Changes in the cell walls on fruit skin of Beurré D´Anjou pears (Pyrus communis L.) associated with sunburn injury

The purpose of this work was to evaluate the effects of sunburn on pear fruit skin at the structural, biochemical, and molecular levels. Pears (Pyrus communis L.) cv. Beurré D'Anjou were collected at the official commercial harvest time and classified into three groups based on levels of sublethal sunburn: no sunburn (S0), mild sunburn (S1), and moderate sunburn (S2). Skin anatomy was characterized by SEM imaging, cell wall components were quantified by spectrophotometric techniques, and the expression of different genes related to cell wall remodeling was measured in S0, S1, and S2 tissues. Sun-damaged skin cells had reduced size and thickened walls. In the cuticular surface, a reduction of microcracking area, correlated with the intensity of the damage, was further noted. Cell wall fractionation revealed changes in the distribution of neutral sugars (NS) following sunburn, although no differences in uronic acid (UA) content were observed in the different pectin fractions. Hemicellulose and lignin contents were significantly (P<0.05) increased in sun-damaged tissues. Expression levels of genes encoding polygalacturonase (PcPG2), β-galactosidase (PcGAL2 and PcGAL4), xyloglucan endotransglycosylase (PcXET), and expansin (PCExp1) were increased in the skin of sun-damaged (S1 and S2) fruit. These results indicate that sunburn alters the anatomy and cell wall structure and composition of pear fruit skin, and stimulates different mechanisms involved in the acquisition of stress tolerance.