HIGH-IRRADIANCE STRESS AND PHOTOCHEMICAL ACTIVITIES OF PHOTOSYSTEM-1 AND PHOTOSYSTEM-2 IN-VIVO

High-irradiance (PII) stress induced changes in the photosynthetic ene rgy 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 o f about 40 % in ESPS2 and an enhancement of about 60 % in ESPS1 The ra te of PSI cyclic electron transport, measured with 705 nm modulated ra diation, also increased in HI-treated leaves. There was a clear state 1-state 2 transition in HI-treated leaves. ESPS1 increased significant ly and ESPS2 decreased drastically in leaves preadapted to state 1 aft er HI (600 W m(-2), 30 min) treatment. Thus, the increase in PS1 activ ity observed immediately after HI-treatment in leaves preadapted to st ate 1 can be due to the coupling of LHC2 to PS1 during the HI-treatmen t. 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.