J. (2008) Photorespiration in C4
grasses remains slow under drought conditions. Plant, Cell and Environment, x, xx-xx. (In Press)
Alfred J. Keys (Department of Plant Sciences, Rothamsted Research) was involved in the planning of the
experiments, in the measurements of amino acids content and interpretation of the results.
Stephen J. Powers (Department of Biomathematics and Bioinformatics, Rothamsted Research) did the
non-linear modelling and applied the equations of C4 photosynthesis mechanistic models.
86
Photorespiration and C4 Photosynthesis under Drought Stress
PHOTORESPIRATION IN C4 GRASSES REMAINS SLOW UNDER
DROUGHT CONDITIONS
ABSTRACT
The CO2-concentrating mechanism present in C4 plants decreases the oxygenase activity of
Rubisco and, consequently, photorespiratory rates in air. Under drought conditions the
intercellular CO2 concentration may decrease and cause photorespiration to increase. The C4
grasses Paspalum dilatatum Poiret, Cynodon dactylon (L.) Pers and Zoysia japonica Steudel
were grown in soil and drought was imposed by ceasing to provide water. Net CO2 assimilation
(A) and stomatal conductance to water vapour decreased with leaf dehydration. Decreased
carbon and increased oxygen isotope composition were also observed under drought. The
response of A to CO2 suggested that the compensation point was zero in all species irrespective
of the extent of drought stress. A slight decrease of A as O2 concentration increased above 10%
provided evidence for slow photorespiratory gas-exchanges. Analysis of amino acids contained
in the leaves, particularly the decrease of glycine after 30 seconds in darkness, supported the
presence of slow photorespiration rates, but slightly faster in C. dactylon than in P. dilatatum and
Z. japonica. Although the contents of glycine and serine increased with dehydration and
mechanistic modelling of C4 photosynthesis suggested slightly increased photorespiration rates
in proportion to photosynthesis, the results provide evidence that photorespiration remained slow
under drought conditions.
KEYWORDS:
CO2- and O2-response curves, amino acids, isotope composition, modelling C4 photosynthesis
87
Chapter III.
INTRODUCTION
The main objective of this study was to investigate whether photorespiration was increased by
drought stress in three species of C4 grasses and could, as a consequence, contribute to metabolic
factors limiting net photosynthesis. Under drought conditions photosynthetic carbon assimilation
decreases in both C3 and C4 plants (e.g. Chaves et al. 2003). Closure of stomata is one of the
major causes of the decrease in photosynthesis but evidence has accumulated that metabolic
limitations also contribute (Du et al. 1996; Saccardy et al. 1996; Ghannoum et al. 2003; Marques
da Silva & Arrabaça 2004; Carmo-Silva et al. 2007). In C4 leaves with Kranz anatomy,
atmospheric CO2 is initially fixed by phosphoenolpyruvate (PEP) carboxylase (PEPC; EC
4.1.1.31) into C4 acids in the mesophyll (M) cells. The C4 acids are transported to the bundle
sheath (BS) cells where they undergo decarboxylation and the released CO2 enters the C3
pathway via ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco; EC 4.1.1.39).
Rubisco is confined to the BS cells and the specialized leaf anatomy decreases leakage of CO2
back to M cells so that CO2 accumulates.
Rubisco acts both as a carboxylase and an oxygenase. Molecules of CO2 and O2 are
competing alternative substrates for reaction with the enediol of RuBP catalysed by Rubisco
(Bowes & Ogren 1972; Laing et al. 1974) and, therefore, rates of photorespiration relative to
photosynthesis are determined by the relative concentrations of O2 and CO2 at the catalytic site
of Rubisco in the chloroplast stroma. In fully hydrated leaves, the CO2/O2 ratio in BS cells of C4
species is 3 to 6 times higher than in M cells under atmospheric levels of CO2 and O2 (Dai et al.
1993; Kiirats et al. 2002). Therefore the oxygenase activity of Rubisco, and consequently the
photorespiratory rate, is slow. The consequences are a low CO2 compensation point in C4 plants
(Forrester et al. 1966) and the absence of an enhancement of net photosynthesis when oxygen in
the gas-phase is decreased from 21 to 2% (Edwards et al. 1985). These observations were
initially interpreted as a lack of photorespiration in C4 plants. More recent studies with several C4
species, including the three main subtypes and both monocotyledons and dicotyledons (Dai et al.
1993; Maroco et al. 1997), have revealed that the maximum rate of net photosynthesis occurred
at O2 concentrations between 5 and 10%. The rise of C4 photosynthesis to a maximum at 5-10%
has been explained by an oxygen requirement for the production of extra ATP needed for the
CO2-concentrating mechanism (Maroco et al. 1997). The decrease in photosynthesis by C4
species when the O2 concentration is elevated above 10% depends on the CO2 concentration (Dai
et al. 1993; Maroco et al. 1997; 1998) and is assumed to be due to photorespiration. To the best
88
Photorespiration and C4 Photosynthesis under Drought Stress
of our knowledge the effect of O2 on C4 photosynthesis by dehydrated leaves has not been
recorded.
Under water deficit conditions the CO2 concentration in the leaves may decrease because
of decreased stomatal conductance and should cause photorespiration to increase. Mechanistic
modelling of C4 photosynthesis is not used as frequently as that of C3 photosynthesis, mostly due
to the additional complexity resulting from the structural and biochemical specialization
characteristic of C4 plants (von Caemmerer 2000). The model of von Caemmerer & Furbank
(1999) uses basic equations to describe the carbon fluxes in C4 photosynthesis and, with careful
assumptions being made, can be used to estimate the CO2 concentration in the bundle sheath, the
rate of oxygenation of RuBP and hence photorespiration.
Photorespiratory metabolism requires the integration of the photorespiratory carbon
oxidation cycle, the photorespiratory nitrogen cycle (Keys et al. 1978) and photosynthetic carbon
assimilation (Keys 1999); consequently there is an interdependence of reactions in different parts
of the overall process. Since photorespiration is relatively little-influenced by metabolite signals,
amino acids can be used as metabolite markers for this pathway (Foyer et al. 2003). Although de
novo assimilation of nitrogen and recycling of ammonia during photorespiration interact (Stitt et
al. 2002), the ratio glycine/serine and both aspartate and alanine levels were strongly correlated
with photorespiration rates in C3 plants (Novitskaya et al. 2002). In the C4 dicotyledon
Amaranthus edulis, increased glycine content in the leaves with increasing O2 in the atmosphere
was taken as indicative of photorespiration (Maroco et al. 2000); although increased serine was
also observed in A. edulis, this amino acid decreased with increasing O2 in C3 plants and was
therefore discounted as an indicator of photorespiration. Post-illumination decreases in glycine
(Kumarasinghe et al. 1977; Rawsthorne & Hylton 1991) are mainly responsible for the postillumination burst of CO2 in C3 plants, a phenomenon leading to the early recognition of
photorespiration. Post-illumination CO2 bursts are generally not seen in C4 plants, mostly
because of the low conductance of BS cell walls to CO2, but the post-illumination decrease in
glycine should be observed if photorespiration is present.
Variations in carbon and oxygen isotope compositions (δ13C and δ18O) reflect the
influence of environmental factors on the kinetics of photosynthesis and the two isotopes are
conveniently measured on the same leaf samples. The CO2-concentrating mechanism present in
C4 plants results in lower discrimination against 13C than in C3 plants and hence an average δ13C
of -13.6‰ in C4 as opposed to -27.8‰ in C3 plants (Troughton 1979). Changes in δ13C under
stress conditions reflect mostly variations in the CO2 concentration at the carboxylation sites and
the coordination between the C3 and C4 cycles (Farquhar 1983). On the other hand, changes in
89
Chapter III.
δ18O are mainly the result of changes in evapotranspiration and reflect the isotope content of the
soil water as well as the fractionation during transpiration (Barbour 2007), which is likely to
change in conditions affecting the evaporative demand.
The experiments described aimed to detect photorespiration in three C4 grasses and to
determine whether the rate increased under drought conditions. The indicators of
photorespiration used were: the response of photosynthesis to CO2, inhibition of photosynthesis
by O2, content of amino acids, post-illumination changes in amino acids, and isotope
fractionation. The three C4 species studied have been reported to belong each to a different
biochemical subtype, according to the main enzyme responsible for the decarboxylation of C4
acids in the BS: Paspalum dilatatum Poiret, NADP-malic enzyme (NADP-ME), Cynodon
dactylon (L.) Pers, NAD-malic enzyme (NAD-ME) and Zoysia japonica Steudel, PEP
carboxykinase (PEPCK).
90
Photorespiration and C4 Photosynthesis under Drought Stress
MATERIAL AND METHODS
Plant material and drought stress induction
The C4 grasses Paspalum dilatatum Poir. cv. Raki (NADP-ME), Cynodon dactylon (L.) Pers var.
Shangri-Lá (NAD-ME) and Zoysia japonica Steudel ‘Jacklin Sunrise Brand’ (produced by
Jacklin Seed Company, USA) (PEPCK) were grown from seeds in pots with peat-free compost,
prepared to Rothamsted Research’s specification by Petersfield Products (Leicester, UK)
supplemented with a slow-release fertiliser (Hydro Agri Ltd, Lincs, UK) in a glasshouse.
Artificial light was provided whenever the natural light was below a photosynthetic photon flux
density (PPFD) of 500 μmol m-2 s-1 during a 16 h photoperiod. Temperature was maintained at a
minimum of 25ºC during the day and at 18ºC during the night.
Seeds of each species were washed with 10% hypochlorite and soaked in water for 1 hour
before sowing. Water was supplied whenever needed during two weeks for P. dilatatum and C.
dactylon and four to five weeks for the slower growing Z. japonica. The seedlings were
transplanted to 1-L cylindrical pots containing equal amounts of soil. Five seedlings were used
per pot. The dates of sowing and transplanting each species were adjusted and the timing of the
drought treatment was chosen in order to have plants at an adequate growth stage for taking
measurements or sampling all three grasses at the same time.
All plants were well-watered until the beginning of the drought stress treatment. Pots
were placed according to a split-plot design, where each column of pots was a main plot of a
particular species and the sampling-days and the treatments (control vs. drought stress) were
randomised in the split-plots. Each pot corresponded to an independent sample. All pots were
watered in the evening and weighed on the following morning in order to ensure that all of them
had similar amounts of water (mean overall weight of 800 ± 50 g). Water deficit was then
imposed on the ‘stress’ pots by ceasing to provide water. The ‘control’ pots were watered once
per day.
Five-week old plants of P. dilatatum and C. dactylon and nine-week old plants of Z.
japonica were analysed. Control and non-watered plants of the three species were either assayed
or harvested on consecutive days, starting when the weight of non-watered pots had been
suitably decreased, in order to obtain leaf samples with different levels of dehydration, and
ending after a maximum of nine to twelve days without watering. The water weight in each pot
(WWP, g) was determined as the weight of the pot at each sampling time less 400 g (the mean
weight of the pots with plants and totally dried soil was 404 ± 16 g).
91
Chapter III.
The youngest fully expanded leaf of each P. dilatatum plant and two young fully
expanded leaves of each plant of C. dactylon or Z. japonica were always analysed. Samples were
collected in the growth environment four hours after the beginning of the photoperiod. In vivo
measurements were made during the first half of the photoperiod. It was assumed that, within
each pot, all the leaves used were identical in terms of developmental stage, physiological and
biochemical properties, and would have experienced the same drought condition.
From each pot, a sample formed by similar leaves to those being used for gas-exchanges
or amino acid analysis was collected to determine the leaf relative water content (RWC). The
fresh (FW), turgid (TW) and dry (DW) weights were measured and used to calculate RWC by
the equation RWC = 100((FW-DW)/(TW-DW)) (Catsky 1960). Leaf area was determined by
scanning the turgid leaves and analysing the image using the software Paint Shop Pro 9 (Jasc
Software, Inc., Minneapolis, MN, USA) and the software Image J 1.33u (National Institutes of
Health, Bethesda, Maryland, USA).
In order to facilitate the analysis of the physiological responses to CO2 and O2, groups of
plants with different drought stress conditions were chosen for each species independently,
considering both the RWC as well as the response in terms of the measured gas-exchanges.
Therefore, control (C) corresponds to all well-watered plants; moderately-stressed (MS) to nonwatered plants with RWC values between 96-80% in P. dilatatum, 95-80% in C. dactylon and
95-85% in Z. japonica; and severely-stressed (SS) to non-watered plants with RWC values lower
than 80 or 85%. Unfortunately, no severely-stressed samples of C. dactylon were obtained in
these experiments as the leaves of this species did not dehydrate as much as the other two grasses.
Gas-exchange measurements (CO2- and O2-response curves)
Gas-exchanges of carbon dioxide and water vapour by attached leaves were measured by infrared gas analysis using a six-chamber system designed and developed at Rothamsted (Lawlor et
al. 1989). Experiments were conducted at 25 ± 2ºC, 35-40% relative humidity and a PPFD of
850 ± 50 μmol m-2 s-1, provided by overhead lamps. The air composition was controlled by a
gas-mixer supplying CO2 and O2, the balance being made up with N2. Two different experiments
were undertaken. In the first, CO2-response curves were determined at a constant O2
concentration (21%) and seven CO2 concentrations (~30, 100, 250, 360, 500, 750 and 1000 μmol
mol-1). In the second experiment, O2-response curves were determined at a constant CO2
concentration (360 μmol mol-1) and six O2 concentrations (2, 5, 10, 15, 21 and 30%).
Measurements on each plant were always taken with time intervals long enough for steady-state
92
Photorespiration and C4 Photosynthesis under Drought Stress
CO2 uptake to be attained at each CO2 or O2 level. Ninety pots were used per experiment,
making a total of 30 samples per species (10 control and 20 non-watered) assayed over five
consecutive days during the drought period (two control and four non-watered per species per
day). Each sample was formed by the middle part of young fully expanded leaves of two plants
from one pot, but, due to their different size, a different number of leaves was used for the three
species: two leaves for P. dilatatum, four leaves for C. dactylon and three leaves for Z. japonica.
After each series of measurements, the area of leaf inside the chamber was determined. The net
CO2 assimilation rate (A) and the stomatal conductance to water vapour (gswa) by the control
and drought-stressed plants of each species were plotted against intercellular CO2 concentration
(Ci) or against the O2 concentration in the gas-phase.
Carbon and oxygen isotope compositions
A similar experimental design to that used to provide material for gas-exchange measurements
was used here. Three leaves of P. dilatatum and ten leaves of C. dactylon or Z. japonica of the
five plants in each pot were used for each sample. After determination of FW and TW, the leaf
samples were dried at 80ºC for more than 48h and weighed to allow calculation of RWC. Three
control and six drought-stressed samples from each species were selected according to their
RWC and sent to Unidade de Análise Instrumental, Faculdade de Ciências da Universidade de
Lisboa, Portugal, for the analyses of the carbon and oxygen isotope compositions (δ13C and
δ18O) on the leaf dry matter. Sub-samples of ground leaf tissue were analysed, to give three
technical replicates of each sample, using stable isotope ratio mass spectrometers (SIRA II, VG
Isogas Limited, Manchester, UK, for carbon, and IsoPrimeTM, Micromass UK Limited,
Manchester, UK, for oxygen, with automatic sample preparation systems EuroEA, EuroVector
S.p.A., Milan, Italy). The Pee Dee Belemnite (PDB) and the Vienne-Standard Mean Oceanic
Water (VSMOW) were used as standards for δ13C and δ18O calculations, respectively. The
results were expressed as parts per thousand deviation from the standards with an analytical
precision of ± 0.12‰.
Amino acid analysis
Amino acids contained in leaves were determined by High-Performance Liquid Chromatography
(HPLC) of o-pthaldialdehyde (OPA) derivatives (Noctor & Foyer 1998). Three control and five
non-watered pots were used per species per day during three consecutive days, making a total of
93
Chapter III.
24 samples per species (nine control and fifteen non-watered pots). The five plants within each
pot were used for the collection of three samples: one light-sample, immediately frozen with
liquid N2 (LN2) in fully illuminated conditions; one dark-sample, identical to the light-sample
but submitted to a period of 30 seconds in darkness before freezing in LN2; and a third sample
for the RWC determination. Taking into account the different leaf sizes, each light- or darksample of P. dilatatum consisted of one leaf from one plant whereas each sample of C. dactylon
or Z. japonica consisted of three leaves from two plants.
Reversed-phase HPLC was performed using a Waters Alliance 2695 Separation Module
and a 474 Scanning Fluorescence Detector operated by the Millenium32 software (Waters,
Milford, USA) with a Waters Symetry C18 4.6×150 mm column (Part No. WAT 054278)
protected by a 4×3 mm guard cartridge (Phenomenex, Torrance, USA). Since the fluorescent
adducts formed by reaction with OPA in the presence of 2-mercaptoethanol are unstable, the
autosampler was set to mix and pre-incubate 10 μL of each sample with 15 μL of OPA reagent
for 2 min before injecting the mixture onto the column. The eluent used for the amino acids
separation was obtained by mixing solvents containing different proportions of methanol,
sodium acetate pH 5.9 and tetrahydrofuran.
Amino acids were extracted from the frozen leaf samples stored at -80ºC. Each sample
was ground in LN2 and then 1.4 mL of 0.1 M HCl was added to the fine powder. The mixture
was ground further during thawing and the homogenate was centrifuged for 10 min at 16000 ×g
and 4ºC. Samples for HPLC were prepared by adding a sub-sample of each supernatant to the
internal standard and pure water, and these mixtures were stored at -20ºC. On the following day,
the mixtures were centrifuged for 40 min at 16000 ×g and 4ºC and then filtered with syringe
filters (0.2 μm) into HPLC autosampler vials. Standard solutions of α-amino-n-butyric acid
(internal standard), serine, glycine, glutamate, glutamine, aspartate, asparagine, and alanine were
prepared in 0.1 M HCl. A stock solution with all the standards was prepared and then diluted in
order to have increasing concentrations for the calibration curves (0, 5, 10, 15, 20 and 25 μM).
Statistical analysis
All the analyses were made using GenStat® 8.2, 2005 (Lawes Agricultural Trust, Rothamsted
Research, UK). Non-linear modelling was used to fit an asymptotic exponential model to the
variation of the leaf relative water content (RWC) with the amount of water in the soil. Using Ftests, non-significantly different (P > 0.05) parameters between species were amalgamated in
order to have a parsimonious model of the data. The responses of the net CO2 assimilation rate
94
Photorespiration and C4 Photosynthesis under Drought Stress
(A) and the stomatal conductance to water vapour (gswa) to the intercellular CO2 concentration
(Ci) and to the given concentration of O2 were modelled similarly. Firstly, non-linear curves
were fitted to the data from each individual plant. Statistically, the best models were: an
asymptotic exponential for the variation of A with Ci, a modified logistic for the variation of
gswa with Ci and an ‘exponential plus linear’ for the variation of A with O2. The latter consisted
of an exponential-associated increase followed by a linear decrease effective after maximal A
was attained. Residual Maximum Likelihood (REML) analysis was then used to predict mean
values of estimated parameters for each species by stress level combination that would occur if
the number of plants in each group (C, MS and SS) was the same (Patterson & Thompson 1971).
Such means were compared using a t-test on the appropriate degrees of freedom from the REML
model and the standard errors of the difference (SED).
Regression analysis was applied to model the variation of the isotope compositions and
amino acids content with RWC, including a squared term in this variable to check for nonlinearity. Non-significantly different (P > 0.05) parameters (t-tests) in the significant (P < 0.05)
model terms were amalgamated in order to attain parsimony. The residuals were checked and
found to generally conform to the assumptions of the analysis. All the absolute values and
percentages presented in the text were calculated from the regression models pertaining to each
data set. The difference between the values obtained for the content of each amino acid in the
samples collected in the light and after 30 seconds in darkness was calculated (these being paired
samples). Regression analysis revealed no significant effect of RWC on this difference for any of
the amino acids studied, and therefore the REML method was used to output predicted mean
values for the difference (dark minus light) for each species. Significance from zero was assessed
through t-tests.
Modelling C4 photosynthesis
The mechanistic model of C4 photosynthesis of von Caemmerer & Furbank (1999), described in
detail by von Caemmerer (2000) and based on the models of Berry & Farquhar (1978) and
Peisker (1979), was applied to the data from the CO2-response curves measured at high
irradiance. A similar approach to that described by Massad et al. (2007) was used on a plant-byplant basis. Firstly, an asymptotic exponential model was found to provide the best description of
the variation of net CO2 assimilation rate (A) with the intercellular CO2 concentration (Ci) for
each plant. The equations for enzyme-limited photosynthesis (von Caemmerer 2000) were then
applied to the individual plants to estimate the maximum Rubisco carboxylation activity (Vcmax)
95
Chapter III.
and the maximum PEPC carboxylation activity (Vpmax), as well as the CO2 concentrations in the
bundle sheath (Cs) and in the mesophyll cells (Cm), for values of Ci from 0 to 560 µmol mol-1
using a step-size of 5 µmol mol-1. Applying the method, all four parameters were primarily
estimated and then, for each plant, Vpmax and Vcmax were fixed at the mean of estimated values
found between 75-175 µmol mol-1 and 300-400 µmol mol-1 Ci, respectively, to re-estimate Cs
and Cm (over Ci), this time free from the instability of the numerical calculation of all four
parameters simultaneously. The model parameters assumed as constant at 25°C (von Caemmerer
2000) are listed in Table III.1 and the equations applied were:
(
(1)
A = g C −C
(2)
An =
(3)
A =
n
i
n
i
m
)
⎛ γ ∗O ⎞
⎟−R
⎜1 −
⎜
Cs ⎟ d
⎛
⎞
⎠
Cs + K c ⎜⎜1 + O ⎟⎟ ⎝
Ko
⎝
⎠
CV
s cmax
C V
m pmax
Cm + K p
(
)
− g C −C − R
bs
s
m
m
In these equations, An represents the net CO2 assimilation rate calculated from the asymptotic
exponential curve. Mean values of Vpmax and Vcmax and mean curves for Cs and Cm versus Ci were
then calculated for the treatment structure of species by stress level over the plants.
Table III.1. Summary of C4 photosynthesis parameters assumed as constant at 25°C (von Caemmerer
2000) and used in the equations for enzyme-limited photosynthesis.
Parameter
Value
Description
Kc
650 µbar
Michaelis-Menten constant of Rubisco for CO2
Ko
450 mbar
Michaelis-Menten constant of Rubisco for O2
Kp
80 µbar
Michaelis-Menten constant of PEPC for CO2
O
210 mbar
O2 partial pressure in the bundle sheath and mesophyll cells
Rd
0.01 Vcmax
Leaf mitochondrial respiration
Rm
0.5 Rd
gbs
gi
*
γ
96
Mesophyll mitochondrial respiration
-2 -1
3 mmol m s
-2 -1
Bundle sheath conductance to CO2
2 mol m s
Mesophyll conductance to CO2
0.000193
Half the reciprocal of Rubisco specificity
Photorespiration and C4 Photosynthesis under Drought Stress
In Figure III.6, the rate of PEPC carboxylation, Vp, was calculated as
(4)
V =
p
C V
m pmax
C +K
m
,
p
and the net CO2 assimilation rate (Ac) was calculated as the solution to the quadratic expression
for enzyme-limited photosynthesis (von Caemmerer 2000), taking the fraction of O2 evolution in
the bundle sheath as zero:
2
aAc + bAc + c = 0 ,
this being
(5)
Ac =
− b − b 2 − 4ac
,
2a
with
a = 1;
{(
b=− V −R +g C
(
p
m
bs
m
)+ (V
cmax
d
)(
)(
)
−R +g
bs
(K (1 + O K ))};
c
o
(
))
c = Vcmax − Rd Vp − Rm + g bs C m − Vcmax g bsγ ∗O + Rd g bs K c 1 + O K o .
In Figure III.7, a rate of photorespiration (Pr) was predicted for each species by stress level
combination using the equation below, which is derived from the equation of overall CO2
assimilation that describes Rubisco carboxylation in the bundle sheath (von Caemmerer, 2000):
(6)
A = Vc − 0.5 Vo − Rd .
