Regulation of Photosynthesis of C3 Plants in Response to Progressive Drought: Stomatal Conductance as a Reference Parameter

We review the photosynthetic responses to drought in field-growngrapevines and other species. As in other plant species, therelationship between photosynthesis and leaf water potentialand/or relative water content in field-grown grapevines dependson conditions during plant growth and measurements. However,when light-saturated stomatal conductance was used as the referenceparameter to reflect drought intensity, a common response patternwas observed that was much less dependent on the species andconditions. Many photosynthetic parameters (e.g. electron transportrate, carboxylation efficiency, intrinsic water-use efficiency,respiration rate in the light, etc.) were also more stronglycorrelated with stomatal conductance than with water statusitself. Moreover, steady-state chlorophyll fluorescence alsoshowed a high dependency on stomatal conductance. This is discussedin terms of an integrated down-regulation of the whole photosyntheticprocess by CO₂ availability in the mesophyll. A study with sixMediterranean shrubs revealed that, in spite of some markedinterspecific differences, all followed the same pattern ofdependence of photosynthetic processes on stomatal conductance,and this pattern was quite similar to that of grapevines. Furtheranalysis of the available literature suggests that the above-mentionedpattern is general for C₃ plants. Even though the patterns describeddo not necessarily imply a cause and effect relationship, theycan help our understanding of the apparent contradictions concerningstomatal vs. non-stomatal limitations to photosynthesis underdrought. The significance of these findings for the improvementof water-use efficiency of crops is discussed.