PHOTOSYNTHETIC ACCLIMATION TO LOW-TEMPERATURE BY WESTERN RED CEDAR SEEDLINGS

Imposition of low, but above freezing, temperatures resulted in a grad ual increase in the cold hardiness of western red cedar seedlings. Thi s was associated with a decrease in the maximum rates of photosyntheti c CO2 fixation and O2 evolution, and changes in chlorophyll a fluoresc ence transients which indicated that photoinhibition had occurred. Max imum -photosynthetic rates declined approximately 40% during cold hard ening. The leaves changed colour from green to red-brown during the ha rdening process. The colour change was due to the synthesis of large a mounts of the carotenoid rhodoxanthin. Lutein levels doubled, while ch lorophyll declined slightly. Dehardening resulted in the rapid recover y of photosynthesis to control levels, the rapid disappearance of rhod oxanthin, and the return of lutein levels to control. It is suggested that rhodoxanthin accumulation at low temperature functions to decreas e the light intensity reaching the photosynthetic apparatus. The combi nation of photoinhibition and rhodoxanthin synthesis probably serves t o protect the photosynthetic capacity of the seedlings at low temperat ure.