Considering that Pr will be half the rate of Rubisco oxygenation, Vo,
(7)
(
)
Pr = 0.5 Vo = − A − Vc + Rd ,
where A is the overall CO2 assimilation and the rate of Rubisco carboxylation, Vc, is calculated
as
V =
c
CsVcmax
⎛
⎞
Cs + K c ⎜⎜1 + O ⎟⎟
Ko
⎝
⎠
.
97
Chapter III.
RESULTS
Water relations
The variation of the relative water content (RWC) in the leaves of P. dilatatum, C. dactylon and
Z. japonica with the water weight in pot (WWP) was described by an asymptotic exponential
model (Figure III.1). In the gas-exchange experiment, RWC decreased to lower values in P.
dilatatum (40%) than in C. dactylon and Z. japonica (75%). In the amino acids experiment,
watering of the pots of Z. japonica and C. dactylon was stopped respectively one or two days
prior to those of P. dilatatum. As a result, lower RWC values were observed for the most
stressed leaves of Z. japonica (40%) than for C. dactylon (60%) and P. dilatatum (80%).
A
B
100
90
90
80
80
RWC (%)
RWC (%)
100
Gas-exchange
experiment
70
60
Amino acids
experiment
70
60
50
50
-0.0256 x
y = 97.5 (1 - 2.78 e
) [s.e. 0.4; 0.33; 0.0015]
-0.0200 x
y = 97.5 (1 - e
40
0
100
200
) [s.e. 0.4; 0.0006]
300
Water Weight in Pot (g)
400
-0.0266 x
y = 98.9 (1 - e
40
500
0
100
200
) [s.e. 0.76; 0.0008]
300
400
500
Water Weight in Pot (g)
Figure III.1. Leaf relative water content (RWC) as a function of the amount of water in the soil,
measured as the water weight in pot (WWP), of the control and non-watered plants of P. dilatatum (black
diamonds), C. dactylon (grey squares) and Z. japonica (open triangles). Each data point corresponds to
one sample, with 30 (A) or 24 (B) samples per species. The asymptotic exponentials fitted correspond to
the best models statistically significant (A, R2 = 89.6%, s2 = 11.9, d.f. = 176; B, R2 = 92.3%, s2 = 19.3, d.f.
= 70).
98
Photorespiration and C4 Photosynthesis under Drought Stress
CO2- and O2-response curves
The net CO2 assimilation rate (A) by the leaves of P. dilatatum, C. dactylon and Z. japonica
increased with the intercellular CO2 concentration (Ci), both in control and drought stress
conditions, as shown in Figure III.2 (A-C). The mean value for the maximal net CO2 assimilation
rate (Amax) in fully hydrated leaves of P. dilatatum and C. dactylon plants was higher than in Z.
japonica plants. In moderately-stressed (MS) leaves of the two former species Amax decreased in
relation to the control, whereas no significant difference (P > 0.05) between MS and control
leaves was observed in Z. japonica. The mean values of Amax obtained for the severely
dehydrated (SS) leaves of P. dilatatum and Z. japonica were slightly less than half of the values
observed for each species under control conditions.
P. dilatatum
30
C. dactylon
Z. japonica
A
B
C
D
E
F
-2
-1
A (μmol m s )
25
20
15
10
5
0
0.4
-2
-1
gswa (mol m s )
0.5
0.3
0.2
0.1
0.0
0
100
200
300
400
Ci (μmol mol-1)
500
0
100
200
300
400
500
Ci (μmol mol-1)
0
100
200
300
400
500
Ci (μmol mol-1)
Figure III.2. Mean values of net CO2 assimilation rate (A; A-C) and stomatal conductance to water
vapour (gswa; D-F) in response to the intercellular CO2 concentration (Ci) in the control (black diamonds),
moderately-stressed (grey squares) and severely-stressed (open triangles) plants of P. dilatatum, C.
dactylon and Z. japonica. The bars correspond to the standard errors of each mean value. Measurements
were taken at ambient O2 (21%), a PPFD of 850 μmol m-2 s-1 and 25ºC.
99
Chapter III.
The stomatal conductance to water vapour (gswa) decreased with increasing Ci for both
fully hydrated and drought-stressed plants of the three species (Figure III.2D-F). The mean
values of gswa in fully hydrated leaves of C. dactylon were generally higher than in the other
two species, especially at low CO2 concentrations, and were decreased by drought stress in all
species. In MS leaves of C. dactylon and in SS leaves of P. dilatatum and Z. japonica the
maximal value of gswa was lower than in control or less dehydrated leaves but no significant
differences (P > 0.05) were observed between the control and MS plants of P. dilatatum and Z.
japonica.
Photosynthesis by P. dilatatum, C. dactylon and Z. japonica plants under control or
drought stress conditions was not dramatically affected by the O2 concentration (Figure III.3AC). However, lower values of A were generally observed at 2% compared to higher O2
concentrations and an ‘exponential plus linear’ model was successfully applied to the variation
of A with O2 (Figure III.3D-F), showing that after the maximal value of net photosynthesis (Amax)
was attained, at O2 concentrations generally between 4 and 10% (O2*), a slight decrease of A
with increasing O2 tends to occur. Note that the predictions of the O2* are poor because there
was no clear definition of the point corresponding to Amax. Apart from an increase of O2* to
values closer to the atmospheric concentration in the dehydrated leaves of P. dilatatum (MS and
SS), no other significant differences (P > 0.05) were observed.
Carbon and oxygen isotope compositions
The carbon isotope composition (δ13C) in fully hydrated leaves of P. dilatatum (-15.2‰) was
less negative than in C. dactylon (-16.3‰) and Z. japonica (-16.6‰). Drought stress had an
identical effect on δ13C for the three species, δ13C being decreased by -0.5‰ when RWC
decreased to 60% (Figure III.4A). No significant differences in the oxygen isotope composition
(δ18O) were found between C. dactylon and Z. japonica (P > 0.05). Higher values were generally
observed for these two species than for P. dilatatum (Figure III.4B), and an increase of ca. 4‰
with dehydration (to 60% RWC) was observed for all species.
100
Photorespiration and C4 Photosynthesis under Drought Stress
P. dilatatum
-2
-1
A (μmol m s )
35
C. dactylon
A
Z. japonica
B
C
30
25
20
15
10
5
0
0
-2
-1
A (μmol m s )
35
10
20
30
0
10
20
30
0
10
20
30
O2 (%)
O2 (%)
O2 (%)
P. dilatatum
C. dactylon
Z. japonica
D
F
E
30
25
20
15
10
α*
C
MS
SS
5
0
0
10
-0.0207
0.0062
-0.0111
20
O2 (%)
α*
O2*
C
MS
SS
7.6
18.4
17.8
30
0
10
-0.0663
-0.0934
---
20
α*
O2*
C
MS
SS
6.0
4.3
---
30
0
O2 (%)
10
-0.0580
-0.0816
-0.0073
20
O2*
5.7
5.1
9.9
30
O2 (%)
Figure III.3. (A-C) Mean values of net CO2 assimilation rate (A) at different O2 concentrations in the
control (C, black diamonds), moderately-stressed (MS, grey squares) and severely-stressed (SS, open
triangles) plants of P. dilatatum, C. dactylon and Z. japonica. The bars correspond to the standard errors
of each mean value. Measurements were taken at ambient CO2 (360 μmol mol-1), a PPFD of 850 μmol m2
s-1 and 25ºC. (D-F) Representation of the ‘exponential plus linear’ model fitted to the variation of A with
O2 in the C (solid lines), MS (long-dashed lines) and SS (short-dashed lines) plants of each species. The
lines correspond to the curves obtained by plotting the best model statistically significant. Also shown are
the mean values estimated for the slope α*, representing the linear decrease of A with O2 after Amax was
attained, and the O2 concentration corresponding to Amax (O2*). The ‘average’ standard error of differences
(SED) considering all data was 0.0683 for α* and 2.5 for O2* (with 55 d.f.).
101
Chapter III.
A
45
B
40
-16
δ 18O (‰)
δ 13C (‰)
-15
-17
35
30
-18
60
y = - 16.8 + 0.0157 x
[s.e. 0.2; 0.0027]
y = - 17.7 + 0.0157 x
[s.e. 0.2; 0.0027]
y = - 18.1 + 0.0157 x
[s.e. 0.2; 0.0027]
70
80
RWC (%)
90
100
25
60
y = 42.7 - 0.1030 x
[s.e. 2.1; 0.0242]
y = 47.0 - 0.1030 x
[s.e. 2.1; 0.0242]
70
80
90
100
RWC (%)
Figure III.4. Carbon (δ13C; A) and oxygen (δ18O; B) isotope compositions as a function of the relative
water content (RWC) in the leaves of P. dilatatum (black diamonds), C. dactylon (grey squares) and Z.
japonica (open triangles). Each data point corresponds to one sample, with 9 samples per species. The
regression lines correspond to the best models statistically significant (P. dilatatum, solid lines; C.
dactylon, dashed lines; and Z. japonica, dotted lines; A, R2 = 90.1%, s2 = 0.40, d.f. = 23; B, R2 = 65.3%,
s2 = 3.23, d.f. = 24).
Amino acids analysis
The variation of the amino acids content in illuminated leaves of the three C4 grasses with leaf
dehydration is shown in Figure III.5. The fully hydrated leaves of P. dilatatum had generally
more serine (Ser) than the other two species. This amino acid increased steeply in all three
species when RWC decreased from 98 to 60% (C. dactylon and Z. japonica) or only down to
80% (P. dilatatum). Below 60% RWC a slight decrease was observed for the leaves of Z.
japonica. Glycine (Gly) had a quadratic variation with RWC. In C. dactylon there was only a
slight increase when RWC started to decrease and then glycine was kept nearly constant. Z.
japonica had less glycine than the other two species in fully hydrated conditions and the amount
increased as the RWC decreased from 98 to 60%, but was not so strongly affected by further
decreases in RWC. In P. dilatatum, although high variability was observed, the regression
showed increased glycine when RWC decreased from 99 to 80%. The ratio Gly/Ser was only
decreased with RWC in C. dactylon. Glutamate (Glu) decreased with decreasing RWC only in C.
dactylon and glutamine (Gln) was not significantly affected by leaf dehydration (P > 0.05), the
same being observed for the ratio Gln/Glu. Aspartate (Asp) decreased linearly with RWC in P.
dilatatum and Z. japonica, but no significant variation with RWC was observed for C. dactylon
(P > 0.05). Alanine (Ala) was not significantly affected by leaf dehydration (P > 0.05) in the
102
Photorespiration and C4 Photosynthesis under Drought Stress
three species, showing high variability, especially in C. dactylon. Conversely, asparagine (Asn)
increased linearly with decreasing RWC, but only in C. dactylon and Z. japonica.
0.40
A Ser
0.20
B Gly
0.80
2
y = 0.256 - 0.000019 x [s.e. 0.021; 0.000002]
C Gly/Ser
y = 0.0044 x [s.e. 0.0003]
2
y = 0.0024 x - 0.000019 x [s.e. 0.0002; 0.000002]
2
0.30
0.15
0.20
0.10
0.40
0.05
0.20
0.10
0.00
-2
Amino Acids Content (mmol m )
y = 0.0021 x - 0.000019 x [s.e. 0.0002; 0.000002]
1.00
2
y = 0.0139 x - 0.000118 x
[s.e. 0.0008; 0.000008]
2
y = - 0.1187 + 0.0139 x - 0.000118 x
[s.e. 0.015; 0.0008; 0.000008]
40
60
80
100
D Glu
0.00
0.50
0.60
40
60
80
100
E Gln
0.00
1.50
40
60
80
100
80
100
F Gln/Glu
y = 0.0082 x [s.e. 0.0001]
0.80
0.40
1.20
0.60
0.30
0.90
0.40
0.20
0.60
0.20
0.10
0.30
0.00
0.80
40
60
80
100
G Asp
0.00
0.80
40
60
80
100
H Ala
0.00
1.60
Asn
y = 1.876 - 0.0136 x [s.e. 0.170; 0.0019]
y = - 0.039 + 0.0046 x [s.e. 0.068; 0.0009]
y = 0.526 - 0.0046 x [s.e. 0.077; 0.0010]
0.60
1.20
0.40
0.40
0.80
0.20
0.20
0.40
40
I
60
y = - 0.560 + 0.0084 x [s.e. 0.354; 0.0037]
0.60
0.00
40
60
80
RWC (%)
100
0.00
40
60
80
RWC (%)
100
0.00
40
60
80
100
RWC (%)
Figure III.5. Variation of the amino acids content with the relative water content (RWC) in the leaves of
P. dilatatum (black diamonds), C. dactylon (grey squares) and Z. japonica (open triangles) collected in
the light: serine (Ser), glycine (Gly), Gly/Ser ratio, glutamate (Glu), glutamine (Gln), Gln/Glu ratio,
aspartate (Asp), alanine (Ala), asparagine (Asn) (A-I, respectively). Each data point corresponds to one
sample, with 24 samples per species. The regression lines and curves, when applied, correspond to the
best models statistically significant (P. dilatatum, solid lines; C. dactylon, dashed lines; and Z. japonica,
dotted lines; A, R2 = 59.8%, s2 = 0.297, d.f. = 69; B, R2 = 56.1%, s2 = 0.035, d.f. = 67; C, R2 = 40.5%, s2
= 0.012, d.f. = 68; D, R2 = 88.5%, s2 = 0.406, d.f. = 69; E, P > 0.05; F, P > 0.05; G., R2 = 56.1%, s2 =
0.035, d.f. = 67; H, P > 0.05; I, R2 = 56.1%, s2 = 0.035, d.f. = 67).
103
Chapter III.
The content of some amino acids in the leaves collected after a period of 30 seconds in
darkness changed significantly compared to corresponding leaves collected in the light (Table
III.2), but the difference ‘dark minus light’ was not affected by leaf dehydration (P > 0.05). In
the three species a decrease in glycine content after 30 seconds in darkness was observed. The
ratio between glycine and serine also decreased, mostly due to the decreased glycine, as no
significant differences were observed for serine. Both glycine and the ratio Gly/Ser showed a
larger decrease in C. dactylon than in P. dilatatum or Z. japonica. Glutamate decreased in the
dark- compared to the light-samples, but only for P. dilatatum and C. dactylon. Conversely,
glutamine content was not significantly changed (P > 0.05), and the ratio Gln/Glu increased only
in P. dilatatum, with no significant changes in C. dactylon or Z. japonica (P > 0.05). Aspartate
decreased after 30 seconds in darkness in C. dactylon and Z. japonica. Concomitantly, in these
two species a substantial increase in alanine was observed. The content in asparagine was not
significantly changed in 30 seconds of darkness (P > 0.05).
Table III.2. Estimated mean values of the difference between the amino acids content (mmol m-2) in the
leaves of P. dilatatum, C. dactylon and Z. japonica collected in the light and after 30 seconds in darkness
(‘dark minus light’) and respective standard errors of differences (in brackets). There was no significant
variation with RWC (P > 0.05) and, therefore, the overall mean values obtained for the samples of each
species (considering all control and non-watered plants, in a total of 24 samples per species) were
analysed by the REML method. NS., not significantly different from zero (P > 0.05).
Species
Ser
Gly
Gly/Ser
Glu
Gln
Gln/Glu
Asp
.Ala
Asn
P. dilatatum
NS.
-0.007
(0.004)
-0.025
(0.019)
-0.029
(0.020)
NS.
0.125
(0.056)
NS.
NS.
NS.
C. dactylon
NS.
-0.019
(0.004)
-0.145
(0.019)
-0.059
(0.020)
NS.
NS.
-0.055
(0.019)
0.115
(0.023)
NS.
Z. japonica
NS.
-0.007
(0.004)
-0.043
(0.019)
NS.
NS.
NS.
-0.031
(0.019)
0.085
(0.023)
NS.
Mechanistic modelling of C4 photosynthesis
The variation of the enzyme-limited net CO2 assimilation rate (Ac) with the CO2 concentration in
the mesophyll cells (Cm) for each of the three C4 grasses was affected by drought stress as shown
in Figure III.6. The mean values estimated for the maximum PEPC carboxylation activity (Vpmax)
and maximum Rubisco carboxylation activity (Vcmax) were higher in C. dactylon than in P.
104
Photorespiration and C4 Photosynthesis under Drought Stress
dilatatum and lower in Z. japonica and were significantly affected (P < 0.01) by the drought
stress level.
P. dilatatum
-2
-1
Ac, Vp, Vcmax (μmol m s )
35
C. dactylon
A
Z. japonica
B
C
30
25
20
15
10
5
0
0
100
200
300
400
500 0
-1
Cm (μmol mol )
100
33.6
29.9
16.6
64.3
57.2
27.7
300
400
-1
Cm (μmol mol )
500 0
100
C
MS
SS
34.3
30.5
---
88.4
78.3
---
200
300
400
500
-1
Cm (μmol mol )
Vcmax Vpmax
Vcmax Vpmax
C
MS
SS
200
Vcmax Vpmax
C
MS
SS
30.8
26.4
17.5
51.8
48.4
32.6
Figure III.6. Enzyme-limited net CO2 assimilation rate (Ac) as a function of the mesophyll CO2
concentration (Cm) in the control (C, black lines), moderately-stressed (MS, dark-grey lines) and severelystressed (SS, light-grey lines) plants of P. dilatatum, C. dactylon and Z. japonica. Also shown are the
rates of PEP carboxylation (Vp, dashed lines) and the maximum Rubisco carboxylation activity (Vcmax,
dotted lines). The mean values of Vcmax and maximum PEPC carboxylation activity (Vpmax) are presented
below each graph. All parameters were calculated by applying a mechanistic model of C4 photosynthesis
(von Caemmerer & Furbank, 1999). For simplification, assumption was made that the fraction of O2
evolution in the bundle sheath is null.
By application of the model equations, the concentration of CO2 in the bundle sheath (Cs)
is predicted to increase steeply with the increase in Ci, reaching values above 1000 μmol mol-1 at
low Ci (Figure III.7A-C). Accordingly, the rate of photorespiration (Pr) for each of the three C4
grasses at the different stress levels was estimated to be always lower than 1.5 μmol m-2 s-1 and
decreased with increasing Ci (Figure III.7D-F). The values predicted for Pr at ambient levels of
CO2 (360-390 μmol mol-1) were slightly higher for the MS plants of each species than in the
corresponding controls whereas slightly lower values were obtained for the SS relative to the MS
plants of P. dilatatum and Z. japonica.
105
Chapter III.
P. dilatatum
14000
C. dactylon
Z. japonica
A
B
C
D
E
F
-1
Cs (μmol mol )
12000
10000
8000
6000
4000
2000
-2
-1
Photorespiration (μmol m s )
0
1.50
1.25
1.00
0.75
0.50
0.25
0.00
0
100
200
300
400
500 0
100
Ci (μmol mol-1)
Ca = 360
μmol mol
-1
Ci
C
MS
SS
Cs
Ac
145 4300 27
80 1950 20
85 1240 9.5
200
300
400
500 0
100
Ci (μmol mol-1)
Pr
0.26
0.42
0.31
Ci
C
MS
SS
Cs
Ac
175 10250 31
85 5250 24
--- -----
200
300
400
500
Ci (μmol mol-1)
Pr
0.12
0.20
---
Ci
C
MS
SS
Cs
Ac
155 4500 23
100 2250 19
110 2000 12
Pr
0.23
0.33
0.23
Figure III.7. Representation of the predicted CO2 concentration in the bundle sheath cells (Cs) and rate of
photorespiration (Pr) as a function of the intercellular CO2 concentration (Ci) in the control (C, black
lines), moderately-stressed (MS, dark-grey lines) and severely-stressed (SS, light-grey lines) plants of P.
dilatatum, C. dactylon and Z. japonica. For the estimation of Cs and Pr, using the equations for enzymelimited C4 photosynthesis (von Caemmerer & Furbank, 1999), Vcmax and Vpmax were fixed at the mean
values obtained for each group of plants (see Figure III.6.) and other model parameters, namely the O2
concentration in the bundle sheath, were assumed to be constant at 25ºC. Also shown are the mean values
of Ci, Cs, Ac and Pr, estimated through the modelling approach, at ambient concentrations of CO2 (360390 μmol mol-1) and O2 (21%).
106
Photorespiration and C4 Photosynthesis under Drought Stress
DISCUSSION
Drought stress condition and photosynthetic responses to CO2
The relative water content (RWC) in the leaves of P. dilatatum, C. dactylon and Z. japonica
decreased with the decrease in water available in the soil (Figure III.1) and was used as a
reference to analyse the effects of drought. The three species showed some differences in the
extent of leaf dehydration. In the gas-exchange experiment, P. dilatatum leaves were more
severely dehydrated than the other two species. In the amino acids experiment, the plants of C.
dactylon and Z. japonica were deprived of water before P. dilatatum in an attempt to get a
similar range of RWC values in the samples of the three species. However, C. dactylon and
(more dramatically) Z. japonica plants were more dehydrated in this experiment. The difficulty
in obtaining similar dehydration levels for the three species was a result of the faster loss of
water by the leaves of P. dilatatum than by C. dactylon and Z. japonica in response to the drying
soil. When water deficit was rapidly imposed in these C4 grasses by the addition of polyethylene
glycol to the nutrient solution (Carmo-Silva et al. 2007), C. dactylon showed a faster decrease in
RWC than P. dilatatum and Z. japonica. The differences are likely to result from the different
methods used for drought stress induction (Chaves et al. 2003).
Stomatal closure and decreased photosynthesis are generally accepted as early
consequences of leaf dehydration (e.g. Chaves et al. 2003) and this was observed among several
C4 grasses (Ghannoum et al. 2003; Marques da Silva & Arrabaça 2004; Carmo-Silva et al. 2007).
The net CO2 assimilation rate (A) decreased in the moderately-stressed (MS) leaves of P.
dilatatum and C. dactylon but not in Z. japonica (Figure III.2) compared to the controls,
suggesting that photosynthesis by the latter species might be more resistant to moderate drought
conditions. In severely-stressed (SS) leaves of P. dilatatum and Z. japonica, A decreased to about
half of the values observed in control conditions. Stomatal closure was also observed in response
to drought stress, as shown by the decrease in the stomatal conductance to water vapour (gswa)
in the MS and SS plants compared to the controls (Figure III.2). The maximal values of gswa in
the MS leaves of P. dilatatum and Z. japonica were not affected compared with the control,
which demonstrates that the effect of the low CO2 concentrations promoting the opening of
stomata was stronger than the drought-induced stomatal closure at that stage. However, for
higher CO2 concentrations, gswa was always lower in dehydrated leaves compared with the
control plants of each species, resulting in generally lower values of Ci for a given CO2
concentration. The decrease of Amax in the most dehydrated leaves of each grass species
107
Chapter III.
compared to their controls suggests that metabolic limitations of photosynthesis are also present.
The same conclusion has been reached for other C4 grasses (Ghannoum et al. 2003). Net CO2
assimilation consistently increased with increasing Ci and the CO2 concentration corresponding
to zero A (the CO2 compensation point) was not significantly different from zero (P > 0.05) for
either control or dehydrated leaves of any of the three species, suggesting little photorespiration.
O2-sensitivity of C4 photosynthesis
Net CO2 assimilation rates (A) at ambient CO2 (360 μmol mol-1) were little affected by the O2
concentration in the gas-phase but A was lowest at 2% O2 and decreased significantly with O2
after the maximal value (Amax) was attained (Figure III.3). Little sensitivity of C4 photosynthesis
to O2 was found in two other grasses, Panicum antidotale (NADP-ME) and Panicum coloratum
(NAD-ME) at low Ci (Ghannoum et al. 1998). The estimation of the O2 concentration
corresponding to Amax in the three C4 grasses of the different metabolic subtypes (O2*, Figure
III.3) agree with previously reported results (Maroco et al. 1997). The inhibition of A at low O2
concentrations is probably due to an extra energy requirement for the regeneration of PEP and
proper functioning of the C4 cycle (Ku et al. 1991; Dai et al. 1993; Maroco et al. 2000). A
reduced ATP production, by the O2-dependent photochemical reactions or even by mitochondrial
respiration (Maroco et al. 1997), might lead to a deficient function of the C4 pathway. The slight
decrease of A at O2 concentrations higher than 10% is likely to be due to the competing
oxygenation of RuBP that initiates the photorespiratory carbon oxidative cycle.
