Heat-induced changes of chlorophyll fluorescence in intact leaves correlated with damage of the photosynthetic apparatus.

Methods were developed to measure chlorophyll fluorescence yield of intact leaf tissue during heat treatment under varying conditions of light intensity and photosynthetic activity. Fluorescence yield of a dark-adapted leaf increases by 2- to 3-fold with an increase of temperature into the region where heat-damage occurs. The temperatures of the fluorescence transition correlate well with the temperatures where quantum yield of CO2 fixation is irreversibly depressed. Fluorescence-temperature (F-T) curves allow ranking of different species according to their heat sensitivity. Within a single species acclimation to different growth temperatures is reflected by shifts of the transition temperatures in the F-T curves. When F-T curves are recorded in the steady light states at increasing light intensities, substantial shifts (up to 6°C) of transition temperatures to higher values are observed. Quantum yield measurements of CO2 fixation confirm that hight-light conditions protect from heat-damage. It is suggested that chlorophyll acts as an intrinsic fluorescence probe of the thylakoid membrane and responds to the same changes which cause irreversible denaturation of photosynthetic enzymes.