HEAT EMISSION AS A PROTECTIVE MECHANISM AGAINST HIGH-IRRADIANCE STRESS IN SUGAR MAPLE LEAVES

High-irradiance (HI) induced changes in heat emission, fluorescence, a nd photosynthetic energy storage (EST) of shade grown sugar maple (Ace r saccharum Marsh.) saplings were followed using modulated photoacoust ic and fluorescence spectroscopic techniques. HI-treatment at 900-4400 mu mol m(-2) s(-1) for 15 min caused an increase in heat emission and a decrease in EST. In some leaves, HI-treatment of 900 mu mol m(-2) s (-1) for 1 min induced a rapid increase in heat emission with a margin al decrease in EST. Parallel to the increase in heat emission, there w as a decrease in fluorescence, and this phenomenon was reversible in d arkness. Quenching of thermal energy dissipation and a recovery in EST were observed during the first 15 min after the W-treatment. This dow n-regulation of photochemical activity and its recovery may be one of the photoprotective mechanisms in shade grown sugar maple plants. The increase in thermal energy dissipation was greater in the red absorbin g long wavelength (640-700 nm) region than in the blue absorbing short wavelength region of photosynthetically active excitation radiation. The photochemical activity was affected more in short wavelengths (400 -520 nm) than in the long wavelength region of the spectrum. This can be due to the migration of light-harvesting chlorophyll (Chl) a/b prot ein complex from photosystem (PS) 2 to PS1 and/or to the disconnection of carotenoid pool from Chls in the pigment bed of photosynthetic app aratus.