DAILY DYNAMICS OF PHYSIOLOGICAL PROCESSES IN SUCCULENT HALOPHILIC PLANTS WITH THE C-3 AND C-4 TYPES OF PHOTOSYNTHESIS FROM THE ARID ZONE OFCENTRAL-ASIA

We studied the daily dynamics of photosynthesis, pH value, and titrata ble acidity of the cell sap in the succulent halophytes Suaeda paradox a, Halostachys caspica, and Lycium rutencium (C3 species); and Climaco ptera transoxana, S. arcuata, and S. microphylla (C4 species) during t he summer in Southern Tadzhikistan. All of the species exhibited a sim ilar dependency of the daily course of photosynthesis: assimilation of CO2 was high throughout the entire light period with a maximum at 10: 00-12:00 h and decline of CO2 fixation at midday. The rate of nighttim e CO2 fixation did not exceed 1.3-3.6% (C4 species) or 1.7-2.9% (C3 sp ecies) of daylight photosynthesis. The level of dark fixation depended on neither the potential activity of PEP carboxylase (C3 and C4 plant s) nor its activity in water-bearing tissue (C4 species). The range of daily oscillations of acidity (DELTApH) was insignificantly higher in the group of C4 plants (0.5-0.8) than in C3 species (0.3-0.7). Regard less of the type of photosynthesis, maximal acidification was recorded during the period of transition from light to darkness (20:00-23:00 h ), acidification being registered during the period of active photosyn thesis before noon in C4 Species. The investigated species do not poss ess CAM photosynthesis, but elevated heterotrophic CO2 fixation and ex tensive pools of organic acids possibly enable them to maintain high o smotic pressure and synthesize protective substances, which is of grea t significance for life under conditions of solonchaks.