TEMPERATURE-SENSITIVE COUPLING AND UNCOUPLING OF ATPASE-MEDIATED, NONRADIATIVE ENERGY-DISSIPATION - SIMILARITIES BETWEEN CHLOROPLASTS AND LEAVES

The effects of temperature on the dark relaxation kinetics of nonradia tive energy dissipation in photosystem II were compared in lettuce (La ctuca sativa L.) chloroplasts and leaves of Aegialitis annulata R. Br. After high levels of violaxanthin de-epoxidation in the light, Aegial itis leaves showed a marked delay in the dark relaxation of nonradiati ve dissipation, measured as nonphotochemical quenching (NPQ) of photos ystem II chlorophyll a fluorescence. Aegialitis leaves also maintained a moderately high adenylate energy charge at low temperatures during and after high-light exposure, presumably because of their limited car bon-fixation capacity. Similarly, dark-sustained NPQ could be induced in lettuce chloroplasts after de-epoxidizing violaxanthin and light-ac tivating the ATP synthase. The duration and extent of dark-sustained N PQ were strongly enhanced by low temperatures in both chloroplasts and leaves. Further, the NPQ sustained at low temperatures was rapidly re versed upon warming. In lettuce chloroplasts, low temperatures sharply decreased the ATP-hydrolysis rate while increasing the duration and e xtent of the resultant trans-thylakoid proton gradient that elicits th e NPQ. This was consistent with a higher degree of energy-coupling, pr esumably due to reduced proton diffusion through the thylakoid membran e at the lower temperatures. The chloroplast adenylate pool was in equ ilibrium with the adenylate kinase and therefore both ATP and ADP cont ributed to reverse coupling, The low-temperature-enhanced NPQ quenched the yields of the dark level (F-o) and the maximal (F-m) fluorescence proportionally in both chloroplasts and leaves. The extent of NPQ in the dark was inversely related to the efficiency of photosystem II, an d very similar linear relationships were obtained over a wide temperat ure range in both chloroplasts and leaves. Likewise, the dark-sustaine d absorbance changes, caused by violaxanthin de-epoxidation (A(508 nm) ) and energy-dependent light scattering (A(536) nm) were strikingly si milar in chloroplasts and leaves. Therefore, we conclude that the dark -sustained, low-temperature-stimulated NPQ in chloroplasts and leaves is apparently directly dependent on lumen acidification and chloroplas tic ATP hydrolysis. In leaves, the ATP required for sustained NPQ is e vidently provided by oxidative phosphorylation in the mitochondria. Th e functional significance of this quenching process and implications f or measurements of photoprotection versus photodamage in leaves are di scussed.