STOMATAL AND NONSTOMATAL LIMITATIONS TO PHOTOSYNTHESIS IN 2 WHEAT CULTIVARS SUBJECTED TO WATER-STRESS

Two wheat (Triticum aestivum L.) cultivars, Trakia (drought-sensitive, DS) and Slavianka 196 (drought-tolerant, DT), were subjected to water stress induced by polyethyleneglycol (PEG) 6000: 15 % PEG for 6 h (mi ld stress); 25 % PEG for 6 h and 15 % PEG for 24 h (severe stress). Ex posure of plants to water stress led to a noticeable decrease in both the initial slope of net photosynthetic rate to intercellular CO2 conc entration (P-N/c(i)) curves and the maximum P-N Ribulose 1,5-bisphosph ate carboxylase (RuBPC) activity was almost unchanged under mild stres s while under severe stress it was reduced by about 26-27 %. The ratio of variable to maximal chlorophyll fluorescence did not change which implied that there was little effect of examined stress conditions on the photosystem 2 electron transport. The relative magnitude of stomat al and nonstomatal factors in limitation of photosynthesis depended on stress severity.