Light-induced decrease of reflectance provides an insight in the photoprotective mechanisms of ripening apple fruit

The magnitude of real-time changes in reflectance of ripening apple (Malus × domestica Borkh., cv. Antonovka) fruit induced by PAR irradiation (500 μE m−2 s−1) was monitored together with pigment composition and non-photochemical dissipation of absorbed light energy. In fruit with high chlorophyll content irradiation induced, within 180 s, a notable decrease of reflectance in the bands centred at 520, 692 and 740 nm. These changes were strongly inter-correlated and exhibited tight relationships with chlorophyll fluorescence intensity, violaxanthin de-epoxidation, NPQ, qN and qP. The decrease of the reflectance in the blue-green is suggested to be related mainly with irradiation-induced conversion of violaxanthin to zeaxanthin whereas the decrease of the reflectance in the red and NIR was obviously due to the quenching of chlorophyll fluorescence. Based on these findings, spectral index AVI (Apple Violaxanthin cycle Index) for non-destructive assessment of violaxanthin cycle operation in apple was developed. According to the results of violaxanthin cycle monitoring with the use of AVI, its activity as well as non-photochemical quenching declined in the course of apple ripening on the background of the increase in the proportion of extrathylakoid carotenoids (mainly fatty acid-esterified xanthophylls) together with the relative amount of light intercepted by these pigments. Comparative importance of violaxanthin cycle and light screening by extrathylakoid carotenoids for photoprotection at different stages of fruit development is discussed.