Chlorophyll- and anthocyanin-rich cell organelles affect light scattering in apple skin

Abstract

Abstract: Apple skin contains several groups of strongly absorbing cell organelles with pigments that change dynamically in type and concentration during fruit maturation. Chlorophylls and carotenoids, both primarily involved in photosynthesis, are found in the grana of chloroplasts, while anthocyanin vacuolar inclusions (AVIs) accumulate for light protection in red-skinned cultivars. A Mie model describing light scattering by absorbing spherical particles in a non-absorbing medium allowed to theoretically investigate the explicit influence of grana and AVIs on the effective scattering coefficient μs′ and the absorption coefficient μa. The reconstruction of the complex refractive indices of the organelles predicted anomalous dispersion, i.e., a local increase in the real part of the refractive index in the spectral regions with high chlorophyll and anthocyanin absorption, in agreement with the Kramers–Kronig relations. As a result, peaks in μs′ were predicted to be shifted to longer wavelengths compared to the corresponding μa bands. This selective scattering effect was confirmed experimentally with integrating sphere measurements for red- or green-skinned apple samples of the cultivars ‘Elstar’, ‘Gala’ or ‘Jonagold’. Comparison between simulations and measurements indicated that the Soret bands of chlorophyll a and chlorophyll b are at 435 nm and 469 nm, respectively, and overlap with the absorption of carotenoids, whose red-most edge is at 488 nm. For anthocyanin absorption, a pronounced blue shift from 550 to 520 nm was observed, indicating structural or chemical changes of AVIs. Graphical abstract: [Figure not available: see fulltext.]