INCREASED XANTHOPHYLL CYCLE ACTIVITY AND REDUCED D1 PROTEIN INACTIVATION RELATED TO PHOTOINHIBITION IN 2 PLANT-SYSTEMS ACCLIMATED TO EXCESSLIGHT

Young and mature canopy leaves of tropical forest trees (Anacardium ex celsum, Castilla elastica) and cold- and nonacclimated leaves of spina ch (Spinacia oleracea) grown in temperate climate were used as systems of differing susceptibility to high light stress to assess the role o f xanthophyll cycle activity and D1 protein turnover in photoinhibitio n of photosynthesis. Previous investigations with spinach revealed two distinct stages of photoinhibition (indicated by a decline in the rat io of variable to maximum chlorophyll fluorescence) which were reflect ed in different kinetics of recovery in low light. An initial fast rec overy phase of unknown biochemical mechanism (about 1 h) was followed by a slow phase (several hours), which may be based upon resynthesis o f the D1 protein in the photosystem II reaction center. All leaves stu died exhibited these two phases. However, the kinetics and relative am plitude of the two phases varied strongly and were dependent on leaf a ge and light acclimation. Recovery was rapid in young canopy sun leave s and cold-acclimated leaves of spinach due to a pronounced initial ph ase. On the other hand, the slow, probably D1 related phase dominated in mature canopy leaves and non-acclimated leaves of spinach. The fast phase of recovery and epoxidation of zeaxanthin via the xanthophyll c ycle were closely correlated in all leaves studied, In addition, recov ery following photoinhibition in the presence of dithiothreitol, which prevented formation of zeaxanthin, occurred only in a slow phase. On the other hand, leaf incubation prior to photoinhibition with streptom ycin, an inhibitor of chloroplast-encoded protein synthesis, eliminate d slow recovery by preventing resynthesis of the D1 protein. The young tropical sun leaves and the cold-acclimated spinach leaves exhibited a higher pool of xanthophyll cycle pigments per chlorophyll and in res ponse to strong light converted a higher percentage of violaxanthin to zeaxanthin than the mature and the non-acclimated leaves, respectivel y. Quantification of D1 protein in photoinhibited spinach leaves by me ans of electrophoresis and Western blotting revealed a strongly dimini shed D1 protein degradation in the cold-acclimated compared to the non -acclimated state, High xanthophyll cycle activity and pool sizes obse rved in response to high light with young canopy and cold-acclimated s pinach leaves may serve to protect these leaves against inactivation o f the D1 protein.