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In the past, the role of light as an energy source was largely ignored in research focused on cold acclimation and freezing tolerance in plants. However, cold acclimation is an energy-requiring process. We summarize research illustrating that photoautrophs as diverse as cyanobacteria (Plectonema boryanum), green algae (Chlorella vulgaris, Dunaliella salina, Chlamydomonas raudensis), crop plants (Triticum aestivum L., Secale cereale L., Brassica napus L.), and conifers (Pinus banksiana) L.) tailor the structure and function of the photosynthetic apparatus to changes in temperature and irradiance to maintain cellular energy balance called photostasis. Modulation of either temperature or irradiance results in a similar imbalance in cellular energy that is sensed through changes in chloroplastic excitation pressure. Thus, concepts of photostasis and excitation pressure provide the context through which one can explain the congruence of phenotypic plasticity and photosynthetic performance associated with cold acclimation and photoacclimation. Photosynthetic organisms can sense changes in temperature and irradiance through modulation of the redox state of the photosynthetic electron transport chain, which, in turn, governs phenotype through the regulation of nuclear gene expression and chloroplast biogenesis. We suggest that elucidation of the molecular mechanism(s) by which excitation pressure regulates phenotypic plasticity and photosynthetic performance will be essential in addressing the challenge of maintaining or perhaps enhancing crop productivity under the suboptimal growth conditions predicted to occur as a consequence of climate change. Key words: algae, cold acclimation, cyanobacteria, excitation pressure, phenotypic plasticity, photoacclimation, photostasis, plants. Auparavant, on ignorait largement le role de la lumiere comme source d'energie dans les recherches portant sur l'acclimatation au froid et la tolerance au gel. Cependant, l'acclimatation au froid constitue un processus demandant de l'energie. L'auteur resume la recherche, illustrant que des autotrophes aussi diversifies que les cyanobacteries (Plectonema boryanum), les algues vertes (Chlorella vulgaris, Dunaliella salina, Chlamydomonas raudensis), les plantes cultivees (Triticum aestivum L., Secale cereale L., Brassica napus L.), et les coniferes (Pinus banksiana) L.), ajustent la structure et la fonction de leurs appareils photosynthetiques selon les changements de temperature et d'irradiance pour maintenir la balance energetique cellulaire appelee photostasie. Une modulation de la temperature ou de l'irradiance induit un debalancement similaire dans l'energie cellulaire lequel est percu par des changements dans la pression d'excitation chloroplastique. Ainsi, les concepts de photostasie et de pression d'excitation fournissent le contexte dans lequel on peut expliquer la congruence de la plasticite phenotypique et de la performance photosynthetique associees a l'acclimatation au froid et la photoacclimatation. Les organismes photosynthetiques peuvent percevoir les changements de temperature et d'irradiance par la modulation de l'etat redox de la chaine de transport des electrons photosynthetiques, laquelle a son tour gouverne le phenotype par la regulation de l'expression des genes de la biogenese des chloroplastes. L'auteur suggere que l'elucidation du ou des mecanismes cellulaires par lesquels la pression d'excitation regle la plasticite phenotypique et la performance photosynthetique sera essentielle pour faire face au defi de maintenir et possiblement augmenter la productivite des recoltes sous des conditions de croissance suboptimales, tel que prevu comme consequence du changement climatique. [Traduit par la Redaction] Mots-cles : algue, acclimatation au froid, cyanobacteries, pression d'excitation, plasticite phenotypique, photoacclimatation, photostasie, plantes., Introduction Historically, the role of light, energy trapping, and transformation through photosynthesis was, by and large, ignored in research focused on cold acclimation and freezing tolerance of plants even though [...]

