Limitations of photosynthesis in Phaseolus vulgaris under drought stress: gas exchange, chlorophyll fluorescence and Calvin cycle enzymes

In this article, the effects of drought stress (DS) on gas exchange, chlorophyll (Chl) a fluorescence and Calvin cycle enzymes in Phaseolus vulgaris are evaluated. Three-week-old plants were exposed to DS by receiving only so much water every evening to ensure 30% field capacity water content overnight. After three days under these conditions, we observed that DS induced a decline of the CO(2) assimilation. Gas-exchange data showed that the closure of stomata during DS did not lead to a concomitant decline in calculated intercellular CO(2) concentration. Moreover, DS plants showed a reduction of the photochemical Chl fluorescence quenching, photosystem II quantum yield and electron transport rate and a higher pH gradient and more heat dissipation as compared to controls. The activity of Calvin cycle enzymes, Rubisco, sFBPase, and Ru5PK, decreased strongly in DS plants as compared to controls. Data analysis suggest that the decrease of CO(2) assimilation under drought conditions is not related to a diminished capacity of the use of NADPH and ATP but probably to the decline of enzyme activity involved in RuBP regeneration (Ru5PK).