SEASONAL-CHANGES IN XANTHOPHYLL CYCLE-DEPENDENT ENERGY-DISSIPATION INYUCCA-GLAUCA NUTTALL

The evergreen species Yucca glauca was characterized at the end of Sep tember and following exposure to low temperatures at the end of Novemb er. In November the diurnal pattern of xanthophyll cycle-dependent ene rgy dissipation was altered such that this thermal dissipation process was engaged at a high level throughout the day, whereas in September it only became engaged when leaves received direct sunlight. An analys is of the diurnal partitioning of the absorbed excitation energy into photochemistry versus thermal dissipation suggested that a smaller fra ction of that energy was utilized in photochemistry and a greater frac tion was dissipated thermally at the end of November compared to Septe mber. Lower ratios of Chi alb and beta-carotene/xanthophylls both sugg ested a decrease in the ratio of reaction centre plus core antenna pro teins compared to light-harvesting proteins, and a lower leaf chloroph yll content suggested a decrease in light-harvesting capacity in Novem ber versus September. Thus adjustments to the photosynthetic apparatus occurred on several levels in response to the increase in excess exci tation energy that Y. glauca experienced during the onset of winter.