Correlation between the Maintenance of Photosynthesis and in Situ Protoplast Volume at Low Water Potentials in Droughted Wheat

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Abstract
Studies were undertaken to examine the relationship between water deficit effects on photosynthesis and the extent of protoplast volume reduction which occurs in leaves at low water potential (.PSI.w). This relationship was monitored in two cultivars (''Condor'' and ''Capelle Desprez'') of cultivated wheat (Triticum aestivum) that differed in sensitivity to drought, and in a wild relative of cultivated wheat (Triticum kotschyi) that has been previously found to be ''drought resistant.'' When subjected to periods of water stress, Condor and T. kotschyi plants underwent osmotic adjustments; Capelle plants did not. Photosynthetic capactiy was maintained to different extents in the three genotypes as leaf .PSI.w declined during stress; Capelle plants were most severaly affected. Calculations of internal leaf [CO2] and stomatal conductance from gas exchange measurements indicated that differences in photosynthetic inhibition at low .PSI.w among the genotypes were primarily due to nonstomatal effects. The extent of protoplast volume reduction that occurred in leaves at low .PSI.w was also found to be different in the three genotypes; maintenance of protoplast volume and photosynthetic capacity in stressed plants of the genotypes appeared to be correlated. When the extent of water stress-induced inhibition of photosynthesis was plotted as a function of declining protoplast volume, this relationship appeared identical for the three genotypes. It was concluded that there is a correlative association between protoplast volume and photosynthetic capacity in leaves of wheat plants subjected to periods of water stress.