Photostability of pigments in ripening apple fruit: a possible photoprotective role of carotenoids during plant senescence Mark N. Merzlyak *, Alexei E. Solovchenko Department of Physiology of Microorganisms, Faculty of Biology, Moscow State University, 119992GSP-2 Moscow W-234, Russia Received 5 April 2002; received in revised form 8 July 2002; accepted 29 July 2002 Abstract 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 B/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 B/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. # 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Apples; Carotenoids; Chlorophylls; Photoprotection; Ripening/senescence; Reflectance spectroscopy 1. Introduction Carotenoids (Car) are essential components of plant photosynthetic apparatus and their roles in light har- vesting, stabilisation of thylakoid membranes, energy distribution and dissipation in pigment /protein com- plexes and photoprotection are well documented [1  /5]. However, little is known on physiological significance of Car retention and/or accumulation often occurring during senescence, the phenomenon that at the terminal stages of chlorophyll (Chl) breakdown is responsible for bright yellow coloration of plant tissues [1,6 /13]. Generally, both in senescing leaves and ripening fruit the Car pool represented by xanthophylls and carotenol fatty acid esters is associated with plastoglobuli in chloroplasts undergoing transformation to geronto- plasts and then to chromoplasts [1,6,12,14  /16]. Plastoglobuli of senescing leaves characterised by high amounts of neutral lipids (including Car) [6,14  /16] have been proposed to consist of a thin monolayer of polar lipids and proteins covering the surface, with apolar components buried in the interior (see [12]). Although deposited in plastoglobuli Car may represent simply rejected products without any significance [1], several hypotheses on their physiological role(s) have been framed out. In plants plastoglobuli may serve as pools for storage of thylakoid lipids (pigments, fatty acids, triacylglycerols, prenyl quinones etc.) [1,6,14  /16] which could be utilised during regreening and gerontoplasts redifferentiation into chloroplast (see [17]). The involve- ment of Car recognised as powerful antioxidants in the metabolism of reactive oxygen species (ROS) generated under light conditions in transforming chloroplasts has been postulated [1,11]. Furthermore, it was suggested that ROS acting as secondary messengers mediate Abbreviations: Car, carotenoids; Chl, chlorophyll; CRI, carotenoid reflectance index; R, reflectance; ROS, reactive oxygen species. * Corresponding author. Tel.:  /7-95-939-2587; fax:  /7-95-939- 3807 E-mail addresses: mnm@6.cellimm.bio.msu.ru, m_merzlyak@mail.ru (M.N. Merzlyak). Plant Science 163 (2002) 881  /888 www.elsevier.com/locate/plantsci 0168-9452/02/$ - see front matter # 2002 Elsevier Science Ireland Ltd. All rights reserved. PII:S0168-9452(02)00241-8