Carotenoid composition and down regulation of photosystem II in three conifer species during the winter

The carotenoid composition of the needles of three conifer species. Ponderosa pine (Pinus ponderosa) . Douglas fir ( Pseudotsuga menziesü) and blue spruce (Picea pun—gens) . growing in full sunlight was found to differ between the summer and the winter, with higher levels of lutein and the carotenoids of the xanthophyll cycle and lower levels of α‐carotene being present during the winter. In addition, the extent of the de‐epoxidation of violaxunthin to antheraxanthin and zeaxanthin at midday was greater during the winter than during the summer. The latter two carotenoids were also found to be retained at high levels overnight on cold days during the winter. Associated with the retention of antheraxanlhin and zeaxanthin were sustained depressions of photosystem II (PSII) efficiency. These decreases in photo system II efficiency were accompanied by changes in chlorophyll fluorescence characteristics that are indicative of increased levels of energy dissipation in the chlorophyll pigment bed. Sustained depressions in PSII efficiency are commonly interpreted as “photoinhibitton”. We therefore suggest that low temperature‐induced photo inhibition in these conifers during the winter was due to a down regulation of photosystem II that involved sustained xanthophyll cycle‐associated energy dissipation. Furthermore, the predawn conversion state of the xanthophyll cycle responded rapidly to day to day variation in temperature. Being less epoxidized on colder days and more epoxidized on both previous and subsequent warmer days. Such flexibility in the response of the santhophyll cycle to changes in temperature may be important in the regulation and protection of the photosynthetic apparatus of such evergreen plants in a climate that experiences relatively rapid changes in temperature.