PHOTOCHEMICAL EFFICIENCY OF PHOTOSYSTEM-II AND XANTHOPHYLL CYCLE COMPONENTS IN ZEA-MAYS LEAVES EXPOSED TO WATER-STRESS AND HIGH LIGHT

The effects of two light treatments (photosynthetically active photon flux density of either 650 or 1950 mu mol m(-2) s(-1)) on the photoche mical efficiency of Photosystem II (PS II) (measured as variable to ma ximum fluorescence ratio) and on the xanthophyll cycle components was studied in wilted Zea mays leaves. For comparison, these parameters we re followed under the same light conditions in well-hydrated leaves ma intained either in normal or CO2-free air. The net CO2 assimilation of dehydrated leaves declined rapidly as their relative water content (R WC) decreased from 100 to 60% while the PS II efficiency measured afte r a prolonged dark period of 16 h declined only when RWC leaves was lo wer than 60%. Furthermore, drought caused an increase in the pool size of the xanthophyll cycle pigments and the presence of a sustained ele vated level of zeaxanthin and antheraxanthin at the end of the long da rk period. The leaf water deficit enhanced the sensitivity of PS II ef ficiency to light exposure. During illumination, strong inhibition of PS II efficiency and large violaxanthin deepoxidation was observed in wilted leaves even under moderate photon flux density compared to cont rol leaves in the same conditions. After 2 h of darkness following the light treatment, the PS II efficiency that is dependent on the previo us PPFD, decreased with leaf water deficit. Moreover, zeaxanthin epoxi dation led to an accumulation of antheraxanthin in dehydrated leaves, All these drought effects on PS II efficiency and xanthophyll cycle co mponents were also obtained in well-hydrated leaves by short-term CO2 deprivation during illumination. We conclude that the increased suscep tibility of PS II efficiency to light in wilted maize leaves is mainly explained by the decrease of CO2 availability and the resulting low n et CO2 assimilation.