The rate of decreasing A with increasing O2 (Fig III.3D-F), partly due to photorespiration,
suggests that rates in ambient air would be in the range 0-2 μmol m-2 s-1 (representing 0-6% of
Amax) and slightly faster in C. dactylon than in the other two C4 grasses, but the variability is
large and there is no clear trend for its variation with leaf dehydration. Since A was affected by
drought (Figures III.2 and III.3), there is a tendency for increased photorespiration in proportion
to photosynthesis when leaf dehydration increases. Photorespiration in Amaranthus edulis (C4
dicotyledon), estimated by the release of NH3, represented 6% of the rate of CO2 assimilation
(Lacuesta et al. 1997). In two C4 grasses, a sharp increase of the CO2 compensation point when
the RWC fell below 60% compared with very low values in fully hydrated leaves, led to the
conclusion that photorespiration was greatly enhanced under drought conditions (Ghannoum et
al. 2003). The results presented here show that photorespiration by the three C4 grasses studied is
slow and not sufficient to explain the decrease observed in A under the drought conditions
attained (Figure III.1).
108
Photorespiration and C4 Photosynthesis under Drought Stress
Effects of dehydration on carbon and oxygen isotope compositions
Drought stress affected leaf carbon and oxygen isotope compositions (δ13C and δ18O) similarly
in the three C4 grasses (Figure III.4), with a decrease of 0.5‰ in δ13C and an increase of 4‰ in
δ18O when the RWC decreased down to 60%. In cotton leaves (C3 plant), stomatal closure after
ABA treatment resulted in
13
C and
18
O enrichment (Barbour & Farquhar 2000). Conversely, a
study with several C4 grasses revealed a decrease of δ13C under drought but no consistent
variation in δ18O (Ghannoum et al. 2002). The less negative values of δ13C in P. dilatatum
(NADP-ME) than in C. dactylon (NAD-ME) and Z. japonica (PEPCK) agree with previously
reported differences among C4 grasses from the different subtypes (Hattersley 1982). On the
other hand, differences in the leaf length and interveinal distance among grasses (Helliker &
Ehleringer 2000) might explain the higher values of δ18O in C. dactylon and Z. japonica than in
P. dilatatum.
Variations in δ13C are mostly due to changes in the ratio of intercellular to atmospheric
CO2 concentrations (Ci/Ca) and/or changes in the fraction of CO2 fixed by PEPC that
subsequently leaks out from the BS without being assimilated by Rubisco (leakiness, φ)
(Farquhar 1983). Variation in φ can result either from alterations in the physical conductance of
BS cells to CO2 or alterations in the balance between PEPC and Rubisco activities (Peisker &
Henderson 1992). In Sorghum bicolor (Williams et al. 2001) and in sugarcane (Saliendra et al.
1996), both carbon isotope discrimination and φ increased under drought conditions, suggesting
that the coordination between the C4 and C3 cycles was affected. Conversely, Buchmann and coworkers (1996) related the decrease of δ13C in several C4 grasses (including P. dilatatum, C.
dactylon and Z. japonica) under drought with decreased stomatal conductance, which affected
the intercellular CO2 concentration. Decreased gswa observed in the dehydrated leaves of each
species (Figure III.2) resulted in decreased Ci/Ca compared to the control plants of each species
(data not shown) and could contribute to the decrease in δ13C (Figure III.4A). However, the data
obtained do not exclude the possibility that impairment of photosynthetic metabolism increased
φ and contributed to the decrease in δ13C with drought in the three C4 grasses.
The increase of leaf δ18O with RWC (Figure III.4B) reflects the variation of δ18O in the
soil water, which becomes enriched in the heavy isotope under drought conditions due to
evaporation, and variation due to evaporative and diffusional effects during transpiration
(Barbour 2007). Decreased gswa in the dehydrated leaves (Figure III.2) probably contributed to
the leaf 18O enrichment because H218O diffuses more slowly and has lower vapour pressure than
H216O, causing the water in the leaf to become enriched in 18O during transpiration. Variations in
109
Chapter III.
evaporation during plant growth are integrated in the oxygen isotope composition of leaf
material (Barbour & Farquhar 2000). Although fractionation of oxygen isotopes occurs during
exchanges of CO2 and O2 with the atmosphere resulting from the sum of photosynthesis,
photorespiration and mitochondrial respiration, these effects are quickly buffered in the leaf due
to rapid isotopic exchange between the carbonyl oxygen in organic molecules and leaf water
(Barbour 2007).
Although some suggestions have been made that photorespiration should be considered
when interpreting δ13C (Gillon & Griffiths 1997) and δ18O (Farquhar et al. 1998) of leaf material,
the rate of photorespiration present in C4 plants is not likely to be of reasonable size to make
considerable contribution for the variations in isotope ratios of leaf dry matter. In the present
study, drought did not cause changes in δ13C in the same direction as observed for C3 species
(Cerling 1999), where photorespiration is rapid.
Effects of dehydration on steady-state contents of amino acids in illuminated leaves
The increased amounts of glycine and serine in the dehydrated leaves of all three species,
especially with decreases in RWC down to 80 and 60% (Figure III.5A,B), might be interpreted
as an increase in photorespiratory metabolism and flux of glyoxylate into the pathway because of
increased oxygenation of RuBP. Alternatively, these changes may reflect an increase in pool
sizes of glycine and serine because of slower transfer to the mitochondria and peroxisome.
Increased glycine and serine contents were also observed in droughted maize leaves (Foyer et al.
1998).
In leaves of C. dactylon, when RWC decreased from 98 to 60%, asparagine increased
from 0.5 to 1.1 mmol m-2 (Figure III.5I) and glutamate decreased from 0.8 to 0.5 mmol m-2
(Figure III.5D). An increase of 2- to 6-fold in the amount of asparagine was previously reported
in C. dactylon plants submitted to drought stress (Barnett & Naylor 1966) and the same authors
reported a concomitant decrease in glutamate and alanine. In Z. japonica the content of
asparagine increased by 4-fold and aspartate decreased when RWC decreased to 40% (Figure
III.5I,G) but glutamate amounts were not affected. No significant effect of RWC on the alanine
content was observed for any of the three C4 grasses (Figure III.5H). The contents in glutamate,
asparagine and alanine were generally higher in C. dactylon than in P. dilatatum and Z. japonica
whereas the content in glutamine was lower in the first species.
110
Photorespiration and C4 Photosynthesis under Drought Stress
Amino acid changes in 30 seconds of darkness
In illuminated leaves, carboxylation and oxygenation of RuBP will produce P-glycerate and Pglycolate. The later enters the photorespiratory carbon oxidative cycle and, as a result, glycine
will be converted into serine in the mitochondria. In darkness the regeneration of RuBP and the
production of both P-glycerate and P-glycolate will stop. The glycine pool will then decrease
because the amino acid is no longer being formed, but it is still converted into serine. Therefore,
the decrease in the amount of glycine in the leaves after 30 seconds in darkness compared to the
amounts in fully illuminated leaves during steady-state photosynthesis will be related to the rate
of photorespiratory production of P-glycolate through the oxygenation of RuBP. The pools of
glycine and serine in the leaves are thought to reflect the photorespiratory fixation of O2 quite
accurately, given that the synthesis of these two amino acids occurs mostly through this process
(Jolivet-Tournier & Gerster 1984). Earlier studies revealed that in C3 plants, steady-state
photosynthesis followed by one or two minutes in darkness results in considerable decrease in
glycine and increase in serine (Roberts et al. 1970, Kumarasinghe et al., 1977). The rate of
incorporation of 18O into glycolate and glycine in maize (C4 monocotyledon, NADP-ME) leaves
increased with increasing O2 concentrations, providing direct evidence for photorespiratory O2
uptake, albeit at a much lower rate than in wheat (de Veau & Burris 1989). In Amaranthus edulis
(C4 dicotyledon, NAD-ME), the decrease of both glycine and serine contents at very low O2 was
consistent with photorespiratory production of both amino acids at atmospheric O2 levels
(Maroco et al. 2000). In the C4 grasses P. dilatatum, C. dactylon and Z. japonica there was no
effect of decreased RWC (P > 0.05) on the difference between the content of amino acids in the
light- and the dark-samples and therefore the mean changes after 30 seconds in darkness for each
species, presented in Table III.2, were estimated considering all the data for the control and nonwatered plants together. A decrease in the glycine content was observed after 30 seconds in
darkness, but serine content was not affected in any of the three species. The changes in glycine
suggest that photorespiration was faster in C. dactylon (0.32 ± 0.07 μmol m-2 s-1) than in P.
dilatatum and Z. japonica (0.12 ± 0.07 μmol m-2 s-1). Maroco et al. (2000) proposed that changes
in the glycine pool would be a better indicator of the occurrence of photorespiration in maize
(C4) than changes in the serine pool. On the other hand, Novitskaya et al. (2002) demonstrated
that the ratio between the two amino acids is strongly correlated with the photorespiration in C3
plants. The decrease in Gly/Ser ratio in the dark was notably higher in C. dactylon than in P.
dilatatum and Z. japonica (Table III.2), suggesting that photorespiration in the NAD-ME grass
might be faster than in the other two species, although still much slower than the values reported
111
Chapter III.
for C3 species (Keys 1986; Novitskaya et al. 2002). Photorespiratory CO2 evolution rates at
atmospheric O2 (21%) and CO2 (350 μmol mol-1) for wheat (C3) and maize (C4) were estimated
to be 27 and 2%, respectively, of net photosynthetic CO2 assimilation (de Veau & Burris 1989).
More extensive studies on isotopic oxygen uptake in the light found slightly faster
photorespiration rates in NAD-ME compared to NADP-ME monocotyledon species (Furbank &
Badger 1982; Siebke et al. 2003).
Glutamine synthetase is responsible for the re-assimilation of NH3 produced in the
photorespiratory carbon oxidation cycle (Keys et al. 1978) and the glutamate produced by the
GOGAT complex during the photorespiratory nitrogen cycle acts as a donor of amino groups
needed for the photorespiratory carbon oxidation cycle (Keys 1999). The GOGAT reaction
would be expected to stop quickly in the dark, justifying the decrease in glutamate content in the
darkened leaves of P. dilatatum and C. dactylon, which was not evident in Z. japonica (Table
III.2). As suggested by Novitskaya et al. (2002), the trends in glutamate and glutamine might
well reflect processes linked to photosynthesis other than photorespiration, including primary N
assimilation, and can not be interpreted as a result of altered photorespiratory NH3 flux.
The decrease in aspartate, the primary C4 acid formed in the M cells of NAD-ME and
PEPCK species, in the darkened leaves of C. dactylon and Z. japonica (Table III.2) is a clear
consequence of stopping the primary fixation of CO2 by PEPC. Accordingly, the increase in
alanine in the same two species is consistent with continuing amination of pyruvate in the C4
cycle. In wheat and potato (C3 plants) decreased contents in both aspartate and alanine were
found under photorespiratory conditions (Novitskaya et al. 2002). However, in durum wheat a
period of 30 seconds in darkness induced no changes in aspartate or alanine although a clear
decrease in glycine and serine was observed (data not shown). Moreover, the post-illumination
changes in aspartate and alanine were restricted to C. dactylon and Z. japonica, with no changes
being observed in P. dilatatum (NADP-ME), suggesting their association with the C4
photosynthetic pathway rather than the photorespiratory metabolism.
Modelling the CO2-response of C4 photosynthesis under drought conditions
A mechanistic model of C4 photosynthesis (von Caemmerer & Furbank 1999) was applied to the
photosynthetic response of the three C4 species to the intercellular CO2 concentration at high
irradiance shown in Figure III.2. This approach allowed the simulation of the effect of moderate
and severe leaf dehydration on the rates of photosynthesis and photorespiration by the three
grasses. At high irradiance, photosynthesis is assumed to be enzyme-limited and mostly
112
Photorespiration and C4 Photosynthesis under Drought Stress
determined by the rates of PEP and RuBP carboxylation by PEPC and Rubisco, respectively, and
by the regeneration of PEP (von Caemmerer 2000). The variation of the enzyme-limited net CO2
assimilation rate (Ac) with the CO2 concentration in the mesophyll cells (Cm) is represented in
Figure III.6. The values predicted seem to agree fairly well with the experimental results (Figure
III.2). However, there might be some overestimation of the modelled values of Ac for C. dactylon
and Z. japonica. For simplification, it was assumed that the fraction of O2 evolution in the BS is
null for all three species but this is not likely to be correct for the NAD-ME or PEPCK species,
in which PSII activity will probably increase the O2 concentration in the BS, decreasing the
actual value of photosynthesis. The mean values estimated for the maximum PEPC
carboxylation activity (Vpmax) and maximum Rubisco carboxylation activity (Vcmax) were higher
in C. dactylon than in P. dilatatum and lower in Z. japonica and decreased with leaf dehydration
(Figure III.6). These results suggest that both enzymes responsible for the carboxylation of PEP
and RuBP are down-regulated under drought conditions. The negative effect of drought on the
photosynthesis response to CO2 is more notable in the SS plants of P. dilatatum and Z. japonica,
but a clear effect of leaf dehydration on the two enzymes and on Ac was simulated for the MS
plants of each of the three species.
The application of the model equations to the experimental data from the CO2-response
curves allowed the simulation of a rate of photorespiration (Pr) for each of the three C4 grasses at
the different stress levels (Figure III.7). Even at the lowest Ci values, the predicted Pr was never
higher than 1.5 μmol m-2 s-1 and at ambient levels of CO2 (360-390 μmol mol-1) Pr was
estimated to be less than 3.5% of Ac for all species and stress levels. The modelled data suggest
that photorespiration is present at higher rates in the MS relative to the well-watered plants of
each species, especially in Z. japonica. The lower values of Pr estimated for the SS compared to
the MS plants of P. dilatatum and Z. japonica indicate that metabolic inhibition of
photosynthesis occurs at the level of Rubisco, affecting both the rates of photosynthesis and
photorespiration. Pr was estimated to be lower in C. dactylon than in the other two species.
However, higher values could have been predicted for either C. dactylon (NAD-ME) or Z.
japonica (PEPCK) if a more realistic O2 concentration in the bundle sheath, likely to be higher
than in P. dilatatum (NADP-ME), had been considered.
Stomatal closure causes decreased Ci under drought conditions (Figure III.2).
Concomitantly, lower values of Cs for a given CO2 concentration are estimated for the
dehydrated leaves compared with the controls (Figure III.7). The slower response of increasing
Cs with increasing Ci under drought stress suggest that metabolic impairment might be present,
affecting the effectiveness of the C4 cycle. As a result, Ac is also decreased and Pr increases, i.e.
113
Chapter III.
the predicted rates of photorespiration increase as a proportion to the modelled net
photosynthesis when leaf dehydration increases for all three species. Many assumptions were
made to allow the simulation of plant photosynthetic and photorespiratory responses by the
application of a mechanistic model and, consequently, much uncertainty is involved. It was
assumed that stomata apertures decreased uniformly and that gm and gbs were not changed with
stress. Furthermore, it was accepted that the estimated values of Vcmax and Vpmax changed with
stress. More accurate predictions will be possible when more measurements of the assumed
parameters are available as, for example, of the kinetic constants of Rubisco from each of the C4
species.
CONCLUSIONS
Drought stress induced decreased leaf water contents, stomatal closure and decreased net CO2
assimilation in the leaves of P. dilatatum, C. dactylon and Z. japonica. The response of net
photosynthesis to CO2 was typical of C4 leaves and extrapolation to zero A gave no evidence of a
significant CO2 compensation point even in droughted plants. Net CO2 assimilation was
decreased only slightly with increasing O2 concentrations above 10% in the atmosphere around
the leaves but this trend was not increased by drought stress. Leaves of both control and
droughted plants showed δ13C and δ18O typical of C4 plants and changes with RWC were not
indicative of changes in photorespiration. Increased contents of glycine and serine in the
dehydrated leaves of the three species provided the only suggestion for increased
photorespiratory metabolism by water deficit. Changes in amino acids content after 30 seconds
in darkness, especially the decrease in glycine, were consistent with slow photorespiration rates
in all three species, but slightly faster in C. dactylon (NAD-ME) than in P. dilatatum (NADPME) and Z. japonica (PEPCK), and not changed by drought stress in either species. Mechanistic
modelling of CO2-response curves as well as the O2-sensitivity of C4 photosynthesis by the three
grasses suggested slightly increased photorespiration rates in proportion to photosynthesis.
However, the overall results presented here suggest that the C4 grasses of the three different
metabolic subtypes are able to maintain levels of CO2 at the Rubisco site sufficiently high to
limit oxygenase activity under drought stress.
114
Photorespiration and C4 Photosynthesis under Drought Stress
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Photorespiration and C4 Photosynthesis under Drought Stress
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119
Chapter IV.
C4 ENZYMES IN DROUGHT-STRESSED GRASSES
Chapter IV.
An integral copy of this chapter has been submitted for publication in Photosynthesis Research:
Carmo-Silva A. E., Bernardes da Silva A., Keys A. J., Parry M. A. J. & Arrabaça M. C. (Submitted) The
activities of PEP carboxylase and the C4 acid decarboxylases are little changed by drought stress in three
C4 grasses of different subtypes. Photosynthesis Research.
Anabela Bernardes da Silva (Centro de Engenharia Biológica, Faculdade de Ciências da Universidade de
Lisboa) was involved in the planning of the experiment and interpretation of the results obtained.
122
C4 Enzymes in Drought-Stressed Grasses
THE ACTIVITIES OF PEP CARBOXYLASE AND THE C4 ACID
DECARBOXYLASES ARE LITTLE CHANGED BY DROUGHT
STRESS IN THREE C4 GRASSES OF DIFFERENT SUBTYPES
ABSTRACT
The efficiency of C4 photosynthesis depends on good coordination between the assimilation of
CO2 by phosphoenolpyruvate carboxylase (PEPC) and the subsequent decarboxylation of C4
acids, which might be affected under water deficit. The effects of gradually-induced drought
stress on the activities of PEPC and the decarboxylating enzymes in Paspalum dilatatum
(NADP-malic enzyme, NADP-ME), Cynodon dactylon (NAD-malic enzyme, NAD-ME) and
Zoysia japonica (PEP carboxykinase, PEPCK) were slight. Moderate leaf dehydration caused
increased physiological activity of PEPC in all three species, decreased activities of NADP-ME
in P. dilatatum and of NAD-ME and PEPCK in C. dactylon, but had no effect on the
decarboxylating enzymes in Z. japonica. Decreased inhibition of PEPC activity by L-malate
under drought suggested an increased PEPC phosphorylation state in all species. Considerable
PEPCK activity in all three species suggests its possible role as a supplementary decarboxylase
in P. dilatatum and C. dactylon or additional involvement in non-photosynthetic metabolism.
KEYWORDS:
NADP-ME, NAD-ME, PEPCK, P. dilatatum, C. dactylon, Z. japonica
123
Chapter IV.
INTRODUCTION
C4 photosynthesis is characterised by the presence of a CO2-concentrating mechanism, which
involves the initial fixation of CO2 by phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) in
the mesophyll (M) cells followed by the decarboxylation of the resulting C4 acids in the bundle
sheath (BS) cells, where the CO2 released is assimilated by ribulose-1,5-bisphosphate
carboxylase/oxygenase (Rubisco, EC 4.1.1.39). The increased CO2 concentration in the BS cells
results in low RuBP oxygenation and, consequently, low rates of photorespiration and increased
rates of photosynthesis by C4 plants (Kanai & Edwards 1999). In the present work, the relative
contribution of the three decarboxylases involved in C4 photosynthesis and the coordination
between the primary carboxylation and subsequent decarboxylation steps were evaluated in three
C4 grasses exposed to drought stress.
The three biochemical subtypes of C4 photosynthesis have been defined by the main
enzyme responsible for C4 acid decarboxylation (Gutierrez et al. 1974; Hatch et al. 1975; Hatch
1987): NADP-malic enzyme (NADP-ME, EC 1.1.1.40), NAD-malic enzyme (NAD-ME, EC
1.1.1.39) and PEP carboxykinase (PEPCK, EC 4.1.1.49). However, variations to these ‘classical’
mechanisms of C4 photosynthesis occur. PEPCK is present in various species of the NADP-ME
subtype (Walker et al. 1997; Voznesenskaya et al. 2006) and acts as a supplementary
decarboxylase in Zea mays (Wingler et al. 1999). A set of anatomical characteristics, including
the shape and position of the chloroplasts in the BS cells, is associated with each biochemical
subtype (Gutierrez et al. 1974; Prendergast et al. 1987; Dengler et al. 1994). Most C4 grasses fit
clearly into one of the three subtypes but Hattersley & Watson (1992) distinguished a total of ten
anatomical-biochemical variants of C4 plants.
The three C4 grasses studied in the present work were classified as belonging to each of
the different biochemical subtypes of C4 photosynthesis: Paspalum dilatatum as a NADP-ME
species (Usuda et al. 1984), Cynodon dactylon as a NAD-ME species (Hatch & Kagawa 1974)
and Zoysia japonica as a PEPCK species (Gutierrez et al. 1974). Dallisgrass (P. dilatatum),
bermudagrass (C. dactylon) and zoysiagrass (Z. japonica) are warm-season species used for
turfgrass purposes throughout the world (Brown 1999). Additionally, the first two species are
important forage and cultivated pasture grasses whilst C. dactylon is also one of the world’s most
serious weeds (Jones 1985).
The understanding of plant responses to drought and the identification of species better
adapted to the expected climate changes is crucial. Plant physiological and biochemical
responses to water deficit depend on the rate of induction and severity of the stress, the leaf age
124
C4 Enzymes in Drought-Stressed Grasses
and state of development, and the species studied (Chaves 1991). In a previous study on the
effects of rapidly-imposed drought conditions, C. dactylon leaves dehydrated faster but its
photosynthesis and instantaneous water use efficiency were more resistant to water deficit than
in Z. japonica and P. dilatatum (Carmo-Silva et al. 2007). Whether decreased CO2 assimilation
during drought stress can be partly attributed to metabolic limitation by the enzymes of the CO2concentrating mechanism in the three ‘classical’ biochemical subtypes of C4 photosynthesis is
still unclear.
The CO2-concentrating mechanism can be affected by water deficit either through effects
on the activities and/or regulation of the C4 cycle enzymes or through restriction of metabolite
transport between the M and BS cells. Studies with several C4 grass species of the NADP-ME
subtype, including Zea mays (Saccardy et al. 1996; Foyer et al. 1998), Sacharum sp. (Du et al.
1996) and Setaria sphacelata (Marques da Silva & Arrabaça 2004), showed contradictory
responses of the C4 enzymes involved in the primary carboxylation, PEPC, and in the
decarboxylation of malate, NADP-ME. Rapidly-induced leaf dehydration decreased PEPC
activity in Zoysia japonica, but did not affect the enzyme in Paspalum dilatatum and Cynodon
dactylon (Carmo-Silva et al. 2004; 2007). The decarboxylating enzymes considered to have an
important role in the decarboxylation of C4 acids by each of these grass species were not affected
to an extent that would limit photosynthesis under rapidly-induced water deficit conditions
(Carmo-Silva et al. 2004). The results obtained in that study suggested that the proportion of
decarboxylation accounted for by PEPCK and NAD-ME in Z. japonica could change with leaf
dehydration.
