Photoinhibition

 Effects of changing light conditions on the ecophysiological condition behind survival were examined on beech from two different populations. Plants were grown in a greenhouse under simulated understorey and canopy gap light conditions. Upon exposure to high light maximum photosynthesis of shade-acclimated leaves increased followed by a reduction over several days to between high- and low-light control rates. In the reciprocal transfer, the decrease in maximum photosynthesis was rapid during the first 2–3 days and then levelled off to values comparable to low-light controls. Seedlings from Sicily (Madonie) showed generally higher maximum photosynthetic rates than those from Abetone. Leaf conductance varied in the same direction as photosynthesis in high- to low-light seedlings but to a lesser degree. Leaves grown under low light and exposed to high light experienced photoinhibition. The Abetone population was more susceptible to photoinhibitory damage than the seedlings from Sicily. Exposure to high light of shade-acclimated seedlings resulted in intermediate chlorophyll concentrations between levels of the high-light and low-light seedlings. Carotenoid concentration was unaffected by treatments. Seedlings grew more in high light, but had a lower leaf area ratio. Light-limited seedlings showed a shift in carbon allocation to foliage. Leaves formed in the new light regime maintained the same anatomy that had been developed before transfer. Seedlings from Sicily had thicker leaves than those of seedlings from Abetone. Seedlings from Abetone were found to be more susceptible to changing light conditions than seedlings from Sicily. We conclude that small forest gaps may represent a favorable environment for photosynthesis and growth of beech regeneration as a result of the limited ability of seedlings to acclimate to sudden increases in high irradiance and because of the moderate levels of light stress in small gaps.

In this study, we have compared photosynthetic performance of barley leaves (Hordeum vulgare L.) grown under sun and shade light regimes during their entire growth period, under field conditions. Analyses were based on measurements of both slow and fast chlorophyll (Chl) a fluorescence kinetics, gas exchange, pigment composition; and of light incident on leaves during their growth. Both the shade and the sun barley leaves had similar Chl a/b and Chl/carotenoid ratios. The fluorescence induction analyses uncovered major functional differences between the sun and the shade leaves: lower connectivity among Photosystem II (PSII), decreased number of electron carriers, and limitations in electron transport between PSII and PSI in the shade leaves; but only low differences in the size of PSII antenna. We discuss the possible protective role of low connectivity between PSII units in shade leaves in keeping the excitation pressure at a lower, physiologically more acceptable level under high light conditions.

1. Climate change is altering temperatures and precipitation patterns all over the world. Melting glaciers
increase surface run-off, thereby increasing the transport of suspended solids through streams.
The increased load of suspended solids affects turbidity, which decreases the availability of photosynthetically
active radiation for primary producers.
2. We analysed how glacial loading of clays influences the light : nutrient ratio and photosynthetic
parameters (measured with a pulse amplitude modulated fluorometer) and the carbon : phosphorus
(C : P) elemental ratio of periphytic primary producers. A field study was conducted in two canopyfree
streams that receive water from the glaciers of Mount Tronador (Patagonia, Argentina), one with
high glacial load and the other with clear water. In addition, we conducted an in situ colonisation
experiment with three different light treatments.
3. We observed that periphytic biomass (chlorophyll a and carbon content) in the streams and in the
experiment varied directly with turbidity. Moreover, photosynthetic parameters varied similarly
because of an increase in the efficiency of electron transfer per open reactive centre in the more
turbid stream and a chronic photoinhibition of photosystem II in the clearer stream.
4. Periphytic C : P also varied with turbidity as we observed a decrease in C : P with an increase in
light in both streams and in the experiment.
5. Our main conclusion is that an increase in glacial melting with its associated increase in glacial
clay load should protect primary producers against high irradiances (photosynthetic active radiation
+ ultraviolet radiation) in canopy-free streams.

