High-irradiance (HI) induced changes in heat emission, fluorescence, and photosynthetic energy storage (EST) of shade grown sugar maple (Acer saccharum Marsh.) saplings were followed using modulated photoacoustic and fluorescence spectroscopic techniques. HI-treatment at 900-4400 µmol 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 µmol m-2 s-1 for 1 min induced a rapid increase in heat emission with a marginal decrease in EST. Parallel to the increase in heat emission, there was a decrease in fluorescence, and this phenomenon was reversible in darkness. Quenching of thermal energy dissipation and a recovery in EST were observed during the first 15 min after the HI-treatment. This down-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 absorbing 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 protein complex from photosystem (PS) 2 to PS1 and/or to the disconnection of carotenoid pool from Chls in the pigment bed of photosynthetic apparatus. and K. Veeranjaneyulu, R. M. Leblanc.
High-irradiance (HI) stress induced changes in the photosynthetic energy storage (ES) of photosystems 1 (ESPS1) and 2 (ESPS2) were studied with 650 nm modulated radiation in intact sugar maple (Acer saccharum Marsh.) leaves. HI-treatment (420 W m-2, 1 h) caused an inhibition of about 40 % in ESPS2 and an enhancement of about 60 % in ESPS1. The rate of PS1 cyclic electron transport, measured with 705 nm modulated radiation, also increased in HI-treated leaves. There was a clear state 1- state 2 transition in HI-treated leaves. ESPS1 increased significantly and ESPS2 decreased drastically in leaves preadapted to state 1 after HI (600 W m-2, 30 min) treatment. Thus, the increase in PS1 activity observed immediately after HI-treatment in leaves preadapted to state 1 can be due to the coupling of LHC2 to PS1 during the HI-treatment. Further, the dissociation of LHC2 from PS2 during the HI-treatment resulted in apparently (about 15 %) greater inhibition than the "true" inhibition of PS2 activity. The presence of LHC2 with PS2 (state 1) at the time of HI-treatment caused no additional damage to PS2 or its coupling to PS1 offered no apparent HI-treatment. Further, the dissociation of LHC2 from PS2 during the HI-treatment resulted in apparently (about 15 %) greater inhibition than the "true" inhibition of PS2 activity. The presence of LHC2 with PS2 (state 1) at the time of HI-treatment caused no additional damage to PS2 or its coupling to PS1 offered no apparent protection to the photosynthetic apparatus. and K. Veeranjaneyulu, M. Charland, R. M. Leblanc.