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

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Abstract
We review the photosynthetic responses to drought in field‐grown grapevines and other species. As in other plant species, the relationship between photosynthesis and leaf water potential and/or relative water content in field‐grown grapevines depends on conditions during plant growth and measurements. However, when light‐saturated stomatal conductance was used as the reference parameter to reflect drought intensity, a common response pattern was observed that was much less dependent on the species and conditions. Many photosynthetic parameters (e.g. electron transport rate, carboxylation efficiency, intrinsic water‐use efficiency, respiration rate in the light, etc.) were also more strongly correlated with stomatal conductance than with water status itself. Moreover, steady‐state chlorophyll fluorescence also showed a high dependency on stomatal conductance. This is discussed in terms of an integrated down‐regulation of the whole photosynthetic process by CO2 availability in the mesophyll. A study with six Mediterranean shrubs revealed that, in spite of some marked interspecific differences, all followed the same pattern of dependence of photosynthetic processes on stomatal conductance, and this pattern was quite similar to that of grapevines. Further analysis of the available literature suggests that the above‐mentioned pattern is general for C3 plants. Even though the patterns described do not necessarily imply a cause and effect relationship, they can help our understanding of the apparent contradictions concerning stomatal vs. non‐stomatal limitations to photosynthesis under drought. The significance of these findings for the improvement of water‐use efficiency of crops is discussed.