The structure and function of photosystem II (PSII) is highly susceptible to photooxidative damage, which is induced by high fluence or fluctuating light. However, many of the mechanistic details of how PSII homeostasis is maintained under photoinhibitory light remain to be defined. We describe analysis of Arabidopsis thaliana gene At5g07020, which encodes an unannotated integral thylakoid membrane protein. Loss of the protein causes altered PSII function under high irradiance light, and hence is named Maintenance of PSII under High light1 (MPH1). The MPH1 protein co-purifies with PSII core complexes and co-immunoprecipitates core proteins. Consistent with a role in PSII structure, mph1 mutant PSII complexes (supercomplexes, dimers, and monomers) are less stable in plants subjected to photoinhibitory light. Accumulation of PSII core proteins is compromised under these conditions in the presence of translational inhibitors. This is consistent with the hypothesis that the mutant has e...

We studied the effects of elevated CO2 concentrations (0.03% vs. 0.1%) on light absorption, membrane permeability, growth, and carbon fixation under photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) exposures in the diatom Thalassiosira ...

The chloroplastic glutamine synthetase (GS, EC 6.3.1.2) activity was previously shown to be the limiting step of photorespiratory pathway. In our experiment, we examined the photosynthetic high-light responses of the GS2-mutant of barley (Hordeum vulgare L.) with reduced GS activity, in comparison to wild type (WT). The biophysical methods based on slow and fast chlorophyll fluorescence induction, P700 absorbance, and gas exchange measurements were employed. Despite the GS2 plants had high basal fluorescence (F0) and low maximum quantum yield (Fv/Fm), the CO2 assimilation rate, the PSII and PSI actual quantum yields were normal. On the other hand, in high light conditions the GS2 had much higher non-photochemical quenching (NPQ), caused both by enhanced capacity of energy-dependent quenching and disconnection of PSII antennae from reaction centers (RC). GS2 leaves also maintained the PSII redox poise (QA-/QA total) at very low level; probably this was reason why the observed photoinhibitory damage was not significantly above WT. The analysis of fast chlorophyll fluorescence induction uncovered in GS2 leaves substantially lower RC to antenna ratio (RC/ABS), low PSII/PSI ratio (confirmed by P700 records) as well as low excitonic connectivity.

High-light illumination of photosynthetic organisms stimulates the production of singlet oxygen by photosystem II (PSII) and causes photo-oxidative stress. In the PSII reaction centre, singlet oxygen is generated by the interaction of molecular oxygen with the excited triplet state of chlorophyll (Chl). The triplet Chl is formed via charge recombination of the light-induced charge pair. Changes in the midpoint potential of the primary electron donor P680 of the primary acceptor pheophytin or of the quinone acceptor QA, modulate the pathway of charge recombination in PSII and influence the yield of singlet oxygen formation. The involvement of singlet oxygen in the process of photoinhibition is discussed. Singlet oxygen is efficiently quenched by β-carotene, tocopherol or plastoquinone. If not quenched, it can trigger the up-regulation of genes, which are involved in the molecular defence response of photosynthetic organisms against photo-oxidative stress.

Australian rainforests extend from tropical climates in the north to temperate climates in the south, thus providing an opportunity to investigate the physiological responses to temperature of temperate and tropical species within the same forest type.  Eight rainforest canopy tree species were selected to cover the 33o latitudinal range of rainforests in eastern Australia.  Temperature tolerance was measured from six-year-old plants grown under a common environment, by exposing leaves to a series of hot temperatures during late summer and a series of freezing temperatures during midwinter.  Damage was evaluated based on chlorophyll fluorescence measurements 2 hr after exposure and by visual assessment a week after exposure.  Leaves of the tropical species had a higher heat tolerance and lower frost tolerance than leaves of the temperate species, which is consistent with their climate distributions.  In contrast, the temperature tolerance of the photosynthetic apparatus was not correlated with the native climates of species.  However, the tropical species had significant photoinhibition during winter.  All species maintained the integrity of the photosynthetic apparatus and avoided tissue damage over a similar span of temperatures (ca. 60 oC), which reflects the similar annual temperature range of their climates.  Chlorophyll fluorescence and visual assessment provided different estimates of the absolute and relative temperature tolerance of the species, thus stressing the importance of visual assessments of injury to the determination of the relative temperature tolerance of species.