Stomatal conductance regulates photosynthesis under progressive drought: from grapevines to a generalised pattern

The present work reviews some findings of our research group on photosynthetic responses to drought in field-grown grapevines, which are extended to other species. Down-regulation of photosynthesis as a function of leaf water potential was dependent on the conditions during growth and measurements in grapevines. However, a uniform response of many photosynthetic parameters (electron transport rate, carboxylation efficiency, intrinsic water-use-efficiency, respiration rate in the light, etc) to the light-saturated stomatal conductance was observed under different conditions. This suggests that, under drought, different photosynthetic processes may be regulated by CO2 availability in the chloroplasts rather than by leaf water status. Moreover, it was shown that stomatal conductance and net photosynthesis correlated with steady-state chlorophyll fluorescence, which was interpreted in terms of an integrated down-regulation of the whole photosynthetic process in response to drought. A later study with different Mediterranean shrubs revealed that all them presented the same pattern of photosynthetic dependence on stomatal conductance, and such a pattern was very similar to that of grapevines. Further analysis of the available literature suggests that the above mentioned pattern is quite general in C3 plants, and it helps solving apparent contradictions about the predominance of stomatal and non-stomatal limitations to photosynthesis under drought.