CHILLING TEMPERATURES AND THE XANTHOPHYLL CYCLE - A COMPARISON OF WARM-GROWN AND OVERWINTERING SPINACH

Photoprotective energy dissipation activity, that was largely associated with the deepoxidation of the xanthophyll cycle, was examined in spinach leaves grown outside during the winter versus leaves that had developed at moderate temperatures in a glasshouse. On a leaf area basis the rates of photosynthesis were higher in leaves from the field at all temperatures examined, but were similar in both sets of leaves on a chlorophyll basis. The rate at which energy dissipation activity increased upon sudden exposure to high light was similar for the warm-grown leaves and those growing outside. This rate was futhermore similar to that of the rate of anther-axanthin and zeaxanthin formation, and was similar throughout the winter as long as the pre-dawn level of photosystem II (PSII) efficiency was at a normal high level. Whereas energy dissipation activity developed more rapidly at higher temperatures, the final extent of energy dissipation activity was greater at lower temperatures, where the rate of energy utilisation through photosynthetic electron transport was much lower. On colder days leaves collected predawn from plants growing outside exhibited sustained decreases in PSII efficiency, which were associated with sustained decreases in both maximal and minimal levels of fluorescence. Such characteristics suggest that the leaves exposed to high light on colder days during the winter exhibited sustained energy dissipation activity that remained engaged throughout the night. It is likely that the xanthophyll cycle was involved in this response, as the sustained high levels of energy dissipation activity were found to be associated with sustained high levels, and thus the retention of, zeaxanthin and antheraxanthin overnight.