The regulation of the C4 enzymes involved in the carboxylation and decarboxylation
reactions was reviewed by Leegood and Walker (1999). PEPC in C4 plants is activated in the
light by reversible phosphorylation of a serine residue in the N-terminus and regulated
allosterically by metabolites (Chollet et al. 1996; Izui et al. 2004). The phosphorylation increases
the maximal activity and makes the enzyme less sensitive to inhibition by L-malate and more
sensitive to activation by glucose-6-phosphate (Huber & Sugiyama 1986). Jiao & Chollet (1989)
showed that PEPC sensitivity to L-malate could be used to assess the degree of phosphorylation
of the enzyme. Studies on drought-induced changes in the PEPC phosphorylation state, judged
by the inhibitory effect of L-malate on the enzyme activity, gave contradictory results in Zea
mays, with either decreased (Foyer et al. 1998) or unchanged (Saccardy et al. 1996) PEPC
sensitivity in dehydrated leaves. Information on the regulation of the decarboxylating enzymes
involved in C4 photosynthesis and on the effects that environmental factors exert on them is
sparse. The activity of NADP-ME in the light is modulated by changes in pH and by the
125
Chapter IV.
concentrations of L-malate and Mg2+ (Iglesias & Andreo 1990). NAD-ME activity is regulated
by adenylates and the ratio NADH/NAD+, requires Mn2+ and is stimulated in the presence of
fructose-1,6-bisphosphate and CoA (Murata et al. 1989). PEPCK has also an absolute
requirement for Mn2+, is regulated by the concentrations of metal ions and ATP (Walker et al.
1997), inhibited by a number of phosphorylated metabolites (Burnell 1986) and in some, but not
all, C4 plants is susceptible to phosphorylation (Walker & Leegood 1996). Adenylates can have
an important role in the coordination of NAD-ME and PEPCK activities (Walker et al. 1997).
The relative contribution of each of the three enzymes to C4 acid decarboxylation in different
species needed further research. The decarboxylation step in the C4 pathway is generally
assumed to be non-limiting for photosynthesis; however the coordination between the primary
carboxylation in the mesophyll and the further decarboxylation in the bundle sheath is crucial to
maximize the efficiency of the CO2-concentrating mechanism in C4 plants.
The aim of the present work was to characterise further three C4 grass species in relation
to the different photosynthetic mechanisms and to assess the response of the carboxylating and
decarboxylating enzymes to gradually-induced drought stress. The activities of PEPC, NADPME, NAD-ME and PEPCK in the leaves of P. dilatatum, C. dactylon and Z. japonica were
studied under water deficit conditions. Evidence for drought-induced changes in the regulation of
the enzymes was obtained. The relative activities of the C4 acid decarboxylases under
physiological conditions suggest different contributions to the photosynthesis by the three
species.
126
C4 Enzymes in Drought-Stressed Grasses
MATERIAL AND METHODS
Plant material and drought stress induction
The C4 grasses Paspalum dilatatum Poir. cv. Raki (NADP-ME), Cynodon dactylon (L.) Pers var.
Shangri-Lá (NAD-ME) and Zoysia japonica Steudel ‘Jacklin Sunrise Brand’ (produced by
Jacklin Seed Company, USA) (PEPCK) were grown from seeds in pots with peat-free compost
(Petersfield Products, Leicester, UK) supplemented with a slow-release fertiliser (Hydro Agri
Ltd, Lincs, UK) in a glasshouse. Artificial light was provided whenever the natural light was
below a photosynthetic photon flux density (PPFD) of 500 μmol m-2 s-1 during a 16 h
photoperiod. Temperature was maintained at a minimum of 25ºC during the day and at 18ºC
during the night. Each pot contained five plants and was well-watered until the beginning of the
drought stress treatment. Water deficit was then imposed on the ‘stress’ pots by ceasing to
provide water and the ‘control’ pots were watered once per day. Each pot corresponded to one
independent sample, with 12 control and 12 non-watered pots being used per species. The
treatments (control vs. drought stress) were randomised in a split-plot design with species as the
main plots (columns). From the full set, three control (C) and five non-watered (S) samples of
each species were selected according to their leaf dehydration level (in order to optimise the
range of drought stress intensities) and the corresponding frozen samples used for biochemical
measurements.
Leaf samples were collected in the growth environment four hours after the beginning of
the photoperiod, eight to ten days after drought stress induction: P. dilatatum was sampled first,
C. dactylon next and Z. japonica last. Five-week old plants of the two former species and nineweek old plants of the slow-growing Z. japonica were analysed simultaneously. Taking into
account the different leaf sizes, each sample of P. dilatatum consisted of two leaves, while each
sample of the other two grasses consisted of five leaves, taken from the same pot. The youngest
fully expanded leaf of each plant of P. dilatatum and two young fully expanded leaves of each
plant of C. dactylon or Z. japonica were always used. It was assumed that, within each pot, all
the young fully expanded leaves were identical in terms of developmental stage, physiological
and biochemical properties, and would have experienced the same drought condition. Therefore,
leaf sub-samples were taken from each pot: the first was quickly frozen in liquid nitrogen (LN2)
and then stored at -80ºC for biochemical assays and the second was used to determine the leaf
relative water content (RWC). The fresh (FW), turgid (TW) and dry (DW) weights were
measured and used to calculate RWC by the equation RWC (%) = 100 × ((FW - DW) / (TW 127
Chapter IV.
DW)) (Catsky 1960). Afterwards, another young fully expanded leaf was taken from each pot for
measurement of the leaf water potential (LWP) using a pressure-chamber (Ritchie & Hinckley,
1975). The soil water content (SWC, %, v/v) was determined in three opposite locations in each
pot using an HH2 moisture meter with a Theta probe (type ML2x, AT Delta-t Devices Ltd.,
Cambridge, UK).
Extraction of C4 enzymes
The frozen leaf samples (0.1 to 0.4 g FW) were ground in a cold mortar with quartz sand, 1%
(w/v) insoluble PVP and 10 (P. dilatatum and C. dactylon) or 15 volumes (Z. japonica) of icecold extraction medium containing 50 mM Bicine-KOH pH 8.0, 1 mM EDTA, 5% (w/v)
PVP25000, 6% (w/v) PEG4000, 10 mM DTT, 50 mM 2-mercaptoethanol and 1% (v/v) protease
inhibitor cocktail (Sigma, St Louis, MO, USA). After grinding to produce a fine suspension,
aliquots were taken for total chlorophyll determination and the remaining homogenate was
centrifuged for 3 min at 14 000g and 4ºC. The supernatant was kept on ice while sub-samples
were taken for measuring each of the enzyme activities at 25ºC in continuous assays monitoring
absorbance at 340 nm (UV-500, Unicam Ltd., Cambridge, UK, with the software Vision 32).
The extraction medium and procedure was optimized in order to achieve the best recovery of the
four enzymes activities and to ensure that they remained stable for 1h. Each value presented is
the mean of at least two replicate measurements using the same leaf extract. The total
chlorophylls content in the leaf homogenates was determined after extraction in 96% ethanol
(Wintermans & de Mots 1965).
PEPC activities and sensitivity to effectors
PEPC activity was measured by coupling the carboxylase reaction with malate dehydrogenase
(MDH), essentially as described by Bakrim et al. (1992). The reaction mixture (1 mL) for the
determination of the maximal activity of the enzyme (Vmax), under optimal pH and substrate
conditions, consisted of 50 mM Hepes-KOH pH 8.0, 10 mM MgCl2, 10 mM NaHCO3, 0.2 mM
NADH (Sigma), 10 U MDH (Sigma), 10 mM PEP (Sigma) and 20 μL of crude extract. The
physiological activity (Vphysiol) was determined under similar conditions but at pH 7.3 and with
2.5 mM instead of 10 mM PEP. The reaction mixtures, with all the components except NADH,
were allowed to equilibrate at 25ºC for 1 min before starting the reaction. The activation state of
PEPC was calculated as the ratio Vphysiol / Vmax × 100.
128
C4 Enzymes in Drought-Stressed Grasses
One sub-sample of the leaf crude extracts was desalted by gel filtration (Sephadex G-25,
Pharmacia Biotech, Uppsala, Sweden, in PD-10 columns), to assess the effects of the inhibitor Lmalate and of the activator glucose-6P on PEPC physiological activity. The assays were
performed in the same conditions as described above for PEPC Vphysiol, using 40 μL of the
desalted leaf extract and measuring the activity in the absence and with increasing concentrations
of L-malate or glucose-6P. The inhibition or activation of the enzyme was assessed by
calculating the ratio of the activity at each concentration in relation to the activity in the absence
of the effector. Inhibition by aspartate was also tested in the leaf extracts of C. dactylon and Z.
japonica.
Activities of C4 acid decarboxylases
PEPCK was assayed in the carboxylation direction by coupling the reaction with malate
dehydrogenase (MDH) according to Walker et al. (2002) with minor modifications. The reaction
mixture (1 mL) for measurement of the enzyme activity under physiological concentrations of
divalent ions contained 100 mM Hepes-KOH pH 7.0, 100 mM KCl, 90 mM KHCO3, 4 mM
MgCl2, 10 μM MnCl2, 1 mM ADP (Sigma), 0.2 mM NADH, 12 U MDH (Sigma), 5 mM PEP
(Sigma) and the reaction was started by the addition of 40 μL of leaf crude extract. Maximal
activity of the enzyme from each of the three species was obtained under the same conditions but
in the presence of 5 mM instead of 4 mM MgCl2 and 2 mM instead of 10 μM MnCl2 and starting
the reaction by the addition of PEP after incubating the enzyme, contained in the leaf crude
extracts, at 25ºC with all the other components for 3 min. The activation state of PEPCK was
calculated as the ratio between the two activities multiplied by 100.
NADP-ME and NAD-ME activities under conditions that approximate the physiological
state were determined using the methods described by Ashton et al. (1990) with minor
modifications. For NADP-ME activity, the reaction mixture (1 mL) contained 50 mM HepesKOH pH 8.0, 10 mM MgCl2, 0.5 mM NADP+ (Sigma), 5 mM L-malate and 40 μL of crude
extract. For NAD-ME activity, the reaction mixture (1 mL) contained 50 mM Hepes-KOH pH
7.2, 4 mM MnCl2, 0.1 mM CoA, 4 mM NAD+ (Sigma), 5 mM L-malate and 40 μL of crude
extract. In both cases, the enzymes contained in the leaf crude extracts were incubated at 25ºC
with all the components except the substrate for 3 min and the reactions were then started by the
addition of L-malate. This procedure was adopted after obtaining greater rates of NADP-ME
activity in P. dilatatum and NAD-ME activity in C. dactylon under these conditions.
129
Chapter IV.
Statistical analysis
All the analyses were made using GenStat® 9.2, 2005 (Lawes Agricultural Trust, Rothamsted
Research, UK). Regression analysis was applied to model the variation of enzyme activities and
activation states with RWC. Non-significantly different (P > 0.05) parameters (t-tests) in the
significant model terms of the regression (F-tests, P < 0.05) were amalgamated in order to attain
parsimony. The resulting best models were plotted and the parameter estimates with their
respective standard errors (s.e.), the percentage of variance accounted for by the model (R2), the
residual mean square (s2) and the degrees of freedom (d.f.) are given with the plots. All the
absolute values and percentages presented in the text were calculated in accordance with the
regression analysis performed. Residual Maximum Likelihood (REML) analysis was used to
verify if there was a significant effect of each effector on the activity of PEPC. The ratio of the
enzyme activity with each concentration of effector in relation to the activity in its absence was
calculated for each sample. The significance of the treatments on this ratio was assessed through
the Wald test (Welham & Thompson 1997). Subsequently, mean values estimated for the control
and drought-stressed plants of each species at the different effector concentrations were
compared using t-tests on the appropriate degrees of freedom from the REML model and the
standard errors of differences (SED) for all possible comparisons. The least significant difference
at the 5% level (LSD(5%)) considering all data for each effector is given as a reference.
130
C4 Enzymes in Drought-Stressed Grasses
RESULTS
Drought stress induction
The soil water content (SWC), in the non-watered pots was four to five times lower than in the
control pots of each species (Table IV.1), resulting in lower values of leaf water potential (LWP)
and relative water content (RWC) in the drought-stressed samples relative to the controls.
Table IV.1. The soil water content (SWC), leaf relative water content (RWC) and leaf water potential
(LWP) of well-watered (C) and drought-stressed (S) plants of P. dilatatum, C. dactylon and Z. japonica.
The mean values and respective standard errors were calculated from measurements taken with three
control and five drought-stressed samples of each species.
Species
P. dilatatum
C. dactylon
Z. japonica
SWC
C
36.7 ± 0.8
36.2 ± 0.9
39.3 ± 0.2
(%)
S
6.4 ± 0.6
8.0 ± 1.0
7.7 ± 1.2
RWC
C
98.1 ± 0.1
98.4 ± 0.2
98.1 ± 0.5
(%)
S
87.1 ± 2.9
93.1 ± 1.5
93.4 ± 1.0
LWP
C
-0.93 ± 0.02
-0.82 ± 0.04
-0.95 ± 0.04
(MPa)
S
-1.21 ± 0.03
-1.16 ± 0.04
-1.65 ± 0.12
PEP carboxylase
PEPC activity was higher in C. dactylon than in the other two species (Figure IV.1). A very
slight, but significant (P ≤ 0.001), increase of Vphysiol was observed with decreasing RWC
whereas Vmax was not significantly affected by leaf dehydration in any of the three species (P >
0.05). PEPC activation state was lower in P. dilatatum (< 40%) than in the other two species (ca.
80-90%). The activation state of the enzyme increased by ca. 16% in P. dilatatum and 6% in C.
dactylon and Z. japonica when RWC decreased to 90%.
PEPC physiological activity in the leaves of P. dilatatum, C. dactylon and Z. japonica
was significantly affected (P < 0.001) by the concentration of L-malate (Figure IV.2). The
enzyme was less inhibited in the non-watered than in the control plants (P < 0.001) and the
extent of inhibition differed among the three species (P = 0.047), being greater in P. dilatatum
131
Chapter IV.
and Z. japonica than in C. dactylon. The inhibition of PEPC Vphysiol by aspartate in the two latter
species was less pronounced than by L-malate, but the enzyme was similarly more inhibited in Z.
japonica than in C. dactylon and the enzyme from control leaves of both species was more
sensitive to the inhibitor than from the dehydrated leaves. The effect of glucose-6P on PEPC
Vphysiol (P < 0.001) was significantly different among the three species (P < 0.001), with greatest
activation in P. dilatatum. The enzyme present in drought-stressed leaves of C. dactylon was
more strongly activated by glucose-6P than in the control leaves, but for the other two species
the activating effect was greater in the fully hydrated leaves. The overall difference between
control and non-watered samples was not significant (P > 0.05).
-1
30
25
-1
PEPC (μmol min mg Chl)
P. dilatatum
20
Z. japonica
A
B
C
y = 10.06 - 0.0553 x (s.e. 8.82; 0.0963)
y = 26.15 - 0.0553 x (s.e. 9.19; 0.0963)
y = 17.12 - 0.0553 x (s.e. 9.20; 0.0963)
D
E
F
y = 85.6 - 0.576 x (s.e. 25.8; 0.282)
y = 140.9 - 0.576 x (s.e. 26.9; 0.282)
y = 140.9 - 0.576 x (s.e. 26.9; 0.282)
15
10
5
0
100
Activation state (%)
C. dactylon
80
60
40
20
0
80
85
90
RWC (%)
95
100
80
85
90
RWC (%)
95
100
80
85
90
95
100
RWC (%)
Figure IV.1. (A-C) PEPC activities (μmol min-1 mg-1 Chl) and (D-F) activation state (%) as a function of
the relative water content (RWC, %) in the leaves of P. dilatatum, C. dactylon and Z. japonica. Activity
was measured under physiological (Vphysiol; closed symbols) and optimal conditions (Vmax; open symbols)
and the activation state (D-F) was calculated as 100×Vphysiol/Vmax. Each data point corresponds to one
sample (with eight samples per species). Regression lines were fitted when the RWC effect was
significant (P < 0.05). Vphysiol, R2 = 91.0%, s2 = 4.325, d.f. = 20; activation state, R2 = 94.7%, s2 = 37.1, d.f.
= 21.
132
C4 Enzymes in Drought-Stressed Grasses
P. dilatatum
PEPC Activity (Ratio)
1.0
C. dactylon
B
A
C
0.8
0.6
0.4
0.2
Malate LSD(5%)
0.0
0.00
4.0
PEPC Activity (Ratio)
Z. japonica
0.25
0.50
0.75
0.00
0.25
Malate LSD(5%)
Malate LSD(5%)
Aspartate LSD(5%)
Aspartate LSD(5%)
0.50
0.75
0.00
0.25
0.50
Inhibitor (mM)
Inhibitor (mM)
Inhibitor (mM)
P. dilatatum
C. dactylon
Z. japonica
E
D
0.75
F
3.0
2.0
1.0
Glucose-6P LSD(5%)
0.0
0.00
1.25
2.50
3.75
Glucose-6P (mM)
Glucose-6P LSD(5%)
0.00
1.25
2.50
3.75
Glucose-6P (mM)
Glucose-6P LSD(5%)
0.00
1.25
2.50
3.75
Glucose-6P (mM)
Figure IV.2. Sensitivity of PEPC physiological activity (Vphysiol; at pH 7.3) to the concentration of the
inhibitor malate (A, B, C) and the activator glucose-6P (D, E, F) in the leaves from control (closed
symbols) and non-watered (open symbols) plants of P. dilatatum, C. dactylon and Z. japonica. The effect
of aspartate was also analysed on the two latter species. The ratio of the enzyme activity at each
concentration of malate (black triangles), aspartate (grey triangles) and glucose-6P (black squares)
relative to the activity in the absence of added effectors (closed circles) was calculated for each sample
and the mean values for each group of plants were analysed by the REML method. The overall least
significant difference at the 5% level (LSD(5%)) is 0.084 for malate (48 d.f.), 0.056 for aspartate (32 d.f.)
and 0.480 for glucose-6P (51 d.f.).
133
Chapter IV.
C4 acid decarboxylases
The activity of PEPCK at optimal concentrations of Mg2+ and Mn2+ was high in all three species
(Figure IV.3), compared with the other C4 acid decarboxylases. Conversely, the activity of the
enzyme in the presence of low but physiological concentrations of the divalent ions was low in P.
dilatatum, with an average activation state of 25.7 ± 2.7%, but high in C. dactylon and Z.
japonica, where the activation state was generally above 80%. PEPCK activity decreased with
leaf dehydration only in C. dactylon and at physiological concentrations of Mg2+ and Mn2+, with
no other significant changes being observed with decreased RWC (P > 0.05). The activation
state of the enzyme was differently affected in C. dactylon and Z. japonica, decreasing by 8.5%
in the former species and increasing by 24% in the latter when RWC decreased to 90%.
10
C. dactylon
A
Z. japonica
B
C
-1
PEPCK (μmol min mg Chl)
P. dilatatum
-1
8
6
4
2
100
Activation state (%)
y = 0.0761 x (s.e. 0.0039)
0
D
E
F
80
60
40
20
y = 0.9664 x (s.e. 0.0285)
0
80
85
90
RWC (%)
95
100
80
85
90
RWC (%)
y = 322.9 - 2.44 x (s.e. 98.9; 1.04)
95
100
80
85
90
95
100
RWC (%)
Figure IV.3. (A-C) PEPCK activities (μmol min-1 mg-1 Chl) and (D-F) activation state (%) as a function
of the relative water content (RWC, %) in the leaves of P. dilatatum, C. dactylon and Z. japonica.
Activity was measured under conditions assumed to represent the closest to the physiological (closed
symbols) and at optimal concentrations of Mg2+ and Mn2+ (open symbols), and the activation state (D-F)
was calculated as the ratio between the two multiplied by 100. Each data point corresponds to one sample
(with eight samples per species). Regression lines were fitted when the RWC effect was significant (P <
0.05). ‘Physiological’, R2 = 82.4%, s2 = 23.67, d.f. = 21; activation state, R2 = 94.5%, s2 = 58.8, d.f. = 20.
134
C4 Enzymes in Drought-Stressed Grasses
In P. dilatatum NADP-ME activity decreased by 8% with decreasing RWC down to 90%
(Figure IV.4). The activity of this enzyme was very low in C. dactylon and Z. japonica (0.50 ±
0.09 μmol min-1 mg-1 Chl) and did not change with leaf dehydration (P > 0.05). On the other
hand, the activity of NAD-ME in fully hydrated leaves of C. dactylon (~3.8 μmol min-1 mg-1
Chl) was higher than in Z. japonica (1.59 ± 0.11 μmol min-1 mg-1 Chl) and very low in P.
dilatatum (0.62 ± 0.11 μmol min-1 mg-1 Chl) (Figure IV.4). In C. dactylon NAD-ME activity
decreased by 8% when RWC decreased to 90%, but no other significant variations with drought
stress were observed for this enzyme.
10
C. dactylon
A
Z. japonica
B
C
E
F
8
6
4
2
0
10
y = 0.0688 x (s.e. 0.0014)
D
-1
NAD-ME (μmol min mg Chl)
-1
-1
NADP-ME (μmol min mg Chl)
P. dilatatum
-1
8
6
4
2
y = 0.0389 x (s.e. 0.0011)
0
80
85
90
RWC (%)
95
100
80
85
90
RWC (%)
95
100
80
85
90
95
100
RWC (%)
Figure IV.4. Activities of (A-C) NADP-ME and (D-F) NAD-ME (μmol min-1 mg-1 Chl) as a function of
the relative water content (RWC, %) in the leaves of P. dilatatum, C. dactylon and Z. japonica. Activities
were measured under conditions assumed to represent the closest to the physiological. Each data point
corresponds to one sample (with eight samples per species). Regression lines were fitted when the RWC
effect was significant (P < 0.05). NADP-ME, R2 = 98.4%, s2 = 0.130, d.f. = 22; NAD-ME, R2 = 95.1%, s2
= 0.089, d.f. = 21.
135
Chapter IV.
DISCUSSION
Effects of leaf dehydration on C4 enzyme activities and regulation
Slowly-induced leaf dehydration had a similar effect on PEPC from P. dilatatum, C. dactylon
and Z. japonica. The physiological activity of the enzyme increased slightly with decreasing
RWC in all three species due to an increase in the activation state of the enzyme (Figure IV.1);
PEPC maximal activity was not affected by leaf dehydration. In Setaria sphacelata both
maximal and physiological activities and the activation state of PEPC increased with slowlyinduced drought stress (Marques da Silva & Arrabaça 2004). Conversely, in Saccharum sp. a
linear decrease of the enzyme activity with decreasing leaf water potential was observed (Du et
al. 1996). In Zea mays PEPC maximal activity was either little (Foyer et al. 1998) or not affected
(Saccardy et al. 1996) by gradually imposed water deficit. When leaf dehydration was rapidly
induced by the addition of polyethylene glycol 4000 to the nutrient solution, PEPC activities
were not affected in P. dilatatum (NADP-ME) and C. dactylon (NAD-ME) and decreased in Z.
japonica (PEPCK) only when RWC was below 70% (Carmo-Silva et al. 2004; 2007). The
different results are attributed to the different methods used for drought stress induction and are
species-dependent (Chaves 1991).
The drought-induced increase in PEPC physiological activity (Figure IV.1) is consistent
with increased phosphorylation of the enzyme as indicated by the decreased sensitivity to the
inhibitor L-malate in all three species (Figure IV.2). Saccardy et al (1996) found no droughtinduced changes on PEPC phosphorylation state, judged by the inhibitory effect of L-malate on
the enzyme from Z. mays, but Foyer et al. (1998) reported a decrease of PEPC sensitivity to the
inhibitor in dehydrated leaves of the same species. In fully hydrated leaves, PEPC from C.
dactylon was relatively less inhibited by L-malate than the enzyme from P. dilatatum and Z.
japonica, suggesting a higher phosphorylation state in the former species (Figure IV.2), whilst
the extent of inhibition of the enzyme from drought-stressed plants was similar for the three C4
grasses. Even though aspartate was a weaker inhibitor of PEPC than L-malate, similar relative
inhibition patterns were observed with the two C4 acids. The activation of PEPC by glucose-6P
was greater in the enzyme from P. dilatatum than from C. dactylon and Z. japonica. In vivo, it is
plausible that changes in the concentrations of metabolite effectors in the dehydrated leaves
might also regulate the activity of the enzyme. Decreases in L-malate and glucose-6P were
previously observed in severely dehydrated leaves of Saccharum sp. (Du et al. 1998).
