Seasonal differences in PSII efficiency (Fv/Fm), the conversion state of the xanthophyll cycle (Z + A)/ (V + A + Z), and leaf adenylate status were investigated in Euonymus kiautschovicus. On very cold days in winter, Fv/Fm assessed directly in the field remained low and Z + A high throughout day and night in both sun and shade leaves. Pre-dawn transfer of leaves from subfreezing temperatures in the field to room temperature revealed that recovery (increases in Fv/Fm and conversion of Z + A to violaxanthin) consisted of one, rapid phase in shade leaves, whereas in sun leaves a rapid phase was followed by a slow phase requiring days. The pre-dawn ATP/ADP ratio, as well as that determined at midday, was similar when comparing overwintering leaves with those sampled in the summer, although pre-dawn levels of ATP + ADP were elevated in all leaves during winter relative to summer. After a natural transition to warmer days during the winter, pre-dawn Fv/Fm and Z + A in shade leaves had returned to values typical for summer, whereas in sun leaves Fv/Fm and Z + A levels remained intermediate between the cold day in winter and the summer day. Thus two distinct forms of sustained (Z + A)-dependent energy dissipation were identified based upon their differing characteristics. The form that was sustained on cold days but relaxed rapidly upon warming occurred in all leaves and may result from maintenance of a low lumenal pH responsible for the nocturnal engagement of (Z + A)-dependent thermal dissipation exclusively on very cold days in the winter. The form that was sustained even upon warming and correlated with slow Z + A to violaxanthin conversion occurred only in sun leaves and may represent a sustained engagement of (Z + A)-dependent energy dissipation associated with an altered PSII protein composition. In the latter, warm-sustained form, uncoupler or cycloheximide infiltration had no effect on the slow phase of recovery, but lincomycin infiltration inhibited the slow increase in Fv/Fm and the conversion of Z + A to violaxanthin.