COMPARATIVE-STUDIES OF PLANT-RESPONSE TO HIGH HARDENING AND INJURING TEMPERATURES

Cucumber and wheat seedlings were employed to investigate the time-cou rse of cell thermotolerance changes in leaves subjected to high harden ing and injuring temperatures in the presence of the inhibitors of RNA (actinomycin D) and protein (cycloheximide) synthesis. Both inhibitor s prevented the increase in thermotolerance under hardening temperatur es (38-degrees-C for cucumbers and 40-degrees-C for wheat) and failed to influence the development of tolerance under injuring temperatures (48-degrees-C for cucumbers and 43-degrees-C for wheat). Under the com bined action of the hardening and injuring temperatures on the seedlin gs, this response was maintained irrespective of the order in which th e two temperatures followed each other. Following a series of succeedi ng injuring and hardening temperature actions, cell thermotolerance un der the physiologically normal temperature was shown to depend conside rably on the length of exposure to the hardening temperature. These ef fects seem to depend on different mechanisms that produce thermotolera nce under hardening and injuring temperatures.