Download SUGAR AND ACID METABOLISM IN CITRUS FRUIT 1

SUGARAND ACID METABOLISM IN CITRUS FRUIT
Karen E. Koch
1
Two important
horticultural
questions
in this
1.
What affects
sugar levels
in citrus
fruit?
2.
What affects
acid
levels
in citrus
fruit?
area
are:
Sugar LevelsPhotosynthetic
activity
in ~arby
leaves has long been thought
to influence
levels
of soluble
so~ds
in fruit,
and these so~ds
are predominantly
sugars.
Classic
work by Sites
aM Reitz
(Fig.
1) showd highest
solids
were in
fruit
picked
from the outside
of the tree
on the southeast,
south,
and west
exposures;
positions
where light
intensity
aM photosynthetic
activity
would likely
have been greatest.
N
1)1
s
low
high
Fta.
1.
Effect
of
8had1ns
Or&ns.8
here
in
leitz,
d1rect10n
of
8unl1aht
expO8Ure
on total
801ub1e
8o11d8
bar¥88ted
fr08
port1oD8
cro..
Mct1on.
(redoM
aIM!
(%) of'Va1enc1a'
of the
tree
8hown
fr08
S1tes
aIM!
1949)
A still
closer
tie has recently
been fooM between photosynthesis
in single
leaves x-.ar fruit,
am sugars translocated
into
fruit
sections
vertically
aligx-.d
with them.
The fruit
quarter
positioned
as shown in Figure 2 usually
receives
over
90% of
the
photosynthetic
examination
of imividual
fro.
a single
leaf
move
sugar distribution
occur
fruit
or
if
1Assistant
GaiMsvi.l1e,
all
other
material
nearby
Professor,
exported
from
the
nearby
leaf.
Further
juice
segments
has shown that
most of these photosynthates
into
only
one or two of the segments.
Similar
patterns
of
from an imividual
leaf
even if
it
is farther
away from the
Fruit
leaves
are
Crops
removed.
Depart1Rent,
FL 32611.
59
liAS,
University
of
Florida,
r1l'
2.
Label.d
product.
of pbotoeynth..1.
8OV8 fra
l..v..
l8ar
fruit
1oto
88ctioYerticelly
al1l_d
with
tb_.
Greater
than 90% of photo.yuthat..
froa a J1v.n
leaf
viII
tYl'ically
be fooin the 1/4
fruit
labe1ad A in thi.
f1lure.
Tre_port
into
1_ividual
juice
88ct1o1. .t1l1
.ore
.pec1f1c.
(troa
l.odI,
1984.)
Sugar movement into
citrus
fruit
is strikingly
affected
by the unusual
distribution
of vascular tissues in the fruit.
The peel has a dense ~twork
of
vascular
buMles
aM receives
photosynthetic
materials
fr~
leaves relatively
rapidly
(about
3 to 6 hours).
Citrus
juice
tissues,
however,
differ
froa
the juice
tissue
of allDost
all
other
crOp
species
in haviag
~ vascular
straMs.
Three
vascular
buMles
outside
each segaent
must therefore
provide
.ill
the solids
to the
juice
vesicles
inside
(Fig.
3).
Dorsal
vascular
bundles
are the main path of sugar
movement,
with
some photosynthetic
products
also
entering
by septal
bundles.
Central
.traMs
supply
only seeds.
Ptl.
J.
three
.e.Clllar
b.&1Id1..
aupp1y
pbot08yatbetic
prllduct.
(prt_rt1y
auaan)
to all
juiC8
ti.8U8.
iD8ide
a liven
.er_at.
C1trv.
fruit
dlff.r
f~
all
other
alro_lc
crop.
In be.1..
web exteD81V8
DOo...CIIlar
anu.
Bach juice
...1el.
1. joiDed
to the .ea-
-at
a
ep1ami.
.a.Clllar
like
...Iel.
at a .1te
b.&1Id1e
by
a
bel
Deer
r-
.talk.
