2 FORMS OF SUSTAINED XANTHOPHYLL CYCLE-DEPENDENT ENERGY-DISSIPATION IN OVERWINTERING EUONYMUS-KIAUTSCHOVICUS

Seasonal differences in PSII efficiency (F-v/F-m), the conversion stat e of the xanthophyll cycle (Z + A)/(V + A + Z), and leaf adenylate sta tus were investigated in Euonymus kiautschovicus, On very cold days in winter F-v/F-m assessed directly in the field remained low and Z + A high throughout day and night in both sun and shade leaves. Pre-dawn t ransfer of leaves from subfreezing temperatures in the field to room t emperature revealed that recovery (increases in F-v/F-m and conversion of Z + A to violaxanthin) consisted of one, rapid phase in shade leav es, whereas in sun leaves a rapid phase was followed by a slow phase r equiring days. The predawn ATP/ADP ratio, as well as that determined a t midday, was similar when comparing overwintering leaves with those s ampled in the summer, although pre-dawn levels of ATP + ADP were eleva ted in all leaves during winter relative to summer. After a natural tr ansition to warmer days during the winter, pre-dawn F-v/F-m and Z + A in shade leaves had returned to values typical for summer, whereas in sun leaves F-v/F-m and Z + A levels remained intermediate between the cold day in winter and the summer day, Thus two distinct forms of sust ained (Z + A)-dependent energy dissipation were identified based upon their differing characteristics. The form that was sustained on cold d ays but relaxed rapidly upon warming occurred in all leaves and may re sult from maintenance of a low lumenal pH responsible for the nocturna l engagement of (Z + A)-dependent thermal dissipation exclusively on v ery cold days in the winter. The form that was sustained even upon war ming and correlated with slow Z + A to violaxanthin conversion occurre d only in sun leaves and may represent a sustained engagement of (Z A)-dependent energy dissipation associated with an altered PSII protei n composition. In the latter, warm-sustained form, uncoupler or cycloh eximide infiltration had no effect on the slow phase of recovery, but lincomycin infiltration inhibited the slow increase in F-v/F-m and the conversion of Z + A to violaxanthin.