Download University of Groningen Photoinhibition of photosynthesis in higher

University of Groningen
Photoinhibition of photosynthesis in higher plants
van Wijk, Klaas Jan
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Publication date:
1992
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Citation for published version (APA):
van Wijk, K. J. (1992). Photoinhibition of photosynthesis in higher plants: From photosystem II paricticle to
intact leaf s.n.
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Summary
Photosynthesis
is the conversionof light energyinto chemicalenergy,followed by
COt-fixation. In plants photosynthesistakes place in the chloroplast. The light is
absorbedby two photosystems(I and II), locatedin the thylakoid membranes,which
are embeddedin the chloroplast.When much more light is absorbedthan can be used
for COt-fixation, inactivation of photosynthesisoccurs. This inactivation is called
photoinhibition.
The primary site of photoinhibitionis the multi-proteincomplexphotosystemII (PS
il). Illumination of PS II by excess light results into inactivation of its electrorr
transport activity and damage to one of the proteins (the D,-protein) involved in
stabilizationof important redox componentswithin PS II.
In this thesisseveralaspectsof photoinhibitionhave been studied.Photoinhibition
of PS II was studied, both on a basic (biophysicaland biochemical) level and on a
more integrated(eco)physiologicallevel. The resultsof the different approacheswere
integratedand discussedwith respectto the mechanismof photoinhibitionof the leaf
under light stress. Depending on the questions raised, isolated PS II particles,
thylakoids, chloroplasts,protoplastsand intact leaves of field lettuce fllalenalgla
locusta)and/or spinach(Spinaciaoleracea)have been used. A variety of techniques
hasbeen applied,varying from electronspin resonanceat low temperatures(5-15 K),
measurementsof electron transport in vitro, room temperature chlorophyll a
fluorescencemeasurements
both in vivo and in vitro, and gas exchangemeasurements
in vivo.
In Chapter 2 the basicmechanismof photoinhibitionof PS II was studied,using
isolated PS ll-particles. In the absence of efficient electron acceptors, strong
illumination of PS II led to a stepwiseinactivation of different electron transport
componentswithin PS II. The sequenceof inactivation steps was explained by
'overreduction'
of the acceptorside of PS iI. Degradationof the D,-proteinwas shown
to be a secondaryevent and it was not accompaniedby further loss of active electron
transportcomponents.
In Chapter 3, the influence of O, on photoinhibition of PS II and overall
photosynthesiswas studied in isolatedprotoplastsand intact leaves of field-lettuce.
Under the experimentalconditions, where energy-turnoverby photorespirationwas
low, photoinhibition of PS II was promoted by oxygen. No oxygen dependenceof
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photoinhibition of gross maximal photosynthesiswas observed.
The PS II population is heterogeneouswith respect to both antennaesize and
functionality of the acceptor side. The hypothesisthat this heterogeneityplays a
physiologicalrole in recovery of PS II, with different PS il populationsrepresenting
different stepsin a repair-cycleof photoinhibitedPS II, was testedin Chapter 4. No
evidencefor such a repair cycle was found, but new results regarding the sensitivity
and inactivation stateof the different PS II populationswere obtained.
In Chapter 5 methodologicalaspectsof the applicationof chlorophyll fluorescence
to calculateelectron transportrates in intact leaveswere evaluated.
In Chapter 6, the mechanismof photoinhibitionof PS II was further analyzedin
vivo. The metabolicdemandfor ATP and NADPH was manipulatedby temperature
at different light levels. Photoinhibitionbecameonly
and COr- and Or-concentrations
significant when the PS II population was nearly completely down-regulatedby the
lighrinduced acidifrcationof the intrathylakoid space.These results were discussed
with regard to the mechanismof photoinhibitionof PS II in vivo.
In Chapter 7, the recovery from photoinhibition of PS II in vivo was analyzed.
Different recovery phases were detected revealing different sensitivities for
temperatureand streptomycin(an inhibitor of chloroplast translation). Hypotheses
explaining the different recovery phaseswere connectedto the stepwise nature of
photoinhibition of PS II and breakdownof the D,-reaction center protein.
The proposal, that long-term cold acclimationof plants should lead to increased
resistanceagainst photoinhibition at low temperatures,was tested for field-lettuce
(Chapter 3) and spinach(Chapters 6,7). Cold-acclimationof spinachdid indeedlead
to diminished photoinhibition at low temperatures.This diminished sensitivity was
subscribedto small changesof the pigment composition of PS II and increased
maximum photosynthesisrates at these low temperatures. Full recovery from
photoinhibition was reachedmore quickly in the cold-acclimatedspinachleavesand
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it was attributed to diminished levels of photoinhibition. No significant alterationof
the Or-sensitivity during high light stress at low temperaturewas found in cold-
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acclimatedfield lettuce.
In Chapter 8 the experimentalresults of the various chaptersare integratedand
discussedwith respectto the physiologicalsituationof the plant under light stress.
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