Figure
3 also
shows that
each juice
vesicle
is attached
to a point
on the
inside
of the segment epiderais
just
opposite
0"
of the three vascular
bundles.
The base of .ost
juice vesicles
il a minute,
hair-like
strand up to 2 C8 long (about
3/4 inch).
Sugar aove.nt
through
this
zo~ is apparently
not facilitated
by any
vascular
tissuel
or other
specialized
structures.
Photosynthetic
materials
froa
leaves
.ove
through
these vesicle
stalks
and sepent
epidemi8
so slowly.
that
complete transfer
may take 3 to 5 day..
The sugars appear to reach the fruit
within
le88 than 1 day, but redistribution
into juice
tissues
requires
an extended tiae.
The primary
sugar transported
fr~
leaves to froit8
of citrus
trees is sucrose
(c~on
table
sugar).
which
can be broken
down into
two other
smaller
sugars.
glucoae and fructose
(ltg.
4).
Even though sucrose is also the ..in
sugar in citrus
juice,
it is apparently
broken down and resynthesized
as it moves into the froit.
A
possible
reason for this
could be to maintain
a -down-hillconcentration
gradient
frC8 leaves to points
where sugars are -unloadedfrC8 the vascular
tissues
in the
fruit.
Sucrose could then contirl.1e
to move into
the fruit
even though very high
1ew18 are acCU8ulati~
in the juice vesicles.
60
F1g.5.
N
A.i
Effect
of direct10n
of sunlight
expo.ure
and shadina
on
fruit
qU8lity
of 'Valencia'
oranle.
harvested
fro.
portion.
of the tree
.hown here
in cros.
section.
A.
Soluble
solids,
B.
Acid (%), and C.
J.tio
of .oluble
.olid/acid.
8
B.
c.
low
high
61
SUCROSE
~
c
-~.Ac
--
Pil.4.
c
-0
\c
c-c
cJI/°,
IL
o -l\c-c/l
1-
GLUCOSE
occur
injury
~
A .i.ilar
breakdown
in peel of grapefruit
then develops.
of
Sucro.e
prtaary
citru.
fruit,
d~
~
(table
sular
trees
8ular)
transported
and .tored
but
can be
into
Iluco.e
1.
the
in
in
broken
and
fracto...
c
FRUCTOSE
sucrose
(but
after
exposure
with~t
to cold.
resynthesis)
A resistance
has been
to later
looM
to
chilling
Phot08ynthe8is
in green citrus
peel may be iaportant
in reducing
losses
of
respiratory
CO frCD fruit.
As much as OM third
or more of the sugars moving fram
leave8
into
fruit
during
a liven
day could
be lost
to the ataosphere
after
conversion
to CO2 duri~
respiration.
In the ligQt,
ho~ver,
s~
of this CO-,- is
-trappedin the peel by photosynthetic
processes
aDd recycled
back to the fhdt
interior
as sugars aDd/or organic
acids.
The aaount of recycling
will
vary during
fruit
developaent
because respiration
occurs very rapidly
during
early
growth,
am
peel photosynthesis
decreases with color change.
Acid Levels
- Levels
of organic
acids
in citrus
juice
(mostly
citric
acid)
differ
dependiogon
the p08ition
of fruit
on the tree.
Figure
5 (previous
page)
.hows that
total
acidity,
like
total
soluble
.olide
i8 highest
in fruit
on the
out.ide
of the tree piCked from southea8t,
.outh,
and west exposure8.
The ratio
of
.oluble
.olide
to acids was therefore
found by Site.
aDd Reitz (also shown in Fig.
5) to be highest
in the northea8t
quadrant
of the tree,
even though the total
solids
in these fruit
wre
low.r.
--_8
OCI8_no
-~-"
s..- IV
"'e"
'il.6.
Diatr1butioft
of
total
acidity
(Oft rilbt)
and ratio
of
801ubl.
aolida/acida
(on
left)
io
edible
portion
of
'Va1eecia'
oranle.
