Effects of low UV-B doses on the accumulation of UV-B absorbing compounds and total phenolics and carbohydrate metabolism in the peel of harvested lemons

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This paper examined the peel (albedo and flavedo) of postharvest lemon fruits after UV-B exposure in order to analyze relationships between soluble carbohydrate metabolism and secondary metabolite accumulation. Lemons ( Citrus limon, cv. Limoneira 8A) were harvested in winter months (June to August), treated with 0.43 W m −2 (22 kJ m −2 d −1 UV-BBE) of UV-B radiation during 0 (control), 0.5, 1.0, 2.0, 3.0, and 5.0 min, and then stored at 25 °C for 24 h. Peel samples from irradiated areas were obtained with a razor blade and frozen in liquid nitrogen until use for measurements. Data obtained showed that 2 and 3 min of UV-B exposure effectively increased the level of UV-B absorbing compounds and total phenolics in flavedo without causing visual alterations of the peel colour as compared with non-irradiated lemons. By contrast, there were no significant changes in albedo secondary metabolite accumulation. The amount of secondary metabolites was depending upon UV-B time–dose. Exposure over 3.0 min did not further improve the accumulation of UV-B absorbing and phenolic compounds. Soluble sugars (sucrose, glucose and fructose) also accumulated in the lemon peel after UV-B exposure, but the distribution patterns were different. After 3 min time–dose, sucrose and hexoses increased in flavedo, whereas in albedo only increased the sucrose and glucose. This effect was related to UVB-induced changes in the activity of sucrose-hydrolyzing and sucrose-synthesizing enzymes: soluble and cell-bound invertase, sucrose synthase (SS) and sucrose phosphate synthase (SPS). Data indicate that lemon peel retains the capacity to modify the enzyme activity of sucrose metabolism in response to UV-B exposure. Our results also suggest that the exposure of postharvest lemons to low supplemental UV-B doses produces changes in the carbon allocation of peel tissues including synthesis, but probably not only limited to them, of UV-B absorbing and phenolic compounds.