PHOTOSYSTEM-II EXCITATION PRESSURE AND DEVELOPMENT OF RESISTANCE TO PHOTOINHIBITION .1. LIGHT-HARVESTING COMPLEX-II ABUNDANCE AND ZEAXANTHIN CONTENT IN CHLORELLA-VULGARIS

The basis of the increased resistance to photoinhibition upon growth a t low temperature was investigated. Photosystem II (PSII) excitation p ressure was estimated in vivo as 1 - q(p) (photochemical quenching). W e established that Chlorella vulgaris exposed to either 5 degrees C/15 0 mu mol m(-2) s(-1) or 27 degrees C/2200 mu mol m(-2) s(-1) experienc ed a high PSII excitation pressure of 0.70 to 0.75. In contrast, Chlor ella exposed to either 27 degrees C/150 mu mol m(-2) s(-1) or 5 degree s C/20 mu mol m(-2) s(-1) experienced a low PSII excitation pressure o f 0.10 to 0.20. Chlorella grown under either regime at high PSII excit ation pressure exhibited: (a) 3-fold higher light-saturated rates of O -2 evolution; (b) the complete conversion of PSII alpha centers to PSI I beta centers; (c) a 3-fold lower epoxidation state of the xanthophyl l cycle intermediates; (d) a 2.4-fold higher ratio of chlorophyll a/b; and (e) a lower abundance of light-harvesting polypeptides than Chlor ella grown at either regime at low PSII excitation pressure. In additi on, cells grown at 5 degrees C/150 mu mol m(-2) s(-1) exhibited resist ance to photoinhibition comparable to that of cells grown at 27 degree s C/2200 mu mol m(-2) s(-1) and were 3- to 4-fold more resistant to ph otoinhibition than cells grown at either regime at low excitation pres sure. We conclude that increased resistance to photoinhibition upon gr owth at low temperature reflects photosynthetic adjustment to high exc itation pressure, which results in an increased capacity for nonradiat ive dissipation of excess light through zeaxanthin coupled with a lowe r probability of light absorption due to reduced chlorophyll per cell and decreased abundance of light-harvesting polypeptides.