136
C4 Enzymes in Drought-Stressed Grasses
PEPCK had appreciable activity in all three grass species (Figure IV.3), but the activation
state of the enzyme was much lower in P. dilatatum than in C. dactylon and Z. japonica. The
activity of PEPCK at optimal concentrations of Mg2+ and Mn2+ was not affected by drought
stress and the activity of the enzyme at physiological concentrations of the divalent ions changed
with leaf dehydration only in C. dactylon, decreasing by 8.5% when the RWC decreased to 90%.
The activation state of both PEPC and PEPCK was low in P. dilatatum and high in C. dactylon
and Z. japonica. In the latter species both increased with decreasing RWC, whilst in C. dactylon
an increase of PEPC activation state and a decrease of PEPCK activation state were observed
under the drought conditions attained. An effective coordination between the activation and
phosphorylation states of PEPC and PEPCK was observed in the PEPCK-type species Panicum
maximum (Bailey et al. 2007). In Z. japonica the increase of PEPCK activation state with leaf
dehydration could indicate that the enzyme from this species is also regulated by
phosphorylation/dephosphorylation, as in P. maximum and some other C4 grasses (Walker &
Leegood 1996), and therefore by the same type of covalent modification as PEPC, but activated
by dephosphorylation (Walker et al. 1997; Walker et al. 2002) under drought conditions.
Conversely, in C. dactylon a decrease of PEPCK activation state with decreasing RWC was
observed concomitantly with the increased PEPC activation state, suggesting that the enzyme
from this species might not be susceptible to modulation by phosphorylation but regulated by
other mechanism, possibly involving alterations in metabolite levels or ATP concentration
(Walker et al. 1997; Leegood & Walker 1999).
NADP-ME activity decreased with leaf dehydration only in P. dilatatum (Figure IV.4).
The enzyme had no appreciable activity in C. dactylon and Z. japonica, consistent with the view
that this enzyme has no important role in C4 acid decarboxylation in the BS cells of these species.
An abrupt decrease in the activity of NADP-ME in P. dilatatum was previously observed with
rapidly-induced leaf dehydration (Carmo-Silva et al. 2004), and may result from proteolysis or
down-regulation under stress conditions. NADP-ME activity also decreased in gradually
dehydrated leaves of S. sphacelata (Marques da Silva & Arrabaça 2004) and Saccharum sp. (Du
et al. 1996) but was not affected by water deficit in Z. mays (Saccardy et al. 1996).
The activity of NAD-ME was highest in C. dactylon, the species classified as belonging
to this subtype. The activity decreased slightly with leaf dehydration in C. dactylon (Figure IV.4)
but was not affected by decreasing RWC in the leaves of Z. japonica and had no appreciable
activity in P. dilatatum, where the major decarboxylating enzyme is NADP-ME. A decrease in
NAD-ME activity in C. dactylon, with no change in the enzyme activity in Z. japonica, was also
found when drought conditions were rapidly imposed (Carmo-Silva et al. 2004). The enzyme
137
Chapter IV.
from C. dactylon might be slightly affected by leaf dehydration, either through a direct effect
resulting in degradation or inactivation of the enzyme or through an indirect effect on the levels
of adenylates or on the NADH/NAD+ ratio (Leegood & Walker 1999). The NAD-ME from some
but not all species is activated by ATP, whereas ADP and AMP invariably inhibit the enzyme
(Furbank et al. 1991). Different regulation mechanisms of the enzyme are likely to be present, at
least under drought conditions, in C. dactylon and Z. japonica. NAD-ME activity in any species
with high PEPCK activities must be compatible with the high concentrations of ATP involved.
The regulation of both enzymes is likely to be much dependent on the levels of Mn2+ and ATP
and the allosteric interaction between them (Furbank et al. 1991).
C4 biochemical subtypes of the grass species P. dilatatum, C. dactylon and Z. japonica
In the classical definition of the biochemical subtypes of C4 photosynthesis (Gutierrez et al.
1974; Hatch et al. 1975), species belonging to the NADP-ME and NAD-ME subtypes were
assumed to have low activities of the other two C4 acid decarboxylases. Conversely, in the
PEPCK subtype the enzyme NAD-ME plays an important role, providing the required ATP and
contributing considerably to the release of CO2 in the BS cells (Kanai & Edwards 1999). More
recently, PEPCK was shown to be present in the BS of some NADP-ME type C4 species (Walker
et al. 1997; Voznesenskaya et al. 2006) and to contribute to the decarboxylation of C4 acids in
the BS of Zea mays (Wingler et al. 1999). Measuring simultaneously the activities of the three
C4 acid decarboxylases under physiological conditions (Figures IV.3 and IV.4), we observed the
presence of considerable activities of PEPCK in all three species, with the least in P. dilatatum.
Considerable activities of NAD-ME were observed in C. dactylon and Z. japonica and of
NADP-ME in P. dilatatum. To date, the activity of PEPCK has been found to be very low or
negligible in NAD-ME-type grasses. Edwards et al. (1971) reported very low activity of PEPCK
in C. dactylon, insufficient to account for C4 acid decarboxylation, and Hatch and Kagawa
(1974) and Prendergast et al. (1987) could not detect the presence of the enzyme in this species.
However, Figure IV.3 shows high activities of PEPCK in C. dactylon, well above the activity
found for NAD-ME. Prendergast et al. (1987) suggested that some species might be intermediate
biochemically, showing either predominance of NAD-ME or PEPCK activity. Studies in
progress showed that the appropriate leaf anatomical characteristics associated with each
biochemical subtype are present in P. dilatatum (NADP-ME), C. dactylon (NAD-ME) and Z.
japonica (PEPCK). In particular, the elongated shape and centripetal distribution of the BS
chloroplasts and the evenness of the BS outline in transverse leaf sections of C. dactylon are
consistent with previous descriptions for this species (Prendergast & Hattersley 1987; Watson &
138
C4 Enzymes in Drought-Stressed Grasses
Dallwitz 1992; Dengler et al. 1994) and support its classification as a ‘typical’ NAD-ME-type
species (Dengler & Nelson 1999).
The subtypes NAD-ME and PEPCK have many characteristics in common and it is
plausible that species belonging to the PEPCK subtype evolved from ancestral species with the
NAD-ME C4 photosynthetic pathway (Gutierrez et al. 1974; Hattersley & Watson 1992; Watson
& Dallwitz 1992) retaining considerable levels of decarboxylation in the mitochondria. The
decarboxylation by PEPCK can be viewed as a relatively simple addition to, or variant of, the
NAD-ME pathway (Kellogg 1999). The high activities of PEPCK in P. dilatatum and C.
dactylon, suggest that this enzyme might act as a supplementary decarboxylating enzyme to
NADP-ME and NAD-ME (Kellogg 1999; Lea et al. 2001) but it is also known to have nonphotosynthetic functions in amino acid, organic acid, sugar, lipid and secondary metabolism
(Leegood & Walker 2003).
CONCLUSIONS
The activities of PEP carboxylase and the C4 acid decarboxylases were only slightly changed by
gradually-induced drought stress. Decreased RWC affected the activity and regulation of PEPC
in a similar manner in the three different C4 grass species studied but had different effects on the
C4 acid decarboxylases, NADP-ME, NAD-ME and PEPCK. Appreciable activity of PEPCK was
observed in all three species, suggesting that this enzyme may act as a supplementary
decarboxylase to NADP-ME and NAD-ME in addition to its role in other metabolic pathways.
The faster primary carboxylation by PEPC and the higher potential for C4 acid decarboxylation,
by both NAD-ME and PEPCK, suggest the presence of a highly effective CO2-concentrating
mechanism in the C4 grass C. dactylon.
139
Chapter IV.
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143
Chapter V.
RUBISCO FROM C4 GRASSES UNDER DROUGHT
STRESS
Chapter V.
A manuscript using the data presented in this chapter is in preparation for submission to Plant Cell and
Environment:
Carmo-Silva A.E., Keys A.J., Andralojc P.J., Powers S.J., Arrabaça M.C. & Parry M.A.J. (In Preparation)
Rubisco properties and regulation in three different C4 grasses under drought stress.
Alfred J. Keys (Department of Plant Sciences, Rothamsted Research) was involved in the planning of the
experiments, in the measurements of RuBP and Rubisco inhibitors and in the interpretation of the results.
P. John Andralojc (Department of Plant Sciences, Rothamsted Research) was involved in the
determination of Rubisco kinetic parameters and did the Rubisco specificity factor measurements.
Stephen J. Powers (Department of Biomathematics and Bioinformatics, Rothamsted Research) was
involved in the planning of the experiment and did the statistical analysis of the data.
146
Rubisco from C4 Grasses under Drought Stress
RUBISCO PROPERTIES AND REGULATION IN
DIFFERENT C4 GRASSES UNDER DROUGHT STRESS
THREE
ABSTRACT
Decreased water availability limits photosynthetic performance partly through impaired
metabolism.
Under
adverse
environmental
conditions,
ribulose-1,5-bisphosphate
carboxylase/oxygenase (Rubisco) activity must be modulated to match the amounts of CO2 and
ribulose-1,5-bisphosphate (RuBP) available in the chloroplast stroma. The regulation of Rubisco
was studied in three C4 grasses of different metabolic subtypes, Paspalum dilatatum Poiret
(NADP-ME), Cynodon dactylon (L.) Pers (NAD-ME) and Zoysia japonica Steudel (PEPCK)
exposed to drought stress. Decreased initial and total activities of Rubisco with unchanged
maximal activities or enzyme amounts suggested increased inhibition of the enzyme with leaf
dehydration. Decreased amounts of RuBP and increased amounts of an inhibitor, probably 2carboxyarabinitol-1-phosphate (CA1P), were observed in the leaves from drought-stressed plants.
The tight-binding inhibitor of Rubisco was present especially in the dark in Z. japonica. Rubisco
from each of the three species was partially purified and the kinetic constants for carboxylation
and oxygenation were determined. All three species had Rubiscos with smaller specificity factors,
larger Km values for CO2 (Kc) and O2 (Ko) and larger maximum carboxylation activities (Vc) than
observed for wheat Rubisco. The overall results suggest that Rubisco from the three C4 grasses
has evolved characteristics that optimise photosynthetic efficiency and down-regulation of
Rubisco occurs under drought stress, possibly by the binding of inhibitors that may confer
protection to the enzyme.
KEYWORDS:
Rubisco activities, RuBP amounts, inhibition, kinetic constants, P. dilatatum, C. dactylon, Z. japonica
147
Chapter V.
INTRODUCTION
Drought stress is one of the major constraints to plant productivity. There is an importance and
urgency to understand plant limitations to photosynthetic performance under drought conditions
so that plant productivity can be optimized for conditions of limited water availability that are
predicted to spread through several regions of the planet (Petit et al. 1999). Limitations to
photosynthesis under water deficit include both stomatal and non-stomatal factors (see
comprehensive review by Chaves et al. 2003). In most C3 species, stomatal closure, and
consequently decreased CO2 availability for carboxylation, seems to be the primary factor
limiting net assimilation under moderate drought stress, whilst metabolic impairment becomes
more important as stress severity increases (Flexas & Medrano 2002). Lawlor & Cornic (2002)
recognised two types of plant responses to drought, with metabolic limitations either becoming
important only under severe water deficit or contributing to decreased photosynthesis from the
earliest stages of water deficit. In both cases, impairment of metabolism is assumed to increase
as the leaf relative water content (RWC) decreases. Decreased capacity for RuBP regeneration is
probably a major metabolic limitation to photosynthesis under water deficit (Tezara et al. 1999;
Flexas & Medrano 2002) and may result from decreased ATP and/or NADPH synthesis or from
decreased turnover rates in the Calvin cycle due to impaired enzyme activities (Lawlor 2002).
The tissue content of RuBP in relation to the amount of Rubisco gives an indication of its
importance as a limitation to photosynthesis with progressive drought stress.
The presence of a CO2-concentrating mechanism that effectively limits photorespiration
(see Chapter III, Carmo-Silva et al. 2008) and the higher photosynthetic rates generally
associated with C4 plants are features of their improved water use efficiency compared to their
C3 counterparts (Long 1999) and contribute to a better performance of C4 species in areas with
increased aridity. Studies on the response of C4 species to water deficit (Du et al. 1996; Lal &
Edwards 1996; Ghannoum et al. 2003; Marques da Silva & Arrabaça 2004a; Carmo-Silva et al.
2007) showed that stomatal closure, with consequently decreased CO2 availability at the primary
carboxylation site, and non-stomatal factors, including decreased internal conductance to CO2
and metabolic impairment, may contribute to drought-decreased net photosynthesis. Specific
plant responses vary with the genotype and developmental stage, and depend on the rate,
duration and severity of the imposed stress (Chaves et al. 2003).
Ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) is the main enzyme
ultimately responsible for CO2 assimilation in all plant species and one of the crucial points of
regulation of photosynthesis in fully hydrated leaves of the C4 species Flaveria bidentis (Furbank
148
Rubisco from C4 Grasses under Drought Stress
et al. 1997). The importance of the most abundant leaf protein in the limitation of photosynthesis
under drought conditions is not well understood. Flexas & Medrano (2002) suggested that
Rubisco capacity was of little importance in drought-induced photosynthesis limitation. None the
less, decreased activity and/or amount of Rubisco were observed in many C3 and C4 species
exposed to water deficit (Majumdar et al. 1991; Parry et al. 1993; Du et al. 1996; Parry et al.
2002; Tezara et al. 2002; Bota et al. 2004; Marques da Silva & Arrabaça 2004b; Carmo-Silva et
al. 2007). As recently proposed by Flexas et al. (2006), effects of water deficit on Rubisco may
involve differential regulation of the enzyme during stress rather than a direct effect of leaf
dehydration. The factors behind this stress-induced regulation remain unclear. Decreased
Rubisco capacity may not be the cause of limited CO2 assimilation when water availability is
reduced but its down-regulation is likely to be a determinant of photosynthetic performance
under these conditions and may allow better recoveries upon stress release and better overall
plant productivities in environments where water scarcity is an increasingly important problem.
Rubisco activity is regulated by the extent of carbamylation of a lysyl residue within the
catalytic site (Lorimer & Miziorko 1980), by ATP-dependent Rubisco activase, which facilitate
carbamylation in vivo when the CO2 concentration is sub-optimal (Portis 1992; 2003), and by
interaction with various chloroplast metabolites (Hatch & Jensen 1980; Badger & Lorimer 1981;
Jordan et al. 1983), including some tight-binding inhibitors (Pearce & Andrews 2003; Kim &
Portis 2004). In some, but not all, plant species (Vu et al. 1984; Seemann et al. 1985; Servaites et
al. 1986; Holbrook et al. 1992; Sage & Seemann 1993), Rubisco is inhibited at night by a
specific tight-binding inhibitor, 2-carboxyarabinitol-1-phosphate (CA1P) (Gutteridge et al. 1986;
Berry et al. 1987; Moore et al. 1992). The regulation by carbamylation in the light is readily
investigated by comparing the activity in freshly made extracts, the initial activity, with activity
following incubation of extracts with an excess of CO2 and Mg2+ to fully carbamylate the
enzyme, the total activity. The effects of tight-binding inhibitors can be measured by preincubating the extract with a high concentration of SO42- ions and subsequent desalting, which
removes tight-binding inhibitors, and then fully carbamylating the enzyme before measuring
again its activity, the maximal activity (Parry et al. 1997). The difference between maximal
activity and total activity gives an indication of the tight-binding inhibitors causing regulation in
the light. Knowing the amount of Rubisco in the tissue extracts, the number of catalytic sites
occupied by inhibitors can be calculated. Tight-binding inhibitors can also be measured by the
inhibition of the activity of a known amount of purified Rubisco caused by acid extracts of
leaves (Keys et al. 1995). This can be conveniently related to the number of catalytic sites by
comparison with inhibition caused by CA1P on the same amount of purified Rubisco. The
149
Chapter V.
inhibitor, CA1P, is not ubiquitous through the plant kingdom (Servaites et al. 1986; Sage &
Seemann 1993) but it inhibits Rubisco during the night in certain species and to varying extents.
The presence of inhibitors during the daytime (Keys et al. 1995) became associated with
misfire products of Rubisco catalysis (Pearce & Andrews 2003; Kim & Portis 2004). The main
contender for inhibition in the light (Kane et al. 1998), D-glycero-2,3-diulose-1,5-bisphosphate
(PDBP), is too labile for detailed study and its importance in regulation is not fully established.
Tight-binding inhibitors, such as CA1P, protect the Rubisco from proteolytic breakdown (Khan
et al. 1999) and may play an important role in the regulation of the enzyme under stress
conditions (Parry et al. 2008).
Photorespiration is initiated by the oxygenase activity of Rubisco and decreases both the
rate and efficiency of photosynthesis in C3 plants. Despite the use of genetic manipulation,
selection of many mutants and engineering of the Rubisco protein, no major improvements have
yet been made in decreasing oxygenase relative to carboxylase activity (Keys & Leegood 2002;
Parry et al. 2007). Some C3 plants have evolved Rubiscos that discriminate more strongly
against O2 (Galmés et al. 2005) and, although comparatively small, such changes point the way
to improve Rubisco specificity towards RuBP carboxylation (Parry et al. 2007). Rubiscos from
C4 plants tend to have higher sensitivity to O2 (von Caemmerer 2000) and higher maximal rates
of carboxylation (Sage 2002) than Rubiscos from C3 plants. The CO2-concentrating mechanism
present in C4 photosynthesis is one of the few successful strategies evolved in nature that
decreases oxygenation of RuBP and the proportion of photorespiration to photosynthesis. This is
because CO2 and O2 are competitive alternative substrates and thus competitive inhibitors each
to the reaction of the other (Bowes & Ogren 1972; Laing et al. 1974). The high CO2
concentration in the bundle sheath of C4 plants means that the oxygenase reaction of Rubisco is
much decreased in these plants. The CO2-concentrating mechanism may become less effective
under water deficit so that increased photorespiration would contribute to the limitation of
photosynthesis. From consideration of the kinetics of the carboxylation and oxygenation of
RuBP catalysed by Rubisco, a constant, often referred to as the specificity factor, has been
recognised to describe the relative reaction towards the two substrate gases when present
together (Laing et al. 1974). Thus the specificity factor (SF) is equal to VcKo/VoKc, where Vc and
Vo represent the maximum velocities of the carboxylase and oxygenase reactions respectively,
and Kc and Ko the Michaelis-Menten constants (Km) for CO2 and O2. As a consequence, at
concentrations of CO2 (C) and O2 (O) the relative rates of carboxylation and oxygenation can be
expressed as vc/vo = VcKoC/VoKcO. The individual kinetic constants are not known with accuracy
for most species and they become of increasing interest for mechanistic modelling of the
150
Rubisco from C4 Grasses under Drought Stress
response of photosynthesis to environmental conditions (von Caemmerer & Furbank 1999; von
Caemmerer 2000). Photosynthesis mechanistic modelling can give valuable information in
predicting plant responses, such as the effects of decreasing water availability on photosynthetic
performance (e.g. Carmo-Silva et al. 2008; Chapter III). Currently, the assumptions that need to
be made in order to apply these models compromise the reliability and validity of the predictions
and would largely benefit from a deeper knowledge of C4 Rubisco properties. The Km for CO2
(Kc) and for RuBP (KRuBP) for Rubiscos from several C3 and C4 grasses have been reported
(Yeoh et al. 1980; 1981). Jordan and Ogren (1983) measured specificity factors and Kc values
for Rubisco from several species including C3, C4 and C3-C4 intermediates. More progress is
now being made to establish Rubisco kinetic parameters variation between the C3 and C4
photosynthetic pathways (Kubien et al. 2008), but the enzyme from C4 plants, especially
monocotyledons, needs to be better understood.
The objectives of this study were to investigate the effects of water deficit on the amount,
activities and regulation of Rubisco in three C4 grass species. This involved also a study of the
effects on the contents of RuBP and tight-binding inhibitors. The kinetic properties of Rubisco
present in the leaves of the three species were measured with a view to future mechanistic
modelling of the effects of water deficit on C4 photosynthesis.
151
Chapter V.
MATERIAL AND METHODS
General methods
Rubisco was purified from young wheat leaves essentially as described by Keys & Parry (1990).
RuBP was prepared essentially as described by Wong et al. (1980) and purified by ion exchange
chromatography. CA1P was prepared from RuBP by reaction with cyanide, hydrolysis of the
cyanohydrins,
separation
of
2-carboxyarabinitol-1,5-bisphosphate
(CABP),
partial
dephosphorylation by alkaline phosphatase and purification by ion-exchange chromatography
(Gutteridge et al. 1989). Radioactivity of
14
C labelled compounds was measured by liquid
scintillation spectroscopy, using a liquid scintillation analyser model 2100 TR and Ultima Gold
scintillation cocktail (Perkin-Elmer, Waltman, Massachussets, USA).
Specific mixtures of nitrogen and oxygen were prepared using a gas mixer (Model 820,
Signal Group, UK). Concentrations of O2 in solution were calculated by taking the solubility at
25ºC in water as 257.5 μM in a saturated atmosphere at 100% relative humidity and correcting
for the atmospheric pressure (P) during measurements [257.5 × (P – 11598) / (101325 – 11589),
where 11589 Pa is the saturated vapour pressure of water at 25ºC and 101325 Pa is the saturated
atmospheric pressure]. The concentration of CO2 in solution in equilibrium with HCO3- was
calculated assuming a pKa1 for carbonic acid at 25ºC of 6.11 and an accurate measure of the pH
of the buffer used.
Values of Michaelis-Menten constants and maximum carboxylation velocity were
estimated using the EnzFitter (Biosoft: Software for Science, Cambridge, UK) package for
curve-fitting.
Plant material and drought stress induction
The C4 grasses Paspalum dilatatum Poir. cv. Raki (NADP-ME), Cynodon dactylon (L.) Pers var.
Shangri-Lá (NAD-ME) and Zoysia japonica Steudel ‘Jacklin Sunrise Brand’ (produced by
Jacklin Seed Company, USA) (PEPCK) were grown from seeds in trays or pots using peat-free
compost (Petersfield Products, Leicester, UK) supplemented with a slow-release fertiliser
(Hydro Agri Ltd, Lincs, UK) in a glasshouse. Artificial light was provided whenever the natural
light was below a photosynthetic photon flux density (PPFD) of 500 μmol m-2 s-1 during a 16 h
photoperiod. Temperature was maintained at a minimum of 25ºC during the day and at 18ºC
during the night. Seedlings from the trays were transplanted five to a pot. All pots were well152
Rubisco from C4 Grasses under Drought Stress
watered until the beginning of the drought stress treatment. Pots were then placed according to a
split-plot design, where each column of pots was a main plot of a particular species and the
sampling-days and the treatments (control vs. drought stress) were randomised in the split-plots.
Each pot corresponded to one independent sample, with eight control and twelve non-watered
pots being used per species. For the RuBP/CA1P experiment, the number of control and stress
samples was duplicated and organized in two blocks in order to allow the imposition of a
light/dark regime, by restricted randomization.
Water deficit was imposed on the ‘stress’ pots by ceasing to provide water consecutively,
with one-day intervals, to the plants of C. dactylon, then Z. japonica and at last P. dilatatum. The
‘control’ pots were watered once per day. Samples were taken from all three species
simultaneously for four consecutive days during the drought period. Leaf samples were collected
in the growth environment under fully illuminated conditions four hours after the beginning of
the photoperiod. Because the Rubisco tight-binding inhibitor CA1P is synthesized during the
night, leaf samples for the estimation of RuBP and CA1P were collected early in the morning
both under fully-illuminated conditions and after a period of 12 hours of darkness.
Five-week old plants of P. dilatatum and C. dactylon and nine-week old plants of the
slow-growing Z. japonica were analysed simultaneously. Taking into account the different leaf
sizes, each sample of P. dilatatum consisted of two leaves, while each sample of the other two
grasses consisted of five leaves, taken from the same pot. The youngest fully expanded leaf of
each plant of P. dilatatum and two young fully expanded leaves of each plant of C. dactylon or Z.
japonica were always used. It was assumed that, within each pot, all the young fully expanded
leaves were identical in terms of developmental stage, physiological and biochemical properties,
and would have experienced the same drought condition. Therefore, two leaf sub-samples were
taken from each pot: the first was quickly frozen in liquid nitrogen (LN2) and then stored at 80ºC for biochemical assays and the second was used to determine the leaf relative water content
(RWC). The fresh (FW), turgid (TW) and dry (DW) weights were determined and used to
calculate RWC by the equation RWC (%) = 100 × ((FW - DW) / (TW - DW)) (Catsky 1960).
