ASCORBATE INDUCED ZEAXANTHIN FORMATION IN WHEAT LEAVES AND PHOTOPROTECTION OF PIGMENT AND PHOTOCHEMICAL ACTIVITIES DURING AGING OF CHLOROPLASTS IN LIGHT

Mechanisms by which higher levels of Zeaxanthin (Zx) in detached wheat leaves, induced by ascorbate in vivo treatment photoprotect isolated chloroplasts aging in light were investigated. Leaves were incubated for 3 h in light at 400 muM-2 S-1 in the following media: ascorbate (Asc), Asc plus dithlotreitol (DTT) and DTT. Pigments were assayed using a reverse phase HPLC, and photochemical activity (PCA) was measured by an oxygen electrode. The Zx level increased by three times when the leaves were incubated in ascorbate for 3 h. When leaves were treated with Asc plus DTT, Zx accumulation that was stimulated by Asc disappeared. Chloroplasts isolated from untreated leaves, followed by in Vitro incubation in light for 4 h, significantly lost pigments and PCA, including whole chain (H2O to methyl viologen) electron transport and photosystem II (PSII). Chloroplasts isolated from Asc treated leaves showed relatively less chlorophyll and carotenoid loss, and PCA loss. The photoprotection was nullified in chloroplasts isolated from Asc plus DTT treated leaves, which showed lower levels of Zx. Chloroplasts isolated from the DTT treated leaves showed higher pigment loss and PCA loss as compared with the control. These results clearly show that Zx is responsible for the photoprotection of chloroplasts aging in a cell free condition in light.