Effect of pre-harvest application of calcium and boron on dietary fibre, hydrolases and ultrastructure in ‘Cara Cara’ navel orange (Citrus sinensis L. Osbeck) fruit

High dietary fibre diets are associated with the prevention, reduction and treatment of some diseases, such as diverticular and coronary heart diseases. However, pre-harvest calcium and boron sprays associated with the dietary fibre compounds in fruits remain unknown. In the present study, specific activities and gene expression levels of hydrolases were determined in order to identify the effect of pre-harvest application of Ca and B alone or in combination on dietary fibre and the ultrastructure of navel orange fruit (Citrus sinenisis L. Osbeck). The results showed that pre-harvest application of Ca, B had significant effect on the cross-linked polymer network of fruit segment membrane. In accordance with the enzyme activities, expression levels of polygalacturonase, pectinesterase and β-galactosidase were significantly reduced by pre-harvest application of Ca, B alone or in combination. Such treatments increased contents of total dietary fibre, insoluble dietary fibre, protopectin and cellulose, decreased soluble dietary fibre and water soluble pectin. Our study indicated that pre-harvest foliar application of Ca, B is useful for improving the tissue structure of segment membrane, reducing transcript levels and activities of polygalacturonase, pectinesterase and β-galactosidase and regulating the content of dietary fibre on Cara Cara navel orange.