The soil water content (SWC, %, v/v) was determined in three opposite locations in each pot
using an HH2 moisture meter with a Theta probe (type ML2x, AT Delta-t Devices Ltd.,
Cambridge, UK).
Rubisco extraction and activities
Rubisco was extracted from the leaves by grinding the LN2 frozen samples (0.1 to 0.4 g FW) in a
cold mortar with quartz sand, 1% (w/v) insoluble PVP and 10 (P. dilatatum and C. dactylon) or
153
Chapter V.
15 volumes (Z. japonica) of ice-cold extraction medium containing 50 mM Bicine-KOH pH 8.0,
1 mM EDTA, 5% (w/v) PVP25000, 6% (w/v) PEG4000, 10 mM DTT, 50 mM 2-mercaptoethanol
and 1% (v/v) protease inhibitor cocktail (Sigma, St Louis, MO, USA). After grinding to produce
a fine suspension, aliquots were taken for total chlorophyll determination and the remaining
homogenate was centrifuged for 3 min at 16 000g and 4ºC. The supernatant was used for
measuring the activities and amounts of Rubisco, with two analytical replicates for each
measurement.
The activities of Rubisco in the leaf extracts were determined immediately after
extraction by incorporation of 14CO2 into phosphoglycerate at 25ºC (Parry et al. 1997, with some
modifications). The reaction mixture (0.5 mL) contained 100 mM Bicine-NaOH pH 8.2, 20 mM
MgCl2, 10 mM NaH14CO3 (0.5 μCi μmol-1) and 0.4 mM RuBP. Initial activity was determined
by adding 25 μL of crude extract to the mixture and quenching the reaction after 60 s with 0.2
mL of 10 M formic acid. Total activity was measured after incubating the same volume of
extract for 3 min with all the reaction mixture components except RuBP in order to allow
carbamylation of all the Rubisco available catalytic sites. The reaction was then started by
adding RuBP and stopped as above. Maximal activity was measured after removal of Rubisco
tight binding inhibitors. For this purpose, 250 μL of crude extract were incubated with (final
concentrations) 200 mM Na2SO4, 10 mM NaHCO3 and 20 mM MgCl2 for 30 min at 4ºC.
Rubisco protein was precipitated with 20% polyethylene glycol 4000 (PEG4000) and 20 mM
MgCl2 and the tight binding inhibitors were removed by washing the enzyme three times with
20% PEG4000, 100 mM Bicine-NaOH pH 8.2, 20 mM MgCl2, 10 mM NaHCO3 and 50 mM 2mercaptoethanol. Rubisco free from tight binding inhibitors was re-dissolved in the extraction
buffer before performing the assay as for total activity. The mixtures were completely dried at
100ºC and the residues re-hydrated in 0.4 mL H2O and mixed with 3.6 mL Ultima Gold
scintillation cocktail (Perkin-Elmer, Waltman, Massachussets, USA). Radioactivity due to
14
C
incorporation in the acid-stable products was measured by scintillation counting (liquid
scintillation analyser model 2100TR, Perkin-Elmer).
Chlorophylls
The total chlorophyll content in the leaf homogenates was determined spectrophotometrically
(NOVASPEC 4049 Spectrophotometer, LKB Biochrom Ltd. Cambridge, England) after
extraction in 96% ethanol (Wintermans & de Mots 1965).
154
Rubisco from C4 Grasses under Drought Stress
Rubisco amounts
The amount of Rubisco in the leaf crude or inhibitor-free extracts, used for the determination of
initial and total or for the maximal activities, respectively, was determined by the [14C]-CABP
binding assay (Parry et al. 1997). For this purpose, 100 μL of crude extract were incubated with
(final concentrations) 100 mM Bicine-NaOH pH 8.0, 20 mM MgCl2, 10 mM NaHCO3, 50 mM
2-mercaptoethanol, 100 mM Na2SO4 and 75 μM [14C]-CABP (1 μCi μmol-1) for 15 min at 4ºC.
The Rubisco with bound [14C]-CABP was precipitated with 20% PEG4000 and 20 mM MgCl2.
The precipitate was washed three times with 20% PEG4000, 100 mM Bicine-NaOH pH 8.2, 20
mM MgCl2, 10 mM NaHCO3 and 50 mM 2-mercaptoethanol, and then re-dissolved in 500 μL
1% (v/v) Triton X-100. A sub-sample (450 μL) of this solution was mixed with 3.6 mL Ultima
Gold scintillation cocktail and the radioactivity due to [14C]-CABP bound to Rubisco catalytic
sites was measured by scintillation counting.
RuBP and Rubisco tight-binding inhibitors
RuBP and Rubisco tight-binding inhibitors contained in the LN2 frozen leaves (0.1 to 0.4 g FW)
were extracted by grinding the samples to a fine powder in LN2 and then add 0.45 M
trifluoroacetic acid (TFA; 0.25 mL per 100 mg FW). The mixture was ground further during
thawing and then duplicate sub-samples of the homogenate (20 μl) were taken for chlorophyll
determination. The remaining homogenate was centrifuged for 5 min at 16 000g and 4ºC. For the
estimation of RuBP, 50 μL of the leaf extracts were taken into a glass vial and dehydrated in
high vacuum over CaCl2 and NaOH. The residues were re-dissolved in 50 μL H2O, dried down
again and re-dissolved in 50 μL H2O. The RuBP contained in each vial was converted into
phosphoglycerate by incubating at room temperature (ca. 25ºC) for 45 min in a reaction mixture
(0.5 mL) containing 100 mM Bicine-NaOH pH 8.2, 20 mM MgCl2, 8 mM NaH14CO3 (0.5 μCi
μmol-1) and 20 μg of activated Rubisco. The reaction was quenched with 0.1 mL of 10 M formic
acid.
Leaf extracts were frozen in LN2 prior to CA1P estimation, by reference to a calibration
curve obtained simultaneously with standard solutions containing increasing concentrations of
the inhibitor (in 0.45 M TFA). From each standard or sample, 20 μL were used and incubated for
5 min with 230 μL of (final concentrations) 100 mM Bicine-NaOH pH 8.2, 20 mM MgCl2, 10
mM NaH12CO3 and 10 μg of activated Rubisco. After precisely 5 min, the reactions were started
by adding 250 μL of (final concentrations) 100 mM Bicine-NaOH pH 8.2, 20 mM MgCl2, 10
155
Chapter V.
mM NaH14CO3 (0.5 μCi μmol-1) and 0.4 mM RuBP and quenched after another 2.5 min with 0.1
mL of 10 M formic acid.
The mixtures were completely dried at 100ºC and the cold residues re-hydrated in 0.4 mL
H2O and mixed with 3.6 mL scintillation cocktail. Radioactivity due to 14C incorporation in the
acid-stable products was measured by scintillation counting.
Rubisco kinetic constants
Plants of wheat (C3 species used as reference) and each of the three C4 grasses were grown in
trays for determination of Rubisco kinetic parameters using young and fresh leaf material. Leaf
samples (0.5 g FW) were quickly frozen into LN2 and used within one day. Rubisco was
extracted from the leaves by grinding the LN2 frozen samples in a cold mortar with quartz sand,
2.5% (w/v) insoluble PVP and 5 volumes (2.5 mL) of ice-cold extraction medium containing 100
mM Bicine-KOH pH 8.2, 0.1 mM EDTA, 6% (w/v) PEG4000, 10 mM DTT, 50 mM 2mercaptoethanol, 2 mM MgCl2, 10 mM NaHCO3, 1 mM benzamidine, 1 mM ε-aminocaproic
acid and 1% (v/v) protease inhibitor cocktail (Sigma). After grinding to produce a fine
suspension, the homogenate was centrifuged for 4 min at 16 000g and 4ºC. Low molecular
weight proteins and salts present in the leaf crude extracts were removed by passage of 1 mL of
the supernatant though a Sephadex G-200 (GE Healthcare Life Sciences) column (20 mL bed
volume, 1.5 × 11.5 cm) pre-equilibrated and developed using desalt buffer, containing 100 mM
Bicine-KOH pH 8.2, 0.5 mM EDTA, 1 mM KH2PO4, 20 mM MgCl2, 10 mM NaHCO3, 1 mM
benzamidine and 1 mM ε-aminocaproic acid. Fractions of 0.5 mL were collected and after
measuring the soluble protein content by the method of Bradford (1976), measuring the
absorbance at 595 nm (SpectraMax 340 PC, Molecular Devices Ltd., Workingham, UK, with the
software SoftMax Pro V5), the three protein-richest fractions were combined and supplemented
with 2.5% (v/v) protease inhibitor cocktail (Sigma), carefully mixed and divided into aliquots for
the analysis to be performed. Some of the aliquots were immediately frozen into LN2 (for later
measurement of Rubisco amount and for appropriate control assays).
All measurements for determination of kinetic parameters were conducted at 25ºC. The
Michalis-Menten constant for CO2 (Kc) was measured essentially as described by Bird et al.
(1982) and Ko was estimated by measuring Kc-apparent at several O2 concentrations (0, 21, 60 and
100%, balanced with N2). The carboxylation activity of Rubisco was determined at several
concentrations of CO2 for each gas-mixture. The assay buffer was pre-treated by bubbling
through a steady stream of the appropriate gas-mixture. Septum-sealed vials with stirring
156
Rubisco from C4 Grasses under Drought Stress
magnets were connected in series through the septa using butyl rubber transfer tubes fitted at
each end with hypodermic needles and flushed with the appropriate CO2-free gas-mixtures at 20
μL min-1 for at least 30 min. Flushing of the vials was disconnected after addition of buffer but
before addition of the NaH14CO3. The reaction mixtures (1 mL) in the vials used for each gasmixture contained (final concentrations) 100 mM Bicine-NaOH pH 8.2, 20 mM MgCl2, 10 mg
mL-1 carbonic anhydrase (freshly dissolved), 0.4 mM RuBP and six different concentrations of
NaH14CO3 (0-10 mM; 1 μCi μmol-1). The reactions were started with 30 s intervals by the
addition of 20 μL of leaf extract previously activated by incubation with 10 mM NaH14CO3 (1
μCi μmol-1) for 30 min. The reactions were quenched after 2 min with 0.1 mL of 10 M formic
acid. Changes in the activity of Rubisco through the course of the assays were monitored by the
use of replicates of the same vial at staggered time points. The acidified mixtures were dried at
100ºC and the residues re-hydrated in 0.4 mL H2O and mixed with 3.6 mL scintillation cocktail.
Radioactivity due to 14C incorporation in the acid-stable products was measured by scintillation
counting.
Several control assays were performed in order to assure the presence of
14
C
incorporation in the conditions of the assay at saturating CO2 and low O2 concentrations
(positive control) and the absence of
14
C incorporation in the absence of the substrate RuBP,
when RuBP had been replaced by phosphoglycerate or when partially purified Rubisco had been
pre-incubated with CABP to block the catalytic sites (negative controls). The amount of Rubisco
in the partially purified extracts was determined essentially as described above.
Rubisco specificity factor
Rubisco was purified from fresh leaves of wheat and of each of the three C4 grasses grown in
trays and used to determine the specificity factor by total consumption of RuBP in the oxygen
electrode (Parry et al. 1989). The purification consisted of Rubisco precipitation from the leaf
extracts with 20% PEG4000, step elution from an anion exchange column and desalting (Haslam
et al. 2005).
Statistical analysis
All the analyses were made using GenStat® 9.2, 2005 (Lawes Agricultural Trust, Rothamsted
Research, UK). Regression analysis was applied to model the variation of the different
measurements with RWC. Non-significantly different (P > 0.05) parameters (t-tests) in the
157
Chapter V.
significant model terms of the regression (F-tests, P < 0.05) were amalgamated in order to attain
parsimony. The resulting best models were plotted and the parameter estimates with their
respective standard errors (s.e.), the percentage of variance accounted for by the model (R2), the
residual mean square (s2) and the degrees of freedom (d.f.) are given with the plots. Additionally,
the method of Residual Maximum Likelihood (REML) was used to model the different
measurements and assess the statistical significance of species, treatment (control vs. drought
stress) and light/dark regime (when applicable). The significance of the different effects was
assessed through an F-test (Welham & Thompson 1997) on the appropriate degrees of freedom.
Mean values were estimated for the control and drought-stressed plants of each species. The least
significant differences at the 5% level (LSD(5%)) considering all data for each measurement are
given as a reference. The residuals from the various models generally conformed to the
assumptions of the analysis.
158
Rubisco from C4 Grasses under Drought Stress
RESULTS
Drought stress induction
The leaf relative water content (RWC) decreased in response to decreased water availability in
the soil, as measured by the soil water content (SWC), in the non-watered pots of all three
species (Table V.1). However, RWC was less decreased in the drought-stressed samples
collected in the dark, at the end of the 12 hours night period, than in samples collected four hours
after the beginning of the photoperiod. The results obtained with the plants for measurement of
Rubisco activities and amounts (not shown) were similar to those obtained with the light samples
used for estimation of RuBP and CA1P contents.
Table V.1. The soil water content (SWC) and leaf relative water content (RWC) of control (C) and nonwatered (S) plants of P. dilatatum, C. dactylon and Z. japonica used for the estimation of RuBP and
CA1P contents, in the light or after a period of 12 hours in darkness overnight. The mean values and
respective standard errors were calculated from measurements taken with eight control and twelve
drought-stressed samples of each species.
Water relations
P. dilatatum
C. dactylon
Z. japonica
C
S
34.7 ± 0.6
37.1 ± 0.6
38.4 ± 0.4
7.6 ± 0.8
9.0 ± 0.6
11.2 ± 0.6
C
S
36.1 ± 0.9
38.9 ± 0.4
38.5 ± 0.6
(%)
8.1 ± 0.9
9.5 ± 0.7
11.4 ± 0.8
RWC in the Light
(%)
C
S
98.2 ± 0.1
97.9 ± 0.1
97.4 ± 0.3
86.8 ± 3.8
95.0 ± 0.5
94.1 ± 1.0
RWC in the Dark
(%)
C
S
99.4 ± 0.1
99.3 ± 0.3
98.8 ± 0.1
97.2 ± 0.6
98.4 ± 0.2
97.1 ± 0.3
SWC in the Light
(%)
SWC in the Dark
Rubisco activities and amounts
The initial and total activities of Rubisco decreased with decreased RWC but maximal activities
of the enzyme were not significantly affected (P > 0.05) by leaf dehydration (Figure V.1). The
initial activity was higher in P. dilatatum than in C. dactylon and lowest in Z. japonica but no
significant differences (P > 0.05) were observed in the total or the maximal activities of the
enzyme from P. dilatatum and C. dactylon, which were higher than in Z. japonica.
159
Chapter V.
2.0
C. dactylon
Z. japonica
B
A
C
1.5
-1
-1
Initials (μmol min mg Chl)
P. dilatatum
1.0
0.5
75
y = 0.01067 x (s.e. 0.00045)
3.0
80
85
90
95
100
D
75
80
85
90
95
100
E
75
80
85
90
95
100
F
-1
-1
Totals (μmol min mg Chl)
y = 0.165 + 0.01067 x (s.e. 0.059; 0.00045)
y = 0.440 + 0.01067 x (s.e. 0.060; 0.00045)
0.0
2.0
1.0
y = 0.212 + 0.02120 x (s.e. 0.065; 0.00057)
75
4.0
80
85
90
95
100
G
75
80
85
90
95
100
H
75
80
85
90
95
100
85
90
95
100
I
-1
Maximals (μmol min mg Chl)
y = 0.02120 x (s.e. 0.00057)
y = 0.212 + 0.02120 x (s.e. 0.065; 0.00057)
0.0
-1
3.0
2.0
1.0
2.57 ± 0.06
0.0
75
80
2.57 ± 0.06
85
90
RWC (%)
95
100
75
80
2.17 ± 0.08
85
90
95
100
RWC (%)
75
80
RWC (%)
Figure V.1. Rubisco initial (A-C), total (D-F) and maximal (G-I) activities (μmol min-1 mg-1 Chl) as a
function of the relative water content (RWC, %) in the leaves of P. dilatatum (A, D, G; black diamonds),
C. dactylon (B, E, H; grey squares) and Z. japonica (C, F, I; white triangles). Initials were determined
immediately after extraction, totals were determined after incubation in the presence of CO2 and Mg2+ and
maximals were determined after removal of tight-binding inhibitors with sulphate. Each data point
corresponds to one sample (with seven (P. dilatatum and Z. japonica) or eight (C. dactylon) control and
twelve non-watered samples per species). The regression lines applied correspond to the best models
statistically significant (Initials, R2 = 50.5%, s2 = 0.033, d.f. = 55, P < 0.001; Totals, R2 = 32.5%, s2 =
0.052, d.f. = 56, P < 0.001; Maximals, no regression on RWC, P > 0.05)
160
Rubisco from C4 Grasses under Drought Stress
P. dilatatum
Activation Status (%)
80
A
Z. japonica
B
C
60
40
20
0
40
Inhibition Status (%)
C. dactylon
67.3 ± 1.5
D
51.2 ± 1.0
y = 62.2 - 0.540 x (s.e. 18.7; 0.200)
E
51.2 ± 1.0
y = 62.2 - 0.540 x (s.e. 18.7; 0.200)
F
y = 62.2 - 0.540 x (s.e. 18.7; 0.200)
30
20
10
0
75
80
85
90
RWC (%)
95
100
75
80
85
90
RWC (%)
95
100
75
80
85
90
95
100
RWC (%)
Figure V.2. Activation status (A-C, %) and inhibition status (D-F, %) as a function of the relative water
content (RWC, %) in the leaves of P. dilatatum (A, D; black diamonds), C. dactylon (B, E; grey squares)
and Z. japonica (C, F; white triangles). The activation and inhibition status were calculated as
100×Initial/Total and 100×Total/Maximal, respectively, using specific activities (μmol min-1 mg-1
Rubisco). Each data point corresponds to one sample (with seven (P. dilatatum and Z. japonica) or eight
(C. dactylon) control and twelve non-watered samples per species). The regression line applied
corresponds to the best model statistically significant (Activation status, no regression on RWC, P > 0.05;
Inhibition status, R2 = 10.0%, s2 = 95.9, d.f. = 56; P = 0.009).
Rubisco activation status, calculated as a percentage of the initial to the total activity of
the enzyme, measured immediately upon extraction or after pre-incubation of the enzyme in the
presence of CO2 and Mg2+, respectively, was higher in P. dilatatum than in C. dactylon and Z.
japonica and was not significantly affected (P > 0.05) by leaf dehydration (Figure V.2A-C).
Conversely, the inhibition status, calculated as a percentage of the total to the maximal activity
of the enzyme, measured in the same conditions but with removal of inhibitors tightly-bound to
the enzyme before assaying maximal activity, was not significantly different (P > 0.05) among
the three species and was similarly increased with decreasing RWC (Figure V.2D-F).
161
Chapter V.
The amount of Rubisco was higher in P. dilatatum than in C. dactylon and lowest in Z.
japonica and remained unchanged (P > 0.05) with decreasing RWC (Figure V.3). However,
when the mean values obtained for control and drought-stressed plants were compared there was
a significant effect (P = 0.005) of drought on the Rubisco amount, with decreased contents
compared to the control (Table V.2). As a consequence, Rubisco initial and maximal, but not
total, specific activities were higher in drought-stressed plants relative to the control.
-1
Rubisco catalytic sites (nmol mg Chl)
P. dilatatum
C. dactylon
A
Z. japonica
B
C
30
20
10
0
24.4 ± 0.8
26.6 ± 0.8
75
80
85
90
95
100
75
RWC (%)
80
21.0 ± 0.8
85
90
95
100
75
80
RWC (%)
85
90
95
100
RWC (%)
Figure V.3. Rubisco catalytic sites (nmol mg-1 Chl), as a function of the relative water content (RWC, %)
in the leaves of P. dilatatum (A, black diamonds), C. dactylon (B, grey squares) and Z. japonica (C, white
triangles). Each data point corresponds to one sample (with seven or eight control and twelve nonwatered samples per species). There was no significant regression on RWC (P > 0.05).
Table V.2. Rubisco initial (I), total (T) and maximal (M) specific activities and Rubisco amounts in the
leaves from control and drought-stressed plants. Values are means of seven (P. dilatatum and Z. japonica)
or eight (C. dactylon) control and twelve non-watered samples for each species, as given by the REML
models. The analysis revealed significant effects of species on total (P = 0.010) and maximal activities (P
= 0.001) and amount (P < 0.001) and significant effects of drought stress on initial (P = 0.045) and
maximal activities (P = 0.030) and amount (P = 0.005). There were no other significant effects neither
significant interactions (P > 0.05) between species and stress. The average standard errors of differences
(SED) for each variable considering all data are presented as a reference.
P. dilatatum
Rubisco
Specific activities
(μmol min-1 mg-1
Rubisco)
Amount
(mg mg-1 Chl)
162
I
T
M
C. dactylon
Z. japonica
SED
CONTROL
DROUGHT
CONTROL
DROUGHT
CONTROL
DROUGHT
(49 d.f.)
0.71
0.83
0.66
0.72
0.65
0.71
0.07
1.17
1.18
1.32
1.36
1.36
1.35
0.09
1.30
1.39
1.45
1.60
1.46
1.50
0.07
1.93
1.83
1.83
1.63
1.63
1.38
0.11
Rubisco from C4 Grasses under Drought Stress
RuBP and Rubisco tight-binding inhibitors
The RuBP content was greater in fully hydrated leaves of C. dactylon than in Z. japonica and
lowest in P. dilatatum (Figure V.4A). In all three species a marked decrease with RWC was
observed, but the decrease was much steeper in C. dactylon than in Z. japonica and very slight in
P. dilatatum, resulting in similar RuBP contents for an RWC of ca. 87%. The contents present in
the dark-samples were very low but still denoted a significant, very slight, decrease with RWC
-1
RuBP (nmol mg Chl)
140
120
A
Light
y = 0.3285 x (s.e. 0.0358)
y = -553 + 6.7 x (s.e. 164; 1.7)
y = -277 + 3.5 x (s.e. 100; 1.1)
100
80
60
40
20
2
Dark
y = -0.409 + 0.0064 x
(s.e. 0.064; 0.0005)
y = 0.0064 x (s.e. 0.0005)
1
0
60
70
85
90
95
100
RWC (%)
Figure V.4. (A) RuBP amounts (nmol mg-1
Chl), as a function of the relative water content
(RWC, %) in the leaves of P. dilatatum (black
diamonds), C. dactylon (grey squares) and Z.
japonica (white triangles) collected in the light
and after 12 hours in darkness. Each data point
corresponds to one sample (with seven (P.
dilatatum and Z. japonica) or eight (C.
dactylon) control and twelve non-watered
samples per species per light regime). The
regression lines applied correspond to the best
models statistically significant (P. dilatatum,
solid lines; C. dactylon, dashed lines; Z.
japonica, dotted lines; Light, R2 = 75.6%, s2 =
216.7, d.f. = 54; P < 0.001; Dark, R2 = 40.5%,
s2 = 0.0556, d.f. = 58; P < 0.001). (B) RuBP
80
B
LSD(5%)
Light
Species
Drought
-1
RuBP (nmol mg Chl)
100
60
40
amounts (nmol mg-1 Chl) in the leaves from
control (black bars) and non-watered plants
(white bars) of P. dilatatum, C. dactylon and Z.
japonica collected in the light and after 12
hours in darkness. Values are means of all
control or non-watered samples for each
species and light regime. REML analysis
20
revealed significant differences (P < 0.001)
0
between species and between treatments
(control vs. drought stress) with no significant
P. dilatatum
C. dactylon
Z. japonica
Dark
0.5
0.0
interaction (P > 0.05) in the light, and no
P. dilatatum C. dactylon Z. japonica
significant differences (P > 0.05) in the dark.
