Effect of high temperature on photosynthesis in beans. II. CO2 assimilation and metabolite contents
1996
Pastenes, C. | Horton, P.
The effect of high temperatures on CO2 assimilation, metabolite content, and capacity for reducing power production in nonphotorespiratory conditions has been assessed in two different bean (Phaseolus vulgaris L.) varieties, Blue Lake (commercially available in the United Kingdom) and Barbucho (a noncommercially bred Chilean variety), which are known to differ in their resistance to extreme high temperatures. Barbucho maintains its photosynthetic functions for a longer period of time under extreme heat compared with Blue Lake. The CO2 assimilation rate was increased by increases in temperature, with a decrease in ratio of rates at temperatures differing by 10 degrees C. It is suggested that limitations to CO2 assimilation are caused by metabolic restrictions that can be differentiated between those occurring in the range of 20 to 30 degrees C and 30 to 35 degrees C. It is likely that changes in the capacity for Calvin cycle regeneration and starch synthesis affect photosynthesis in the range of 20 to 30 degrees C. But following an increase in temperature from 30 to 35 degrees C, the supply of reducing power becomes limiting. From analysis of adenylate concentration, transthylakoid energization, and, indirectly, NADPH/NADP+ ratio, it was concluded that the limitation in the assimilatory power was due to an oxidation of the NADPH/NADP+ pool. In the range of 30 to 35 degrees C, the photosystem I quantum yield increased and photosystem II maintained its value. We conclude that the reorganization of thylakoids observed at 30 to 35 degrees C increased the excitation of photosystem I, inducing an increase in cyclic electron transport and a decrease in the supply of NADPH, limiting carbon assimilation.
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