Regulation of photosynthesis during water deficit in three grapevine cultivars

Abstract

The main limitation imposed by water deficit on photosynthesis is a decreased CO₂ diffusion to the chloroplast mediated by stomatal closure. However, a decrease in mesophyll conductance, electron transport rate through photosystem II or a relative increase in photorespiration have also been observed in different studies. The degree of importance of these processes to photosynthesis limitation under water deficit may be different depending on its intensity and on the cultivar-specific response. In this study, we analyzed the effect of water deficit in different mechanisms (e.g. stomata closure, mesophyll diffusion, biochemistry) that limit photosynthesis in three grapevine cultivars. For this aim, we established an experiment in Mendoza (Argentina) in one year old potted grapevines of Syrah, Malbec, and Bonarda. Irrigation treatments consisted of fully irrigated vines (100% of their daily consumption) and deficit irrigated vines which were submitted to progressive drought by withholding irrigation during a month. We measured leaf water potential and soil water content. Combined measurements of chlorophyll fluorescence and gas exchange were used to calculate mesophyll conductance, efficiency of photosystem II (FPSII), electron transport rate (ETR) and non-photochemical quenching. Finally, we performed a quantitative limitation analysis in order to evaluate the different limitations to photosynthesis. Our results showed a steep decrease in leaf water potential and soil water content after a month of water deficit. At the beginning, stomatal conductance was rapidly affected, whereas mesophyll conductance and photosynthesis were only moderately regulated. Only when drought became severe, mesophyll conductance, FPSII and ETR were highly affected. Our findings showed that under severe water stress, diffusion within the mesophyll and biochemical impairment limited photosynthesis.