Photoprotective effect of kinetin on pigment content and photochemical activities of wheat chloroplasts aging in vitro

The effects of kinetin (Kn) on pigment content and electron transport activities (ETA) in wheat leaves in vivo and chloroplasts in vitro aging in light was investigated. Excised wheat leaves were infiltrated with Kn for 3 h under irradiation. The treatment increased zeaxanthin (Zx) content by 40% and also increased chlorophyll (Chl a, Chl b) and major carotenoid (Car) contents in the leaves (per fresh mass unit). Chloroplasts isolated from Kn treated leaves, when incubated in light for 4 h showed relatively lower pigment loss and slower loss of ETA compared to the chloroplasts of untreated leaves. These observations suggest photoprotective action of Kn. The photoprotection was more prominent when Kn was applied directly to the irradiated chloroplasts in vitro. Moreover, chloroplasts aging in vitro under irradiation without Kn treatment lost pigments and ETA. Within 3 h of irradiation, both whole chain (H2O to methylviologen) electron transport as well as photosystem (PS) 2 activity were completely lost. However, in the chloroplasts treated with Kn, the loss of pigments was slow and even after 4 h of irradiation the chloroplasts retained 15% of PS 2 and 9% of whole chain ETA. In the untreated chloroplasts, the loss of Zx after 4 h of irradiation was 49% whereas in Kn treated samples its level was 1.3 times higher than that of control. Since a higher level of Zx was maintained in Kn treated chloroplasts, photoprotective action of Kn is possibly mediated through Zx.