[Freezing tolerance of winter wheat plants depends on adaptation of photosynthesis and respiration in different time intervals].

This study is devoted to CO2 gas exchange (true photosynthesis at light saturation (P), dark respiration (R), and P/R ratio) in vegetating and cold-hardened winter wheat (Triticum aestivum L.) plants (cultivar Mironovskaya 808) in relation to their freezing tolerance. Under natural cultivation conditions, freezing tolerance of plants depended on adaptive changes in the shape of P and R curves in the temperature range from 20 to -2 degrees C. These changes, induced by cold hardening and treatment of plants with the photosynthesis inhibitor diuron, were observed within month and week ranges. Under laboratory conditions, the P/R ratio in vegetating plants increased three times within an hour range as the temperature decreased from 22 to 0 degree C. The P/R ratio also decreased within a minute range as a result of partial inhibition of photosynthesis with diuron and immediately decreased when CO2 concentration in the air was reduced from 419 to 0 microl/l. The P/R ratio decreased primarily at the expense of a decrease in P. The decrease in P/R was more pronounced at low temperatures, indicating variability of low-temperature tolerance of photosynthesis within a minute range. The possibility of plant adaptation to nonsimultaneous temperature changes under natural conditions via adaptive changes in temperature tolerance of the photosynthetic apparatus is discussed.