The overall least significant difference at the
5% level (LSD(5%)) is 7.823 for species and
6.375 for treatment (104 d.f.).
163
Chapter V.
(P = 0.029). Analysis of the mean values obtained for the control and the drought-stressed
samples revealed lower RuBP contents under drought conditions in the light but no significant
differences (P > 0.05) in the dark (Figure V.4B).
The amount of CA1P present in the leaves collected after a period of 12 hours in
darkness was much greater in Z. japonica than in C. dactylon or P. dilatatum (Figure V.5A) and
increased with decreasing RWC, in a similar extent for the three species (with no significant
interaction (P > 0.05) between species and treatment). Much lower amounts of the nocturnal
inhibitor were found in the leaves collected under fully illuminated conditions (Figure V.5B),
with no significant differences (P > 0.05) between the three species and a very slight but
significant (P = 0.002) increase with decreasing RWC. Comparing the mean values of CA1P
present in the control and drought-stressed plants, no significant differences (P > 0.05) between
species or treatments were observed in the light. The inhibitor amount in the dark-samples was
much greater in Z. japonica than in C. dactylon and lowest in P. dilatatum, and was higher in the
leaves from drought-stressed than from well-watered plants (Figure V.5C).
In a preliminary assay, the total and maximal activities of Rubisco were determined for
one single sample of control or drought-stressed plants of each species collected in a similar
light/dark regime to that used for the CA1P experiment above. The results of the assay (Table
V.3) suggested a much greater difference in the inhibition status (determined as a percentage of
the total to maximal activity) between the dark and light samples in Z. japonica than in P.
dilatatum or C. dactylon. This suggests the presence of the nocturnal inhibitor in Z. japonica and,
additionally, the possibility for increased amounts under drought conditions.
Table V.3. Rubisco inhibition status (%) in leaves sampled after a period of 12 hours in darkness or in
fully illuminated conditions. Results obtained for a preliminary assay using one single sample of control
or non-watered plants of P. dilatatum, C. dactylon and Z. japonica grown and harvested in identical
conditions to those used for CA1P quantification. The inhibition status was calculated as a percentage of
the total to the maximal specific activities (μmol min-1 mg-1 Rubisco).
INHIBITION
(%)
164
P. dilatatum
CONTROL
C. dactylon
DROUGHT
Z. japonica
CONTROL
DROUGHT
CONTROL
DROUGHT
Dark
98
95
89
85
87
73
Light
109
95
94
79
115
113
Light - Dark
11
0
5
-6
28
40
Rubisco from C4 Grasses under Drought Stress
-1
CA1P (nmol mg Chl)
A
10
Dark
y = 36.0 - 0.359 x (s.e. 10.2; 0.104)
y = 36.8 - 0.359 x (s.e. 10.3; 0.104)
y = 40.4 - 0.359 x (s.e. 10.2; 0.104)
8
6
4
2
0
94
-1
Inhibitor (nmol mg Chl)
2.0
96
B
98
100
y = 2.27 - 0.0201 x
(s.e. 0.57; 0.0060)
Light
1.5
1.0
1.138, d.f. = 56; P < 0.001; Light, R2 = 15.0%,
0.5
0.0
60
70
85
90
95
RWC (%)
C
Dark
5
LSD(5%)
Species
-1
CA1P (nmol mg Chl)
6
4
Figure V.5. (A-B) CA1P amounts (nmol mg-1
Chl), as a function of the relative water content
(RWC, %) in the leaves of P. dilatatum (black
diamonds), C. dactylon (grey squares) and Z.
japonica (white triangles) collected after a
period of 12 hours in darkness overnight (A) or
in fully illuminated conditions (B). Each data
point corresponds to one sample (with seven
(P. dilatatum and Z. japonica) or eight (C.
dactylon) control and twelve non-watered
samples per species per light regime). The
regression lines applied correspond to the best
models statistically significant (P. dilatatum,
solid lines; C. dactylon, dashed lines; Z.
japonica, dotted lines; Dark, R2 = 79.1%, s2 =
Drought
3
100
s2 = 0.0805, d.f. = 56; P = 0.002). (C) CA1P
amounts (nmol mg-1 Chl) in the leaves from
control (black bars) and non-watered plants
(white bars) of P. dilatatum, C. dactylon and Z.
japonica collected in the light and after 12
hours in darkness. Values are means of all
control or non-watered samples for each
species and light regime. REML analysis
revealed significant differences (P < 0.001)
between species and between treatments
(control vs. drought stress) with no significant
interaction (P > 0.05) in the dark, and no
2
significant differences (P > 0.05) in the light.
1
The overall least significant difference at the
5% level (LSD(5%)) is 0.525 for species and
0.428 for treatment (103 d.f.).
0
P. dilatatum
C. dactylon
Z. japonica
Light
0.5
0.0
P. dilatatum C. dactylon Z. japonica
165
Chapter V.
Rubisco kinetic parameters
Rubisco purified from each of the three C4 grasses was characterized by lower specificity factors
(SF) than in wheat (C3 species used as reference) (Table V.4). The Michaelis-Menten constants
of Rubisco for CO2 and O2 (Kc and Ko) estimated for each of the C4 species were in the same
range and were higher than the values estimated for wheat. The maximum Rubisco carboxylation
activity (Vc) was also higher in the C4 than in the C3 species and among the grasses was higher in
Z. japonica than in C. dactylon and lowest in P. dilatatum. Conversely, the maximum Rubisco
oxygenation activity (Vo) was not much different between wheat, P. dilatatum and C. dactylon
but was highest in Z. japonica. The replication was reasonably good, with low standard errors
associated with the mean values estimated for each of the kinetic parameters.
Table V.4. Kinetics parameters of Rubisco from wheat and from the three C4 grasses: P. dilatatum, C.
dactylon and Z. japonica, at 25ºC. For the Michaelis-Menten constants of Rubisco for CO2 and O2 (Kc and
Ko, μM) and the maximum Rubisco carboxylation and oxygenation activities (Vc and Vo, μmol min-1 mg-1
Rubisco) the mean values and respective standard errors were calculated from measurements taken with
three biological and three analytical replicates. The specificity factor (SF) was determined in Rubisco
purified from each species using a minimum of five analytical replicates.
Species
166
SF
(VcKo/VoKc)
Kc
(μM)
Ko
Vc
Vo
-1
(μM)
-1
(μmol min mg Rubisco)
Wheat (C3)
100.0 ± 9.2
10.9 ± 0.9
341 ± 33
2.54 ± 0.07
0.79 ± 0.03
P. dilatatum
88.0 ± 7.1
19.9 ± 0.8
415 ±
5
3.11 ± 0.04
0.71 ± 0.03
C. dactylon
89.2 ± 9.0
21.0 ± 1.3
402 ± 27
3.41 ± 0.18
0.73 ± 0.06
Z. japonica
84.1 ± 7.7
18.5 ± 1.2
403 ± 27
3.78 ± 0.08
0.98 ± 0.08
Rubisco from C4 Grasses under Drought Stress
DISCUSSION
Rubisco activities and substrate availability under drought stress
In all three C4 grasses, Rubisco initial activity showed down-regulation under drought stress with
decreasing activity as leaf dehydration increased (Figure V.1). The down-regulation of Rubisco
can be explained by decreased carbamylation or by the presence of competitive and tight-binding
inhibitors. Carbamylation may be impeded by negative effectors (Hatch & Jensen 1980; Badger
& Lorimer 1981; Jordan et al. 1983) or RuBP bound into the non-carbamylated sites (Brooks &
Portis 1988). Total activity was also decreased with leaf dehydration suggesting an increase in
tight-binding inhibitors. In fact, both initial and total activities decreased by ca. 20-25% when
the RWC decreased from control values to 75% in drought-stressed plants. Similar results were
obtained for C. dactylon and Z. japonica in a previous study on the effects of water deficit
rapidly-imposed by the addition of PEG4000 to the nutrient solution (Carmo-Silva et al. 2004;
2007). The discrepancy between the results reported here and the unchanged activities of
Rubisco in P. dilatatum in that previous study, when leaves were rapidly dehydrated, support the
view that plant responses to drought depend greatly on the rate of water deficit imposition
(Chaves et al. 2003; Flexas et al. 2004).
The initial activity of Rubisco was higher in P. dilatatum than in C. dactylon and Z.
japonica, reflecting both the higher activation state and higher amounts of the enzyme in the
former species (Figures V.2 and V.3). The lowest activities of Rubisco were found in Z. japonica
and this is consistent with the lower amounts of Rubisco protein in the leaves of this species
compared to the other two C4 grasses. As a consequence, the specific activities of the enzyme
were either lower in P. dilatatum than in C. dactylon and Z. japonica (total and maximal) or not
different between the three species (initial, due to higher activation state in P. dilatatum). The
maximal activities (Figure V.1) and enzyme amounts (Figure V.3) showed no statistically
significant trend as RWC decreased. However, in the comparison of mean values obtained for all
control with all drought-stressed plants, Rubisco amount was significantly affected (P = 0.005),
decreasing under drought, but the decrease was very slight. Marques da Silva & Arrabaça
(2004b) reported decreased Rubisco initial and total activities with unchanged enzyme amount in
slowly-dehydrated leaves of Setaria sphacelata, whilst Lal & Edwards (1996) found decreased
initial activities concomitant with decreased enzyme amounts in Amaranthus cruentus but not in
Zea mays. Overall, the results obtained for the Rubisco activities and amounts suggest that, in the
167
Chapter V.
three C4 grasses studied, the main effect of drought on the enzyme activity is a slightly increased
inhibition of the enzyme.
The decrease in RuBP content as leaf dehydration increased (Figure V.4) was large in C.
dactylon and Z. japonica but small in P. dilatatum. Whilst Tezara et al. (1999) observed a
dramatic drop in RuBP in the C3 plant, Helianthus annuus and Bota et al. (2004) found
decreased RuBP contents in several C3 species under severe drought conditions, Lal & Edwards
(1996) reported a slight increase of RuBP in Zea mays as water deficit was established. Although
the drought-induced decrease in ATP was less dramatic than the decrease in RuBP in H. annuus,
it has been concluded that decreased ATP synthesis in response to stress conditions caused
decreased RuBP regeneration (Tezara et al. 1999; Lawlor 2002). The mean nmol of catalytic
sites (Et) of Rubisco in P. dilatatum per mg of chlorophyll was 27 (Figure V.3) whilst the
amount of RuBP as nmol per mg chlorophyll (Rt) was only marginally above this at ca. 34 even
in well watered plants (Figure V.4). Even taking into account that only some 60-70% (Figure
V.2) of the catalytic sites were active the ratio Rt/Et is less than 2 and so approaching the range
where RuBP can be considered limiting (Lawlor 2002). The RuBP content in C. dactylon and Z.
japonica was much higher in control plants, giving Rt/Et ratios of ca. 3.5 and 2.9, respectively,
but the values fell steeply as RWC decreased, to give ratios similar to those for P. dilatatum at
85% RWC. These results support the conclusion that the amounts of RuBP in the leaves of all
three species at the lowest values of RWC (Figure V.4) are close to the critical value for RuBPdependent limitation of photosynthesis (Lawlor 2002).
Rubisco regulation by tight-binding inhibitors
Inhibition of Rubisco activity in Figure V.5 is interpreted as due to CA1P but could be in part
due to inhibition by PDBP, especially in the light (Parry et al. 2002). The drought-induced
increase in the amounts of tight-binding inhibitors, estimated by inhibition of purified Rubisco,
agrees fairly well with the increased inhibition of Rubisco from each species, as estimated from
the percentage of total to maximal activities in a preliminary experiment (Table V.3). The
inhibitor in the acid extracts of leaves collected in the dark is most likely to be CA1P, with most
in Z. japonica and least in P. dilatatum, but the identity needs to be confirmed. Sage & Seemann
(1993) have previously reported regulation of Rubisco at low irradiance in Z. japonica that may
have been due to accumulation of CA1P. No indications were then obtained for the presence of
CA1P in P. dilatatum or C. dactylon.
The leaf relative water content is re-established during the night, through decreased
evaporation from the leaves and continued water absorption from the soil. As a result, samples
168
Rubisco from C4 Grasses under Drought Stress
collected from non-watered pots at the end of the 12 h dark period had higher RWC values than
samples collected in the light period (Table V.1), but generally greater Rubisco inhibition.
Therefore, the increased inhibitor content in the non-watered plants (Figure V.5) is more likely
to reflect down-regulation of leaf metabolism under drought conditions than a direct effect of
leaf dehydration. Increased synthesis of tight-binding inhibitors may protect Rubisco from
proteolysis in the stressed leaves (Khan et al. 1999). The extent of inhibition in the dark samples
measured in vitro, compared with the number of Rubisco catalytic sites present in the leaves in
vivo, predicts inhibition to be of the order of 25%, 5% and 2.5% respectively for Z. japonica, C.
dactylon and P. dilatatum (Figures V5.A and V.3) and to increase in all species with drought
stress. A single preliminary assay (Table V.3) confirms a much higher inhibition in dark leaf
samples of Z. japonica (above 25% and increasing with drought) than in P. dilatatum and C.
dactylon (inconclusive), as estimated from the difference between total and maximal activities. It
is most likely that inhibition in these samples is due to CA1P although to the best of our
knowledge the presence of CA1P in Zoysia species has not been unequivocally demonstrated.
There was a slight increase in estimated tight-binding inhibitors in the light with
decreasing RWC but this was mostly caused by the high values obtained for a few samples with
lower RWC values. The predicted amounts based on the CA1P standard were less than 1.5 nmol
per mg chlorophyll (Figure V.5B) and are therefore sufficient to account for less than 6%
inhibition of Rubisco while the inhibition suggested by the difference between total and maximal
activity appears to be larger than this (Figure V.2.B). Rubisco total activity decreased in wheat
and tobacco under drought stress (Parry et al. 1993; 2002) and this was attributed partly to
increased amounts of the daytime inhibitor (PDBP), rather than CA1P (Parry et al. 2002). The
extent of inhibition of Rubisco in vivo by PDBP is very difficult to assess accurately because of
its extreme lability. Varying rates of Rubisco degradation, and turnover, in different species
under normal conditions (Esquível et al. 1998) may be associated with different inhibitor
contents in the leaves, and the presence of these inhibitors may confer competitive advantage
under unfavourable environmental conditions (Parry et al. 2008).
The modulation of Rubisco activity in vivo, both through carbamylation and the binding
of inhibitors, is affected by Rubisco activase, the enzyme that promotes Rubisco activation
(Spreitzer & Salvucci 2002; Portis 2003). Rubisco activase plays a central role in photosynthesis
limitation under heat stress (Crafts-Brandner & Salvucci 2000; Salvucci & Crafts-Brandner
2004) due to its low thermal stability (Salvucci et al. 2001). To the best of our knowledge, the
effects of drought stress on Rubisco activase have not been reported. Due to the ATP
requirement for Rubisco activation through Rubisco activase (Portis et al. 2008), it is likely that
169
Chapter V.
the event of impaired ATP synthesis by water deficit (Lawlor 2002) and the resulting high
ADP/ATP ratio, would decrease the activity of Rubisco activase, thereby affecting Rubisco
activation. The results presented here support the view that in the three C4 grasses studied
Rubisco modulation occurs in order to match the capacity for RuBP regeneration with the
capacity for RuBP utilization. This modulation is essentially due to Rubisco inhibition as the
enzyme amount is not affected and the activation status remains unchanged.
Kinetic properties of Rubisco from the C4 grasses
The data presented in Table V.4 support the view that Rubisco from C4 plants is less specific for
CO2 but has the potential for faster rates of carboxylation than the enzyme from C3 species (von
Caemmerer 2000; Kubien et al. 2008). Using appropriate controls, it was possible to estimate Kc,
Ko and Vc using partially purified extracts of Rubisco from the leaves of wheat and each of the
three C4 grasses. Attempts were made to measure Vo by an HPLC method based on that of
Yaguchi et al. (1996) to measure the amounts of 3-phosphoglycerate and 2-phosphoglycolate
produced by the oxygenation of RuBP at increasing concentrations of O2, using similar enzyme
extracts. However, these attempts were unsuccessful. As a consequence, Vo was estimated
indirectly by measuring the specificity factor and solving the equation SF = VcKo/VoKc. The
errors on the estimates of SF were too high to allow much precision to be credited to the
estimates of Vo. The Ko of Rubisco is the kinetic parameter for which there is the greatest
variation reported in the literature (see von Caemmerer 2000), with estimates generally in the
range of 200-650 μM being obtained for C3 species. The values obtained in the present study for
the three C4 grasses (ca. 400-420 μM) were higher than the value obtained for wheat, supporting
the conclusion that Rubisco from C4 species may have higher Ko, as well as higher Kc, than
Rubisco from C3 species (von Caemmerer 2000). In view of the effects of drought on the
amounts of RuBP in the leaves, a measure of Km for RuBP (KRuBP) would be a necessary
addition to the kinetic constants to be considered in future studies. Values for KRuBP for many C4
plants have been published (Yeoh et al. 1981) showing a wide range, which suggests that
measurements are needed for any species where the objective is the mechanistic modelling of
photosynthesis.
170
Rubisco from C4 Grasses under Drought Stress
CONCLUSIONS
Rubisco activity was down-regulated in all three C4 grasses and the extent of down-regulation
was increased under drought stress conditions. A decreased capacity for RuBP regeneration with
drought is likely to be a major factor controlling the photosynthetic rate, at least in C. dactylon
and Z. japonica. Evidence was found for the presence of tight-binding inhibitors in the leaves of
the C4 grasses analysed, but the values measured were insufficient to account for the difference
between total and maximal activities of Rubisco in plants in the light. The nature of the inhibitors
that play a role in the inhibition of Rubisco in the light in the three species needs further
investigation. In Z. japonica there was strong evidence for a tight-binding inhibitor in the dark,
possibly CA1P. The kinetic properties of the Rubiscos from the three C4 grasses show a lower
affinity for CO2 and a higher Vc than the enzyme from C3 plants. These characteristics fit the
enzymes for high carboxylation capacity in the environment of the bundle sheath with its high
CO2 concentration.
171
Chapter V.
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Chapter VI.
GENERAL DISCUSSION AND CONCLUSIONS
General Discussion and Conclusions
GENERAL DISCUSSION AND CONCLUSIONS
Drought resistance in C4 grasses
The use of plant species and varieties better adapted to conditions of low water availability and
the production of new varieties with improved performance by traditional or molecular breeding
will improve the use of water available. The C4 grasses Paspalum dilatatum, Cynodon dactylon
and Zoysia japonica and their adaptive response to water deficit were studied in order to
understand the mechanisms underlaying their photosynthetic performance under conditions of
decreased water availability. The three species showed some differences in their drought-stress
physiology and biochemistry but also some responses to gradually induced drought stress
conditions that are common to many plant species (see comprehensive review by Chaves et al.
2003). The decreased shoot growth and photosynthesis are an adaptive response to decreased
water availability and allow energy to be diverted to the production of molecules involved in
stress defence mechanisms (Chaves & Oliveira 2004). Plants adopt diverse strategies to maintain
water status under water deficit (Schulze 1986). In all three C4 grasses, stomatal closure was an
early event in the response to water deficit and decreased stomatal conductance contributed to
the maintenance of control RWC values in the earlier stages of water deficit. Decreased LWP
also contributed to minimize water loss. Additionally, in Z. japonica more severe dehydration is
impeded by leaf curling simultaneously with continuous lowering of the LWP, which increases
the capacity for water absorption from the soil. Deep root systems have been observed in C.
dactylon and Z. japonica (Marcum et al. 1995; Carrow 1996) and we may suggest that increased
root/shoot ratio through sustained root growth concomitantly with decreased shoot growth rates
may help to retrieve more water from the soil. Decreased shoot growth and stomatal conductance
are among the primary effects of decreased water availability in the soil that commonly occur in
many plant species (Chaves et al. 2003) and were observed in the three C4 grasses.
The leaf structure of Z. japonica and the response of this species to water deficit in terms
of leaf water relations (Chapter II) together with the presence of rigid cells walls in association
with its drought resistance (White et al. 2001) support the view that this species is well adapted
to xeric environments. The faster decrease in water availability in the soil observed with P.
dilatatum reflects the high productivity of these plants at the expense of higher water
consumption. This is also a consequence of the different phenotype, with bigger and fewer
leaves, and the lower ratio of dry weight to turgid weight than in the other two C4 grasses.
181
Chapter VI.
The lower drought resistance of P. dilatatum (NADP-ME, subfamily Panicoideae)
compared to C. dactylon (NAD-ME, subfamily Chloridoideae) and Z. japonica (PEPCK,
subfamily Chloridoideae) agree to some extent with several reports on C4 grass distributions
suggesting that dominance of NADP-ME species in comparison to the other C4 subtypes is
positively correlated with precipitation gradients (see Cabido et al. 2008). It has been suggested
that the C4 subtype of photosynthetic pathway may be relevant for the ecology and management
of desertified grasslands, with persistent and drought resistant NAD-ME grasses potentially more
fitted to arid regions (Hattersley 1992). However, the lower resistance of P. dilatatum is also in
agreement with the high correlation of the grass subfamily Panicoideae distribution with higher
precipitation levels compared to the subfamily Chloridoideae (Taub 2000). In C3- and C4-like
subspecies of Alloteropsis semialata (Ueno & Sentoku 2006) responses to drought were
contradictory to the paradigm of higher drought resistance in C4 relative to C3 species, with C4like individuals coping poorly with decreased water availability compared to the C3-like
individuals (Ripley et al. 2007). Therefore, the intrinsic characteristics of each species determine
its resistance to drought more than the photosynthetic pathway or taxonomic background.
The accumulation of Proline (Chapter II) may be involved in plant resistance to drought
conditions. Besides the possible roles of this amino acid in osmoregulation and in sustaining
homeostasis, it has been suggested to act as an energy storage compound (Hare & Cress 1997).
The synthesis of Proline involves the consumption of NAD(P)H and the degradation of the
amino acid upon stress release supplies energy to re-establish growth and allow plants to recover
from the harmful effects. Increased content of other amino acids, like methionine and
phenylalanine, with leaf dehydration may be related to increased synthesis of secondary
metabolites with a role in plant stress defence mechanisms. Additionally, in C. dactylon and Z.
japonica an unusual amino acid, 5-hydroxy-L-norvaline (HNV) was induced by water deficit. As
far as we are aware, this amino acid has not been previously reported to be present in plant
leaves and its biosynthetic pathway in plants is unclear. The presence of HNV in well watered
plants of Z. japonica and its increased content in this species and in C. dactylon with water
deficit suggest that this amino acid may have an important role in C4 grass resistance to drought.
The properties of this amino acid suggest its possible involvement in plant defence mechanisms
that may be triggered under abiotic stress. This hypothesis needs further investigation and may
prove beneficial for plant engineering towards increased drought resistance.
182
General Discussion and Conclusions
Rapid vs. slow water deficit
In the present work, drought stress was slowly induced by ceasing to provide water to the plants
of the three C4 grass species (Chapter II). As a consequence, water availability in the soil
decreased gradually through the experiment, affecting plant growth, leaf water relations, leaf
gas-exchanges and photosynthetic metabolism. Several differences were identified in the
different aspects of plant responses to water deficit in comparison with a previous study, where
leaf dehydration was imposed rapidly, by the addition of polyethylene glycol 4000 (PEG4000) to
the nutrient solution used to water the plants (Carmo-Silva et al. 2004; 2007). It is known that
plant responses can be markedly different depending on the rate of leaf dehydration (Chaves et al.
2003; Flexas et al. 2004).
The most striking difference observed in the three C4 grasses studied was the dramatic
decrease of RWC in C. dactylon in response to the artificial PEG-induced water deficit (CarmoSilva et al. 2007). The addition of solutes to the nutrient solution allows the decrease of its water
potential and consequent decrease in water availability to be taken up by the plant. Due to its
high molecular weight, PEG will not penetrate the root system and its use as an osmolite for
water deficit induction has been validated by several authors (Zhang et al. 2001; Ober & Sharp
2003). Moreover, PEG-induced water deficit did not cause dramatic effects on photosynthesis,
stomatal conductance, photochemistry or enzyme activities in C. dactylon (Carmo-Silva et al.