(redone
froa
Tinl.
1969)
.-Total
acidity
within
an ora~e
can vary dramatically
fr~
0.. part of the juice
tissues
to another.
aM
the
right
side
of Figure
6 shows
the most acid
part
of
Valencia
ora~e
pulp
is
in its
center.
The resulti~
soluble
solids/acid
ratio
(left
side
of Figure
6) is also
lowest
in the middle
of the fruit.
62
Levels of organic acids have long been known to be reduced in grapefruit
(aoo
other citrus)
by application of arsenic-based c~pouM..
Results are often evident
as early as Augu8t aM the change in 80luble 80lids/acid
ratio is due specifically
to effects on acidity.
. low auga,
. highacid
HUMID TROPICS
ARID DESERT
COOL NIGHTS
W ARM NIGHTS
HIGH RAINFALL
./110/1
LOW RAINFALL
8UO.'
..cl~ drOO8 '.Oldly
wlt/l ~.v.'oom.nt
rl"
7.
Lons-term
climatic
Additional
infotmation
enviroaaental
coaditions,
Loaa-tera cll..tlc
effect.
ou .uaar
aud .cid
effects
on sugar aI¥i add
levels
bas begun
to
appear
regarding
but reports
iu citrua
fruit.
are shown in Figure
short-term
effects
are conflicting.
7.
of
.
1. SYNTHESIS
~--r
3. COMPARTMENTA~IZATION
(In vacuol..>
2. UTILIZATION
1il'
8.
Thr..
proc
which caD ia-
flu.nc.
the ext.nt
of acid
accU8Ulat1oD in citru8
fruit.
All
utilization,
three of; the processes
shown in
and coapartaentalization.
Figure
8 8USt
be considered;
synthesis
Evidence
suggests
that at lea8t
s~of
the organic
acids in citrus
juice
may
be synthesized
A) in leaves aoo translocated
to fruit,
B) in peel aoo moved inward
to pulp,
aoo C) inside
juice
vesicles.
Organic acid8 fr<8 leaves were initially
d18coonted a8 a aajor
soorce of citru8
fruit
acidity
years ago after
an experiment
show.d that
fruit
of sweet and soor le_on Won still
"SWoet" or "soor"
if grafted
aM grown on leafy
branche8 of the opp08ite
type.
Still,
8<8ewbat less draaatic
effects
of leaves on fruit
acidity
may still
be i_portant.
In grapes,
for example,
at least
o~ of the aajor
type8 of organic
acids in the fruit
(tartaric
acid)
has
been foooo to be synthe8ized
in 1eave8 aDd transported
to fruit.
Acid levels
in
citrus
leaves are also extreaely
high, even -ore so than the fruit.
63
~
PEP
CO2
VEPCa'bOXy,aao>
OAA
Fig.
9.
The enzyae PEP earboxyla.e
begins a serie.
of reaction.
l.ading
to for8ation
of organie acid. fro. CO2'
1
malic acid
citric
1
acid
The possibility
of organic
acid transport
from peel to pulp is also suggested
by several
observations.
The first
of these is that the arsenic
treatments
noted
above affect
juice
acidity
without
the arsenic
ever reaching
the pulp.
Only leaves
and peel retain
levels
of this compound that can be readily
measured.
Known effects
of arsenate
on metabolic
pathways
may therefore
be occurring
only
in peel and
leaves.
In addition,
transport
of labeled
materials
frM
peel to pulp has been
measured, and high levels
of organic
acids occur in peel.
Lastly,
an enzyme known
to synthesize
organic
acids
(PEP carboxylase)
is present
in peel tissues
(see Fig.
9).
Juice
vesicles
may produce
their
own organic. acids,
either
by _tabolizing
sugars,
or through activity
of the enzyme shown in Figure
9. Greater activity
of
this enzyme occcurs in peel,
but juice
vesicles
have an extensive
capacity
to take
up CO2 (presumably as shown in Fig.
9).
ff-
11'001,...
