ENHANCED EMPLOYMENT OF THE XANTHOPHYLL CYCLE AND THERMAL-ENERGY DISSIPATION IN SPINACH EXPOSED TO HIGH LIGHT AND N STRESS

The involvement of the xanthophyll cycle in photoprotection of N-defic ient spinach (Spinacia oleracea L. cv Nobel) was investigated. Spinach plants were fertilized with 14 mM nitrate (control, high N) versus 0. 5 mM (low N) fertilizer, and grown under both high- and low-light cond itions. Plants were characterized from measurements of photosynthetic oxygen exchange and chlorophyll fluorescence, as well as carotenoid an d chlorophyll analysis. Compared with the high-N plants, the low-N pla nts showed a lower capacity for photosynthesis and a lower chlorophyll content, as well as a lower rate of photosystem It photosynthetic ele ctron transport and a corresponding increase in thermal energy dissipa tion activity measured as nonphotochemical fluorescence quenching. The low-N plants displayed a greater fraction of the total xanthophyll cy cle pool as zeaxanthin and antheraxanthin at midday, and an increase i n the ratio of xanthophyll cycle pigments to total chlorophyll. These results indicate that under N limitation both the light-collecting sys tem and the photosynthetic rate decrease. However, the increased dissi pation of excess energy shows that there is excess light absorbed at m idday. We conclude that spinach responds to N limitation by a combinat ion of decreased light collection and increased thermal dissipation in volving the xanthophyll cycle.