2004; 2007), suggesting that a toxicity effect of the osmolyte on the plants of this species should
be disregarded. The lack of a faster response in terms of stomatal closure may be the explanation
for the dramatic drop in RWC of C. dactylon observed under rapidly-induced water deficit. As
referred above, the present results suggest that decreased stomatal conductance may be the major
factor controlling water loss in order to minimize leaf dehydration in C. dactylon.
The leaf relative water content (RWC) was not much decreased in the samples taken for
biochemical analysis. This was partly due to collection early in the day. Tissue water status
recovered during the night because of stomatal closure and hence decreased evaporation from the
leaves but continued water absorption from the soil. Because samples were taken four hours after
the beginning of the photoperiod, assuming that steady-state photosynthesis would then have
been attained and plant responses would reflect differences in water availability, the RWC was
not as much decreased as if it would have been in the middle of the day. In fact, the drought
samples taken at the end of a 12-hour-nocturnal period (Chapter V) had RWC values that were
not as much decreased as those taken from plants that had been exposed to four hours in the light
(Table V.1). None the less, the samples taken from non-watered pots, eight to twelve days after
183
Chapter VI.
stress imposition, were effectively experiencing stress conditions, as revealed for instance by the
decreased leaf water potentials (Figure II.2).
The photosynthetic responses observed under rapidly-induced water deficit in the three
C4 grasses are likely to result from direct effects of decreased water content in the leaves whilst
the responses observed under slowly-induced water deficit include the effects of adaptive
strategies adopted by the plants to cope with decreased water availability in the soil. The
disparity of results between the two water deficit induction systems adopted provides evidence
for the need to promote conditions more accurately representing those observed under natural
conditions. The gradual decrease in water availability in the soil promoted by ceasing to provide
water to the plants of the three species is therefore a more useful method to understand the
drought stress physiology and biochemistry of these grass species and provide information to the
exploitation of their capacities under water-limited conditions.
Stomatal and metabolic limitations to photosynthesis in C4 grasses
Net CO2 assimilation rates under water deficit may be limited by stomatal closure, metabolic
factors or a combination of both. The relative importance and the timing of the different effects
caused by water deficit that will have a negative impact on photosynthesis have been the target
of much debate some years ago (Flexas & Medrano 2002; Lawlor & Cornic 2002). In P.
dilatatum, C. dactylon and Z. japonica decreased water availability in the soil caused decreased
net CO2 assimilation rate and decreased stomatal conductance to CO2 (Figure III.2). The faster
decrease observed in stomatal conductance than in photosynthesis reveals that stomatal closure is
effectively induced by water deficit, minimizing water loss and slightly enhancing water use
efficiency. Decreased photosynthesis in several C4 grasses has previously been attributed to
stomatal closure (Du et al. 1996; Lal & Edwards 1996; Marques da Silva & Arrabaça 2004;
Carmo-Silva et al. 2007).
The decrease of the maximal rate of net CO2 assimilation attained at saturating CO2
concentrations in moderately and severely-stressed plants suggests that metabolic limitations to
photosynthesis are also present in the three C4 grasses. Moreover, these are likely to become
more important with increased severity of drought stress. An increased relative importance of
metabolic to stomatal limitations to photosynthesis was observed in other C4 grasses (Du et al.
1996; Marques da Silva & Arrabaça 2004) and suggests down-regulation of the photosynthetic
metabolism when CO2 availability is diminished (Chaves et al. 2003; Flexas et al. 2004).
Decreased capacity for RuBP regeneration, possibly as a result of impaired ATP
synthesis, has been considered as one of the major metabolic limitations to photosynthesis in C3
184
General Discussion and Conclusions
plants (Tezara et al. 1999; Flexas & Medrano 2002). An effective decrease of RuBP content was
observed with water deficit in the leaves of the three C4 grasses (Figure V.4), suggesting limited
capacity for the regeneration of this substrate, possibly as a consequence of decreased energy
levels. Concomitantly, decreased Rubisco initial and total activities were also observed and were
associated with increased contents of Rubisco tight-binding inhibitors in the drought-stressed
plants (Chapter V). The increased inhibition of Rubisco supports the hypothesis of ATP being a
limiting factor in the leaves of plants exposed to drought stress. The ATP-dependent Rubisco
activase facilitates the activation of Rubisco both by promoting the carbamylation of the enzyme
and by removing the tight-binding inhibitors (Portis 2003). Thus, if ATP decreases, Rubisco
activase will be less active and more inhibitors may remain bound to Rubisco. The binding of
inhibitors to Rubisco may protect the enzyme against proteolytic breakdown as may occur under
stress conditions (Khan et al. 1999).
The activation state of Rubisco was not affected by water deficit, suggesting that the CO2
concentration at the Rubisco site is not decreased to an extent that will affect substantially the
enzyme carbamylation. The CO2 concentration available for carboxylation by PEPC will be
decreased due to decreased stomatal conductance. However, the physiological activity of PEPC
increased slightly with water deficit, as a result of an increased phosphorylation state of the
enzyme (Figures IV.1 and IV.2). The increased PEPC activity in the mesophyll may be a
strategy to maximize the primary fixation of the less abundant CO2 in the mesophyll.
Accordingly, the activities of the C4 acid decarboxylases in each species were always
considerably higher (at least 4-fold) than Rubisco activity (Figures IV.3, IV.4 and V.1). Thus,
Rubisco is supplied with CO2 concentrations high enough to avoid a considerable increase in
photorespiration under water deficit (Chapter III).
Taken together, the results present here support the view that C4 photosynthesis in the
three grasses studied is primarily limited by decreased CO2 availability resulting from stomatal
closure. Biochemical adjustments are also observed. Decreased ATP synthesis may occur and
contribute to limit photosynthesis, but this hypothesis needs further investigation. RuBP
regeneration is considerably affected and the substrate becomes limiting for CO2 assimilation.
The photosynthetic metabolism is down-regulated in order to avoid severe impairment to occur
in response to the unfavourable conditions. The general effects of water deficit observed on the
photosynthetic metabolism involving the C4 and the C3 cycles may be summarized as:
→ increased PEPC physiological activity (higher activation state),
→ decreased Rubisco initial and total activity (increased enzyme inhibition),
→ and decreased RuBP contents.
185
Chapter VI.
Pyruvate,orthophosphate dikinase (PPdK), the enzyme responsible for the production of
PEP from pyruvate, is one of the control points of C4 photosynthesis (Furbank et al. 1997) and
was reported to limit photosynthesis in drought-stressed leaves of Saccharum sp. (Du et al.
1996). Therefore, both the activity of PPdK and the PEP content should be further investigated
together with determination of the ATP content under drought stress conditions. These studies
should provide the necessary additional information to allow a better understanding of the
drought-stress response of the C4 photosynthetic pathway in these three grasses.
Photorespiration and photosynthetic performance of the C4 pathway
The CO2-concentrating mechanism provides C4 plants with a more efficient photosynthetic
pathway, especially at high irradiance and temperatures. The oxygenase activity of Rubisco is
limited due to the high CO2 environment in the BS cells, resulting in higher CO2 assimilation
rates, which are associated with increased water use efficiencies (Edwards et al. 1985; Long
1999). Stomatal closure is one of the earliest and most common responses to water deficit and
results in decreased water loss at the expense of decreased CO2 availability for assimilation.
Under these conditions, photorespiration may increase and decrease the efficiency of C4
photosynthesis.
Photorespiration consumes ATP and reducing power and has been suggested as an
efficient process conferring protection under stress conditions against imbalanced redox
potentials by maintaining electron flow (Keys & Leegood 2002). The limitation of CO2
assimilation results in lower energy requirements for the Calvin cycle and this has been
suggested in the past to result in photoinhibition. However, most studies reveal that
photoinhibition is an unlikely event that seems to occur only in some plant species and under
very severe drought stress conditions (see Flexas & Medrano 2002). Photorespiration remained
slow under drought conditions, increasing only slightly in proportion to photosynthesis in the
three C4 grass species (Chapter III).
The photosynthetic metabolism of all three C4 grasses seems to adjust to the
environmental conditions in order to maximize efficiency and minimize damage under
unfavourable conditions. Cynodon dactylon tends to present higher photosynthetic rates of CO2
assimilation (Chapter III) and higher activities of the carboxylating and decarboxylating enzymes
(Chapter IV), suggesting that this species may have a more efficient photosynthetic performance.
Additionally, the highest content of RuBP (Chapter V) agrees with the presence of a highly
effective photosynthetic metabolism in C. dactylon. The hypothesis that metabolic impairment
may occur under severe drought stress conditions can not be disregarded, but the data here
186
General Discussion and Conclusions
presented suggest that photosynthesis slows down but considerable photosynthetic activity is
maintained under moderate water deficit.
The three biochemical subtypes of C4 photosynthesis in the grasses studied
The three ‘classical’ mechanisms of C4 photosynthesis (Hatch 1987) occur among the Poaceae
(Hattersley & Watson 1992) and species may be classified into one of these biochemical
subtypes by the relative activities of the three C4 acid decarboxylases: NADP-ME, NAD-ME and
PEPCK (Hattersley 1988). Species belonging to the ‘classical’ NADP-ME and NAD-ME
biochemical subtypes (Gutierrez et al. 1974; Hatch et al. 1975) have low activities of the other
two C4 acid decarboxylases. Conversely, in the PEPCK subtype the enzyme NAD-ME
contributes considerably to the release of CO2 in the BS cells and provides the required ATP for
PEPCK activity (Kanai & Edwards 1999). By measuring simultaneously, in the same leaf
extracts, the activities of the three C4 acid decarboxylases under conditions that were assumed to
be close to the physiological state, we observed the presence of PEPCK activity in all three
species, whereas NAD-ME activity was present in C. dactylon and Z. japonica but was very low
in P. dilatatum, and NADP-ME was present in P. dilatatum but was very low in the other two
grasses (Figures IV.3 and IV.4).
The high activity of NADP-ME found in P. dilatatum agrees with previous description of
this species as belonging to this biochemical subtype (Usuda et al. 1984). Accordingly, the
activity of NAD-ME was very low in the leaves of this species but, curiously, PEPCK had
considerable activity at physiological concentrations and high activity at optimal concentrations
of Mg2+ and Mn2+ (ca. 2.3 and 8.9 μmol min-1 mg-1 Chl, respectively). PEPCK has been
previously shown to be present in some NADP-ME type C4 species, such as Zea mays and
Paspalum notatum (Walker et al. 1997). Subsequent studies in Zea mays revealed that PEPCK
decarboxylates oxaloacetate derived from aspartate whilst NADP-ME is responsible for the
decarboxylation of malate, with both enzymes contributing to the release of CO2 in the BS cells
of this species (Wingler et al. 1999).
High values for the activity (generally ≥ 6 μmol min-1 mg-1 Chl) and the activation state
(generally ≥ 80%) of PEPCK were observed in both C. dactylon and Z. japonica (Figure IV.3).
Considerable activities of NAD-ME were also present in the leaves of the two species (Figure
IV.4), but this enzyme was not as active as PEPCK. These results agree with the typing of Z.
japonica as a PEPCK species (Gutierrez et al. 1974), retaining considerable decarboxylation of
malate in the BS mitochondria by NAD-ME. However, questions arise on what concerns the
187
Chapter VI.
previous characterization of C. dactylon as a NAD-ME species (Hatch & Kagawa 1974). As
referred above, the presence of PEPCK was previously shown in several NADP-ME type
monocots (Walker et al. 1997; Wingler et al. 1999; Voznesenskaya et al. 2006). However, to the
best of our knowledge, this is the first time the presence of high activities of PEPCK has been
found in a NAD-ME-type grass species, C. dactylon.
Edwards and co-workers (1971) were first to report the presence of high PEPCK levels in
several C4 grasses. In that study, very low levels of PEPCK, insufficient to account for C4 acid
decarboxylation, were found in C. dactylon. Subsequently, Hatch & Kagawa (1974) found high
NAD-ME activities in several species, including C. dactylon, and were only able to detect the
presence of PEPCK in species with significantly lower NAD-ME activity. These results led to
the typing of C. dactylon as a NAD-ME species. However, the value obtained for the activity of
this enzyme in C. dactylon (1.4 μmol min-1 mg-1 Chl) was less than half the activity present in
the other species classified as belonging to the same subtype (≥ 3 μmol min-1 mg-1 Chl).
Prendergast et al. (1987) have also reported the absence of PEPCK activity and the presence of
high activities of NAD-ME in this grass species (2.0-3.3 μmol min-1 mg-1 Chl, depending on
plant growth conditions). High activity of NAD-ME in C. dactylon was similarly found in the
present work (≥ 3 μmol min-1 mg-1 Chl), but PEPCK was more active in the same leaves (≥ 6
μmol min-1 mg-1 Chl). C4 grasses with relative activities of the two enzymes as close as 2 NADME to 3 PEPCK have been classified as belonging to the PEPCK subtype (Gutierrez et al. 1974).
Prendergast et al. (1987) suggested that some species might be intermediate biochemically,
showing either predominance of NAD-ME or PEPCK activity. The same authors found masked
activity of PEPCK in the C4 grass Tridens brasiliensis, suggesting the presence of some
compound in the leaves of this species that interfered with PEPCK detection and measurement
by the methods then used. Therefore, special care is taken to introduce the most suitable
components to the extraction medium to maximize the extraction and recovery of the enzyme
activity. The methods adopted were similar to those recently reported by other authors (Bailey et
al. 2007; Marshall et al. 2007; Sudderth et al. 2007) and the values obtained for the activities of
the three C4 acid decarboxylases are in the same range as previously reported data for grass
species (Gutierrez et al. 1974; Hatch et al. 1975; Hatch et al. 1982; Ueno & Sentoku 2006).
The high capacity of PEPCK found in all three C4 grasses, previously reported as
belonging to each of the different biochemical subtypes, suggests the possible role of the enzyme
as a secondary decarboxylase to NADP-ME and NAD-ME (Kellogg 1999; Lea et al. 2001).
However, the hypothesis that the high activity of PEPCK may be related to other aspects of plant
metabolism must not be disregarded. Leegood & Walker (2003) described roles in the cytosol
188
General Discussion and Conclusions
metabolism and in the interface between diverse metabolic pathways in the leaves, such as those
involving amino acids, organic acids, sugars, lipids and some secondary metabolites. Further
investigation is therefore required to clarify the functional significance of the high PEPCK
activities found in the three C4 grass species.
A set of anatomical characteristics is generally associated with the biochemical
mechanism present in a particular species. One of the distinctive characteristics among the three
‘classical’ subtypes is the position of the BS chloroplasts: centrifugal in NADP-ME, centripetal
in NAD-ME and centrifugal or even in PEPCK (Dengler & Nelson 1999). Optical microscopy of
transverse leaf sections stained with Toluidine Blue O revealed the presence of centrifugal or
scattered chloroplasts in the BS cells of P. dilatatum and Z. japonica and centripetal chloroplasts
in the BS cells of C. dactylon (Figure II.5), supporting previous descriptions for the species
(Prendergast & Hattersley 1987; Watson & Dallwitz 1992; Dengler et al. 1994). Moreover, the
elongated shape of the chloroplasts and the evenness of the BS outline in cross section in C.
dactylon support the anatomical classification of this species as a ‘typical’ NAD-ME species
(Dengler et al. 1994; Dengler & Nelson 1999). Exceptions to the characteristics associated with
the different anatomical-biochemical subtypes may occur. As proposed by Gutierrez et al. (1974)
a possible evolutionary pattern in C4 grasses could be from NAD-ME species with centripetal
chloroplasts to PEPCK species, with even or centrifugal chloroplasts and some activity of NADME being retained, to NADP-ME species, with centrifugal chloroplasts (to optimise the
exchange of metabolites between M and BS cells through plasmodesmata) and varying degrees
of grana reduction in the BS chloroplast. However, the three C4 subtypes do not seem to
represent a sequence of progressive biochemical modifications (Hattersley 1988). None the less,
the ‘classical’ subtypes NAD-ME and PEPCK have many characteristics in common in what
concerns their leaf anatomy and it is plausible to consider that species belonging to the PEPCK
subtype evolved from ancestral species with the NAD-ME C4 photosynthetic pathway
(Hattersley & Watson 1992; Watson & Dallwitz 1992) retaining considerable levels of
decarboxylation in the mitochondria. As proposed by Kellogg (1999), the decarboxylation by
PEPCK can be viewed as a relatively simple addition to, or variant of, the NAD-ME pathway.
The carbon isotope composition of leaf dry matter shown in Figure III.4 was different for
each of the three grass species. The anatomical variations among the C4 subtypes, and especially
the presence or absence of a suberin lamella in the BS cells, may be associated with their
conductance properties. The carbon isotope composition tends to be more negative in NAD-ME
than in NADP-ME grass species, with PEPCK species showing intermediate values (Hattersley
1982). This is thought to reflect a faster rate of leakage of CO2 from the BS in the former species,
189
Chapter VI.
which lack a suberin lamella in the BS cells (Hattersley & Browning 1981; Prendergast &
Hattersley 1987; Dengler et al. 1994). The fraction of CO2 fixed by PEPC that subsequently
leaks out of the BS without being assimilated by Rubisco, estimated from short-term carbon
isotope discrimination measured simultaneously with gas-exchanges (Henderson et al. 1992),
was higher for a NAD-ME-type monocot (0.30), than for other monocots belonging to the
NADP-ME or PEPCK subtypes (≤ 0.25). The carbon isotope composition of leaf dry matter was
lower in C. dactylon (-16.3‰) than in P. dilatatum (NADP-ME; -15.2‰) but slightly higher
than in Z. japonica (PEPCK; -16.6‰) (Figure III.4; Carmo-Silva et al. 2008), suggesting that the
former species might have evolved compensatory features that avoid considerable leakage of
CO2 from the BS (Hattersley & Browning 1981) and optimize the photosynthetic performance of
the species.
Heterogeneity in biochemistry and/or leaf structure can be the result of natural
hybridization between photosynthetically different individuals or derive from active evolution of
the photosynthetic pathway taking place. These were suggested as possible explanations to the
variability of C3 and C4 mechanisms observed among different individuals of Alloteropsis
semialata (Hattersley & Watson 1992). This hypothesis was supported by a recent study
showing that plants of A. semialata were not typically C3 or C4 but C3-like or C4-like (Ueno &
Sentoku 2006), showing considerable variability in the expression of the C4 syndrome.
Nowadays, the production of grass seeds and development of new varieties through breeding
techniques might result in some heterogeneity among different individuals of the same species
and explain the results obtained with C. dactylon.
Paspalum dilatatum, Cynodon dactylon or Zoysia japonica?
In practical terms, the goal beyond the fundamental research presented here was to show which
of the three grass species is the best in terms of water economy. The “best” grass will obviously
depend on the perspective and situation. The results obtained support the hypothesis that both C.
dactylon (bermudagrass) and Z. japonica (zoysiagrass) are very resistant, more than P. dilatatum
(dallisgrass), to environmental factors causing plant stress, namely decreased water availability.
Both C. dactylon and Z. japonica could therefore be viewed as better alternatives for the use as
turfgrass in golf courses. However, even though C. dactylon performed very well under
moderate drought conditions, the plants of this species can be severely damaged upon intense
desiccation (Utrillas & Alegre 1997). Zoysia japonica has characteristics that make it more fit to
xeric conditions, notably the higher relative leaf dry matter, superior compaction of leaf tissues
190
General Discussion and Conclusions
and more rigid cell walls that allow lower leaf water potentials to be reached. As a consequence,
zoysiagrass will probably be able to cope better and survive under conditions of severe drought,
as may be observed in future! However, “there is no rose without thorns” and these structural
features result in the harder and stiffer leaves of Z. japonica. Due to its coarse texture, this
species can not be used for the greens (on golf courses), where a great smoothness is needed. Its
higher content in fibers will also compromise its use as forage.
The “best” grass among the three studied when the high productivity is the target for use
as forage or pasture grass when water is not too restricted is Paspalum dilatatum. The potential
benefits associated with the use of mixtures of C3 and C4 forage and turf grasses (Johnston 1996)
should be further investigated as this may provide a better option.
In terms of drought resilience, when the intention is to have a green lawn, looking good
for most winter and summer seasons, and surviving severe summer droughts, the best option will
most likely be the use of Zoysia japonica, but this conclusion should be investigated further in
relation to severe drought conditions.
Overall the best grass for the use as turf and forage grass seems to be Cynodon dactylon,
for its great photosynthetic performance both under well-watered conditions and when water
availability is decreased and for the presence of good strategies to avoid excessive water loss. It
will perform very well under moderate levels of drought stress but will most likely be more
affected than Z. japonica in the case of extreme droughts as observed by Utrillas & Alegre
(1997). Nonetheless, given its overall characteristics, including the fine texture, there is great
advantage in its use for golf courses in Portugal in the present and near future.
191
Chapter VI.
CONCLUDING REMARKS
The three C4 grasses of the different metabolic subtypes showed some differences in their
responses to gradually induced water deficit, even though a generalized pattern could be
observed. The presence of drought resistance mechanisms that minimize water loss was more
effective in Cynodon dactylon and Zoysia japonica than in Paspalum dilatatum. The leaf
structure of Z. japonica makes this species more fit to cope with severe drought conditions. An
unusual amino acid (5-hydroxy-L-norvaline) was drought-responsive in both C. dactylon and Z.
japonica and is likely to be involved in stress defence mechanisms.
Photosynthesis was affected in all three C4 species under water deficit. Stomatal closure
and decreased RuBP contents were the major factors affected by drought stress that may
contribute to limit photosynthesis. Analysis of the CO2-response of photosynthesis and changes
in amino acids content suggested slightly increased photorespiratory rates as a proportion of
photosynthetic rates under water deficit. Nevertheless, the results provided evidence that
photorespiration remains slow in the three C4 grasses under water deficit conditions.
The carboxylating enzymes were similarly affected by water deficit in the three species,
with slightly increased PEPC physiological activity and decreased initial activity of Rubisco.
Down-regulation of Rubisco is likely to be advantageous in terms of protection of the enzyme
when its natural substrate is present at lower concentrations. The activities of the C4 acid
decarboxylases, NADP-ME, NAD-ME and PEPCK, changed only slightly with water deficit and
were more variable and species specific. High activities of PEPCK are present in the three
species, from each of the C4 biochemical subtypes.
Overall, the results provide evidence that an efficient photosynthetic metabolism is
present in the three C4 grasses. Water deficit decreased plant growth and photosynthesis and both
physiological and biochemical adjustments were observed. These responses allow the plants of
each species to perform well under adverse conditions and cope with moderate gradually induced
drought conditions. The plants of the three species and of C. dactylon in particular maintain
considerably high photosynthetic activity at moderate levels of drought stress.
192
General Discussion and Conclusions
FUTURE PERSPECTIVES
Given the broad field of plant responses to drought stress, it would not be possible to cover all
the aspects that needed investigation in the scope of the work here presented. The results
obtained suggest that the immediate next target should be to investigate the effects of water
deficit on photophosphorylation and ATP synthesis.
The applicability of the results obtained should be further investigated in the field,
through the imposition of different irrigation strategies in established lawns of each of the three
species. Special emphasis should however be given to C. dactylon and Z. japonica. Additionally,
other grass species and/or varieties should be included to broaden the screening of drought
resistance in grass species.
Some interesting details arose from the studies performed and two in particular must be
further investigated. The high PEPCK activity found in all three species belonging to each of the
classical biochemical subtypes of C4 photosynthesis should be complemented with studies of
cDNA sequencing, SDS-PAGE and immunoblotting to characterize and quantify the enzyme
type present in each of the three species. It is known (essentially from studies done by Leegood
and co-workers) that structural properties of PEPCK are related to the regulation of the enzyme
by (de)phosphorylation and to its coordination with PEPC. The functional significance of a high
activity of PEPCK in the leaves of the three species studied and in other C4 grasses must be
clarified.
The characterization of the stress-inducible amino acid 5-hydroxy-L-norvaline and its
metabolic pathway must be elucidated in order to exploit the potentialities associated with its
presence and enhancement in response to water deficit conditions.
193
Chapter VI.
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