PHOTOINHIBITION, XANTHOPHYLL CYCLE AND IN-VIVO CHLOROPHYLL FLUORESCENCE QUENCHING OF CHILLING-TOLERANT OXYRIA-DIGYNA AND CHILLING-SENSITIVEZEA-MAYS

The relationship between susceptibility to photoinhibition, zeaxanthin formation and chlorophyll fluorescence quenching at suboptimal temper atures was studied in chilling-sensitive maize and in non-acclimated a nd cold-acclimated Oxyria digyna, a chilling-tolerant plant of arctic and alpine habitats. In maize, zeaxanthin formation was strongly suppr essed by chilling. Zeaxanthin formed during preillumination at 20 degr ees C did not protect maize leaves from photoinhibition during a subse quent high-light, low-temperature treatment, as judged from the ratios of variable to maximal fluorescence, F-v/F-m. However, such preillumi nation significantly increased non-photochemical quenching (q(N)) at l ow temperatures, mainly due to an enhancement of the fast-relaxing q(N ) component (i.e., of energy-dependent quenching, q(E)). In O. digyna, cold-acclimation resulted in an increased zeaxanthin formation in the temperature range of 2.5-20 degrees C. Cold-acclimation substantially decreased the susceptibility towards photoinhibition at 4 degrees C, but q(N) remained nearly unchanged between 2 and 38 degrees C, as comp ared to control plants. Effects of cold acclimation on photosynthesis, photochemical quenching and quantum efficiency of photosystem II were small and indicated a slight amelioration only of the function of the photosynthetic apparatus at suboptimal temperatures (2-20 degrees C). It is concluded, that the xanthophyll cycle is strongly influenced by cold acclimation, while effects on the photosynthetic carbon assimila tion only play a minor role in O. digyna.