THE AFTEREFFECT OF HARDENING TEMPERATURES ON THE NET PHOTOSYNTHESIS RATE OF INTACT PLANTS

The effects of cold and heat hardening treatments, differing in their duration, on the net photosynthesis rate of intact plants of cucumber (Cucumis sativus L.) and Pe-tsai cabbage (Brassica pekinensis (Lour.) Rupr.) in the phase of three leaves were studied in two-factor experim ents at a illuminance of 8 klx and an ambient CO2 concentration of 0.0 5%. When, after an exposure to hardening for no more than five hours, the acclimated plants were returned to the initial growth temperature, cold hardening was found to decrease the CO2 uptake of cucumber and t o increased the net photosynthesis rate, as compared to the initial le vel. Warm hardening temperatures also suppressed photosynthesis and in creased its level after recovery. However, in contrast to cold hardeni ng, the duration of the heat hardening treatment had only a slight eff ect on the subsequently recovered net photosynthesis rate. In two plan t species, the boundaries of the temperature zones, as determined by m easuring tissue thermoresistance, coincided with the characteristic po ints of the generalized curve of temperature aftereffect on net photos ynthesis.