CO2iifP:ep
_""~HJt"
malic
,
e
...,;
",~it~~W;' (01"8'.
.,n.h..a..)
.old
f
.
".""'1-
aconitic acid
lil'tic
acid cycle'
,~
Pil.
10.
The fir.t
h.re
with
citric
acid
.te,
.n
in
X.
wher.
'
citric
acid
Act iYi ty of
exaained
thu.
utilization
thi.
enz,..
1. the
appear.
enaY8e aconita..
.hovn
to influence
le.el.
of
far.
Rates
of citric
acid
utilization
could
ultimately
affect
levels
of this
acid
which
accwaulate.
aoo the enzyme aconitase
is primarily
responsible
for
its
direct
breakdown
of citrate
in .ost
tissues
(Fig.
10 on previous
page).
Research
in Japan
on sweet aoo sour maooarins
aoo in Florida
on low-acid
grapefruit
sutants
(developed
by C. J. Hearn.
U.S.D.A.
Orlaooo)
has iooicated
that
activity
of this
enzyme of the
citric
acid
cycle
may be low in fruit
where the most acid accumulates.
64
Co8part8entalization
of organic
acids
can prevent
their
utilization.
This
"8torage"
occurs
when acids
are transported
across
a me.brat-.
into
the central
vacuole
of a cell.
Vacuoles
are known to be the site
of acid storage
and occupy
over 90% of the volume in most cells.
Investigations
into
properties
of the
vacuolar
.mbrat-.
in citrus
are just
beginning,
but may clarify
part of the basis
for fruit
acidity.
The ultimate
sugar/acid
levels
this ratio.
goal
of
in citrus
current
fruit
is
research
at the University
of
to provide
alternative
means of
65
Florida
on
controlling
References
1.
Erickson,
Reuther,
L. C.
1968.
The geMral
physiology
L. D. Batchelor,
H. J. Webber (eds.).
University
of California
K. E.
Koch,
3.
Koch.
Plant.
Cell
4.
Koch,
citrus
K. E.
fruit
K.
Riverside,
1984a. Translocation
2.
to aligned
Press.
juice
E.
The path
and Euvirooment.
Physiology.
aDd W. T.
segments
citrus.
The Citrus
CA.
of photosynthate
translocation
Avig~.
relative
1984.
Localized
to source-leaf
citrus
fruit
photosynthate
deposition
in
position.
Plant
and Cell
25:859-866.
Purvis,
A. C. and W. Grierson.
1982.
Accuaulation
resistance
of grapefruit
peel
to chilling
injury
temperatures.
J. Amer. Soc. Hort. Sci.
107(1):139-142.
6.
Sinclair,
W. B. 1984.
University
citrus
fruits.
7.
Sites,
oranges
W. aM H. J.
fro.
different
spot-picking
Soc. Hort.
into
7:647-653.
5.
J.
pp. 86-127.
In: W.
rndustry:
Vol. II.
of photosynthetic
products from source leaves
fruit.
HortScience
19:260-261.
segments in citrus
1984b.
of
for
Sci.
quality.
The biochemistry
of California
Reitz.
locations
Part
I.
and physiology
Press, Riverside,
of
as
reducing
related
of the
CA.
sugar am
to winter
lemon and other
1949.
of
The variation
in iMividual
Valencia
the tree
as a guide
to sampling
and
Soluble solids in the juice. . Proc. Amer.
54:1-10.
8.
Sites,
.1. w. am H. .1. Reitz.
1950.
The variation
in individual
Valencia
oranges
from different
locations
of the tree
as a guide
to sampling
and
spot-picking
for quality.
Part
I.
Titratable
acid
and soluble
solids!
titratable
acid ratio
of the juice.
Proc. Amer. Soc. Hort. Sci.
55:73-80.
9.
Ting,
edible
S. V.
portion
Distribution
of citrus
of
fruits.
soluble
cCDpoMnts
am quality
factors
J. Amer. Soc..Hort.
Scl.
94:515-519.
66
in
the