Photostability of pigments in ripening apple fruit: a possible photoprotective role of carotenoids during plant senescence

Light-induced pigment destruction was studied in ripening apple (Malus domestica Borkh., cv. Antonovka) fruits with reflectance spectroscopy. The reflectance spectral changes attributable to light-induced transformation of xanthophyll cycle carotenoids (Car) preceded the pigment degradation. In green fruits, the destruction of chlorophyll and Car proceeded synchronously up to complete disappearance of both pigments. In ripening fruits with a molar chlorophyll/carotenoid ratio <2.5–3, significant amounts of peel Car were retained at the deep stages of chlorophyll degradation. Car were especially resistant to irradiation in yellow fruits (a chlorophyll/carotenoid ratio <0.3); the extent of their bleaching after prolonged irradiation did not exceed 20%. Irradiation of pigment solutions showed that apple fruit Car alone exhibit much higher light stability than in the presence of chlorophyll. The extent, kinetics and stoichiometry of light-induced pigment destruction in apples are consistent with the existence of two carotenoid pools, (i) closely associated with chlorophyll in chloroplast thylakoid membranes and (ii) exhibiting higher light stability localised in plastoglobuli appearing in chloroplasts undergoing transformation to gerontoplasts–chromoplast. It is suggested that the induction of carotenoid synthesis during senescence provides the protection of plastoglobuli and light-sensitive constituents of plant tissues from irradiation in the blue part of the visible spectrum.