| China is one of the important origin area of citrus. The planting acreage and yield of pomelo fruits are top the list of the world. However, the study of the storage and fresh-keeping techniques are limited, and this backward make pomelo fruits vulnerable to decay infection, physiological disease, and thus, the fruits quality was easy to decrease after harvest. Therefore, more and more attentions were put into developing the storage and fresh-keeping technologies for pomelo fruits. In this study, the effects and mechanisms of pre-harvest foliar application of mineral element (Ca, B), plant hormones (GA3, SA), and postharvest Ca, hot treatments to control fruit texture quality during storage were investigated. Meanwhile, the cell wall degradation, cell wall enzyme activities, and cell wall degradation enzymes gene expression of the treated and untreated’Rio Red’grapefruit fruits were comparatively analyzed after harvest. These technologies and their mechanisms resulted from the study may provide theoretical foundation in grapefruit fruit postharvest conservation. The main results obtained from the study are listed as follows:(1) Pre-harvest applications of calcium, boron alone or in combination could maintain fruits firmness, restrain fruits weight loss, and improve fruits SSC content during postharvest storage period. The treatments retarded the increase of WSP, ESP, and4KSF in cell wall of grapefruit fruits, and restained the degradation of cell wall materials, including AIR, SCSP, and24KSF. In addition, calcium and/or boron treatments inhibited activities cell wall enzymes effectively in different storage period (a-L-Af, early period; PME, middle period; PG and Cx, middle-later period;(3-Gal, all storage stage). Meanwhile, gene expression levels were significantly regulated by pre-harvest application of calcium and/or boron treatments in grapefruit fruits. The change tendencies of PG, PME, and Cx enzyme expression accorded with their corresponding enzyme activity, while expression levels of a-L-Af and β-Gal enzymes preceded their enzyme activity changes. The expression levels of PG、 a-L-Af enzymes were regulated significantly in earlier storage stage and the expression levels of Cx were regulated in middle-later storage stage. The PME exhibited lowest transcript levels in later storage stage with Ca treatment, while the expression level of β-Gal was restrained by calcium and/or boron treatment during whole storage period. In conclusion, applications of calcium, boron alone or in combination provided significant regulation effect in grapefruit fruit texture quality, and the most desirable effect were detected in middle-later period of postharvest storage. Calcium and boron combined treatment showed the best regulating effect on grapefruit quality, followed by boron treatment, and calcium application showed the weakest effect than the others.(2) Pre-harvest applications of gibberellin, salicylic acid alone or in combination could maintain fruits firmness and restrain fruits weight loss during postharvest storage period. However, the treatments did not showed an obvious influence on SSC of grapefruit fruits. The degradation process of cell wall material, including pectin and hemicelluloses, was restained by gibberellin and/or salicylic acid treatment, and so the increase speeds of WSP, ESP, and4KSF contents, and the decrease speeds of SCSP and24KSF contents were reduced by the technical treatments. The activities of cell wall hydrolytic enzymes were inhibited by gibberellin and/or salicylic acid applications in different postharvest storage stages (PG, early period; PME, middle period; Cx, a-L-Af, and β-Gal, middle-later period). In addition, notable regulation differences were observed between different treatment techniques, and the activities of PG, Cx, a-L-Af, and β-Gal enzymes were regulated notably by gibberellin and salicylic acid combined treatment, while the activity of PME was obviously controlled by GA3treatment. The expression levels of PG, Cx, a-L-Af and β-Gal accorded with their corresponding enzyme activity changes, but the expression levels of PME preceded its’enzyme activity change. The expression levels of PG were significantly regulated in early storage stage, PME was regulated in earlier-middle storage stage, and Cx, a-L-Af, and β-Gal were regulated in later storage stage. Furthermore, the gene expression of PG, a-L-Af and β-Gal were controlled effectively by gibberellin and salicylic acid combined treatment, and the gene expression of PME, Cx were obviously regulated by gibberellin treatment. The above results indicated that pre-harvest application of gibberellin, salicylic acid alone or in combination inhibited cell wall gene expressions and cell wall degradation enzyme activities, and the treatments retarded cell wall material degradation. The notable inhibition effect of the treatments was observed in middle-later storage stage. Gibberellin and salicylic acid combined treatment showed the best regulating effect, followed by gibberellin treatment, and the salicylic acid application showed inferior effect than the others. These effects are beneficial to improve grapefruit fruit texture quality.(3) Postharvest applications calcium, hot alone or in combination restrained firmness decrease, and retarded weight loss increase, but showed no desired effect in regulation SSC content during storage. The cell wall material WSP, ESP contents of grapefruit fruits were significantly regulated in middle-later storage stage, and changes of SCSP,4KSP,24KSP contents were inhibited during whole storage period by postharvest calcium and/or hot treatments. The activities of cell wall hydrolytic enzymes were regulated by calcium and/or hot treatments in different postharvest storage stages (PME, Cx, and a-L-Af, early-middle period; PME, middle period; PG and β-Gal, middle-later period). In addition, the tendencies of gene expression levels of PG, a-L-Af, β-Gal in grapefruit fruits accorded with their corresponding enzyme activist changes, while PME, Cx gene expressions exhibited time-differences with the their enzyme activist changes. The expressions of PG, PME, β-Gal genes were suppressed obviously by calcium and/or hot treatments in middle-later stage, while Cx and a-L-Af gene expressions were regulated notably in early and later stages during fruit storage. Based on the above results, it could be concluded that the calcium and/or hot treatments significantly inhibited fruit cell wall hydrolytic enzyme activities and gene expressions increase, and retarded pectin and hemicelluloses degradation. The remarkable regulation effects of the technical treatments were observed in middle-later storage stage, and it is helpful to maintain an ideal texture quality of grapefruit fruits in postharvest storage. Calcium and hot combined treatment showed the best regulating effect on grapefruit quality, followed by calcium treatment, and hot application showed inferior effect than the others. |
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