TEMPERATURE-DEPENDENT MODULATION OF THE PHOTOINHIBITION-SENSITIVITY OF PHOTOSYSTEM-II IN SOLANUM-TUBEROSUM LEAVES

High temperature (38 degrees C) dramatically exacerbated the photoinhibition of photosystem II in 23 degrees C-grown potato leaves. This work shows the existence of an adaptive mechanism that cancels the synergism between heat and strong light and markedly enhances the resistance of photosystem II to photodamage. Photoinhibition-resistance of photosystem II was triggered by exposing leaves in weak light to moderately elevated, non-injurious temperatures in the range 32-38 degrees C and was established within about 20 min, thus providing a rapid process of photosynthetic adaptation to simultaneous increase in temperature and light intensity. The reversal at 23 degrees C occurred on a much longer time scale of several hours. Temperature-induced increase in PSII photoresistance did not require de novo protein synthesis, as judged from its insensitivity to cycloheximide and chloramphenicol, and was not accompanied with any apparent change in the antenna size of photosystem II. From the presented chlorophyll fluorescence data, one can postulate that temperature-dependent regulation of the photoinhibition-sensitivity of photosystem II could primarily involve a change in the PSII reaction center, the nature of which remains to be established.