CHANGES IN PHOTOSYNTHETIC PIGMENTS AND IN-VIVO CHLOROPHYLL FLUORESCENCE PARAMETERS UNDER PHOTOINHIBITORY GROWTH-CONDITIONS

Changes in the chlorophyll (Chl) and carotenoid (Car) contents and photosynthetic activity of leaves of three-year-old maple trees were studied under an extremely high irradiance (HI) (5 000 and 7 000 mumol m-2 s-1) and subsequent low irradiance (LI) (10 mumol m-2 s-1). Special attention was paid to a possible linear correlation between zeaxanthin (z) accumulation and the decrease in variable Chl fluorescence (ratio F(v)/F(m)) during photoinhibitory treatment. HI-induced violaxanthin (v) transformation into z was a fast one-step response proceeding within 2-5 min. Changes in the Chl a and b contents or in non-xanthophyll cycle Car were either not observed (30 min at 5 000 mumol m-2 s-1) or a slight decline of the Chl a, lutein, beta-carotene and z contents only occurred at prolonged exposure (60 min at 7 000 mumol m-2 s-1). Photosynthetic activity measured via the ratios (F(v)/F(m), F(v)/F(o), R(fd) 690, R(fd) 735) sank during this treatment to a much higher extent at 7 000 than at 5 000 mumol m-2 s-1. Ratios F(v)/F(o), R(fd) 690 and R(fd) 735 proved to be better indicators of photoinhibition with a larger amplitude than the ratio F(v)/F(m). Unlike literature reports, no linear correlation was found between HI-induced decrease of fluorescence ratios and the z accumulation. Decrease of the fluorescence ratios appears to be biphasic in nature: fast initial fall which parallels the z formation is followed by a phase when ratios F(v)/F(m), F(v)/F(o), R(fd) 690 and R(fd) 735 sink further without any changes in the z pool. Regeneration of photosynthetic activity under LI also proceeded in two steps: a relatively fast increase in ratios with little or no changes in the z pool followed by a slow many hours' restoring of the former fluorescence ratios, paralleled by the z transformation into v. The back-reaction of z to v under LI was increasingly retarded with higher irradiance. Z formation and decrease in the fluorescence ratios are thus probably two HI-induced processes which may be mutually independent.