CAROTENOID CATABOLISM DURING LEAF SENESCENCE AND ITS CONTROL BY LIGHT

Reviews on the biosynthesis, distribution and function of carotenoids in chloroplasts of higher plants and algae are available. However, the data, although limited, on degradation of the pigment during leaf senescence are not integrated. This review is an attempt to summarise the data available so far, generalise them, and address a few unanswered questions in the field. Both Chl and carotenoids degrade during leaf senescence; the latter process, however, is demonstrated in many plant systems to remain relatively stable. Senescence brings about changes in carotenoid composition and causes formation of xanthophyll esters and epoxides in some cases. The esters and free carotenoids are predominantly located in plastoglobuli formed during the process. In the context of compositional changes, the possibility of the operation of the xanthophyll cycle during senescence is discussed. The existing experimental findings suggest the involvement of enzymes for degradation of the pigment. Light is shown to stimulate carotenoid degradation in the background of dark treatment during aging of isolated chloroplasts and plants treated with herbicides. The mechanism of stimulation is mostly explained through the participation of free radicals. In contrast, photoinduced retardation of the pigment loss during leaf senescence is attributed to the action of phytochrome.