Effects of calcium treatments on cell wall material metabolism and related enzyme activities and gene expression in grapefruit(Citrus paradisi Macf.)

【Objectives】Study on the effect of Ca treatment on the cell wall components, the variations of cell wall degrading enzyme activities and the expression of relevant genes will help to understand the relationship between the Ca and metabolism of cell wall materials, explore the mechanisms of Ca to fruit softening, and provide base for the improvement of fruit benching time. 【Methods】A field experiment was carried out in grapefruit orchard in Yuxi City, Yunnan Province in 2012. Grapefruit cultivar ‘Rio red’ was used as tested crop. The rootstock of the cultivar was grafted in 2005 on a local cultivar in density of 3 m×3 m. The Ca treatments were composed of two parts: during fruit growing and after harvest. Growing treatment was foliar spraying 2% CaCl2 solution at very young fruit, end of young fruit, early fruit expansion, end of fruit expansion and color developing stage. After harvest treatment: the harvested fruits were soaked in 2% CaCl2 solution for 5 min, then stored at room temperature. Fruit samples were taken every 15 days and each time with 10 fruits during the storage for the measurement of relative items.【Results】The tightly bound pectins(covalent bound pectin)were dissembled into loosely bound pectins(water soluble pectin, ion bound pectin)with grapefruit fruit ripening. The contents of tightly bound hemicelluloses(24% KOH-soluble fraction)decreased while loosely bound hemicelluloses(4% KOH-soluble fraction)increased. The activity and the expression of cell wall enzymes genes increased as fruit softened. PME expressed high levels after harvest. The level of PG activity increased in the early stage of storage while PG expression changed similar to enzyme tendency. The activities of Cx, α-L-Af and β-Gal accelerated in the middle and later stage of storage respectively,and their expression were obviously enhanced during storage. Calcium treatment had significantly inhibited cell wall enzyme activities and gene expression. Calcium treatment dramatically regulated the activity of α-L-Af, β-Gal activity in middle stage of storage and later stage, respectively. The enzyme activities, gene expressions of α-L-Af and β-Gal had noticeably lower levels in calcium treatments. 【Conclusions】Calcium treatments could reduce the activities of wall-degrading enzymes, the expression of relevant genes, and inhibit the dissembling of wall materials. Therefore, calcium treatment is effective in maintaining fruit texture quality and prolonging the benching period.