PHOTOSYNTHETIC LIGHT-HARVESTING FUNCTION OF CAROTENOIDS IN HIGHER-PLANT LEAVES EXPOSED TO HIGH LIGHT IRRADIANCES

Exposure of barley (Hordeum vulgare L.) leaves to strong white light ( 1500 mu mol photons m(-2).s(-1)) decreased the quantum yield of photos ynthetic oxygen evolution in green light preferentially absorbed by ca rotenoids (Phi-510) but not in red light exclusively absorbed by chlor ophylls (Phi-650). This phenomenon was observed to be (i) rapidly indu ced (within a few minutes), (ii) slowly reversible in darkness (within about 1 h), (iii) insensitive to dithiothreitol and (iv) maximally in duced by photon flux densities higher than about 1000 mu mol.m(-2).s(- 1). Determination of the carotenoid composition of the major light-har vesting complex of PSII (LHCII) and analysis of the thylakoid membrane lipid fluidity before and after strong illumination of barley leaves in the presence or the absence of dithiothreitol showed that the light -induced decrease in the Phi-510/Phi-650 ratio did not require the phy sical detachment of carotenoids from the pigment antennae. Compared to barley plants grown under moderate light and temperature conditions, plants grown in sustained high irradiance at elevated temperature exhi bited (i) a lower Phi-510/Phi-650 ratio, (ii) a reduced size of the fu nctional PSII pigment antenna in green light (but not in red light) an d (iii) a marked increase in the amount of free carotenoids found in n on-denaturing Deriphat-containing electrophoretic gels of thylakoid me mbranes. Similarly, the Phi-510/Phi-650 ratio of the LHCII-deficient c hlorina-f2 barley mutant was very low compared to the wild type. Separ ation and quantification of the cis/trans carotenoid isomers of barley leaves revealed that strong illumination did not induce pronounced ci s-trans isomerization of xanthophylls. Taken together, the data sugges t that the efficiency of energy transfer from carotenoids to chlorophy lls varies with the light environment both in the short term and in th e long term, with excess light energy noticeably inhibiting the photos ynthetic light-harvesting function of carotenoids. The photoprotective significance of this carotenoid decoupling from the chlorophyll anten nae is discussed.