PHOTOSYNTHESIS IN WATER-STRESSED WHEAT - PROTECTIVE EFFECTS OF CYTOKININS

The intensity of photosynthetic CO2 assimilation, the activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), the contents of chlorophylls a and b, and the size of the endogenous pool of free abscisic acid were studied in winter wheat Triticum aestivum L. subjected to water stress induced by discontinuing irrigation (for 10 days) or administering 1, 2, or 3% polyethylene glycol-6000 (for 7 - 14 days). Photosynthetic CO2 assimilation and the activity of Rubisco were considerably suppressed. These effects correlated with the increase in the pool of free abscisic acid in above-ground and underground organs of the plant. The chlorophyll alb ratio decreased because of predominant decreases in the level of chlorophyll a. Photosynthetic CO2 assimilation increased over 7 days of recovery (when irrigation was resumed), but did not return to the baseline level. The cytokinin 6-benzylaminopurine, as well as Thidiazuron and Kartolin-2 (the latter produced a stronger effect than the former), which both act as cytokinins, considerably decreased the inhibitory effects of water stress on photosynthesis.