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Sunlight, the ultimate energy source for life on our planet, enters the biosphere as a direct consequence of the evolution of photoautotrophy. Photoautotrophs must balance the light energy absorbed and trapped through extremely fast, temperature-insensitive photochemistry with energy consumed through much slower, temperature-dependent biochemistry and metabolism. The attainment of such a balance in cellular energy flow between chloroplasts, mitochondria and the cytosol is called photostasis. Photoautotrophs sense cellular energy imbalances through modulation of excitation pressure which is a measure of the relative redox state of QA, the first stable quinone electron acceptor of photosystem II reaction centers. High excitation pressure constitutes a potential stress condition that can be caused either by exposure to an irradiance that exceeds the capacity of C, N, and S assimilation to utilize the electrons generated from the absorbed energy or by low temperature or any stress that decreases the capacity of the metabolic pathways downstream of photochemistry to utilize photosynthetically generated reductants. The similarities and differences in the phenotypic responses between cyanobacteria, green algae, crop plants, and variegation mutants of Arabidopsis thaliana as a function of cold acclimation and photoacclimation are reconciled in terms of differential responses to excitation pressure and the predisposition of photoautotrophs to maintain photostasis. The various acclimation strategies associated with green algae and cyanobacteria versus winter cereals and A. thaliana are discussed in terms of retrograde regulation and the "grand design of photosynthesis" originally proposed by Arnon (1982). [ABSTRACT FROM AUTHOR]

Maximum photosynthetic capacity indicates that the Antarctic psychrophile Chlamydomonas raudensis H. Ettl UWO 241 is photosynthetically adapted to low temperature. Despite this finding, C. raudensis UWO 241 exhibited greater sensitivity to low-temperature photoinhibition of PSII than the mesophile Chlamydomonas reinhardtii P. A. Dang. However, in contrast with results for C. reinhardtii, the quantum requirement to induce 50% photoinhibition of PSII in C. raudensis UWO 241 (50 μmol photons) was comparable at either 8°C or 29°C. To our knowledge, this is the first report of a photoautotroph whose susceptibility to photoinhibition is temperature independent. In contrast, the capacity of the psychrophile to recover from photoinhibition of PSII was sensitive to temperature and inhibited at 29°C. The maximum rate of recovery from photoinhibition of the psychrophile at 8°C was comparable to the maximum rate of recovery of the mesophile at 29°C. We provide evidence that photoinhibition in C. raudensis UWO 241 is chronic rather than dynamic. The photoinhibition-induced decrease in the D1 content in C. raudensis recovered within 30 min at 8°C. Both the recovery of the D1 content as well as the initial fast phase of the recovery of Fv/ Fm at 8°C were inhibited by lincomycin, a chloroplast protein synthesis inhibitor. We conclude that the susceptibility of C. raudensis UWO 241 to low-temperature photoinhibition reflects its adaptation to low growth irradiance, whereas the unusually rapid rate of recovery at low temperature exhibited by this psychrophile is due to a novel D1 repair cycle that is adapted to and is maximally operative at low temperature. [ABSTRACT FROM AUTHOR]

In the past, the role of light as an energy source was largely ignored in research focused on cold acclimation and freezing tolerance in plants. However, cold acclimation is an energy-requiring process. We summarize research illustrating that photoautrophs as diverse as cyanobacteria ( Plectonema boryanum), green algae ( Chlorella vulgaris, Dunaliella salina, Chlamydomonas raudensis), crop plants ( Triticum aestivum L., Secale cereale L., Brassica napus L.), and conifers ( Pinus banksiana) L.) tailor the structure and function of the photosynthetic apparatus to changes in temperature and irradiance to maintain cellular energy balance called photostasis. Modulation of either temperature or irradiance results in a similar imbalance in cellular energy that is sensed through changes in chloroplastic excitation pressure. Thus, concepts of photostasis and excitation pressure provide the context through which one can explain the congruence of phenotypic plasticity and photosynthetic performance associated with cold acclimation and photoacclimation. Photosynthetic organisms can sense changes in temperature and irradiance through modulation of the redox state of the photosynthetic electron transport chain, which, in turn, governs phenotype through the regulation of nuclear gene expression and chloroplast biogenesis. We suggest that elucidation of the molecular mechanism(s) by which excitation pressure regulates phenotypic plasticity and photosynthetic performance will be essential in addressing the challenge of maintaining or perhaps enhancing crop productivity under the suboptimal growth conditions predicted to occur as a consequence of climate change. [ABSTRACT FROM AUTHOR]