Branch girdling at fruit green mature stage affects fruit ascorbic acid contents and expression of genes involved in L-galactose pathway in citrus

Although citrus is an important source of ascorbic acid (AA), and even though girdling is an important agronomic practice in citrus production, the effect of girdling on ascorbate (ASC) accumulation has rarely been studied. In the present study, branch girdling (BG) was carried out on trees of Citrus unshiu cv. Guoqing No. 1' at 40 days before harvest to investigate its effect on fruit ASC accumulation and on the expression of genes involved in the AA l-galactose biosynthetic pathway. BG increased total ASC and AA contents in fruit peel and pulp. In parallel, soluble sugars in the fruit pulp increased. Moreover, the expression of all genes, except for l-galactose-1-P phosphatase (GPP), in the l-galactose pathway were induced in fruit pulp at least during the first 20 days of treatment while their expression (except for GPP) in fruit peel was reduced by BG treatment. Taken together, the expression profiles of six l-galactose biosynthetic genes did not coincide closely with the changes in fruit ASC content following BG. ASC content may respond to girdling via other biosynthetic, catabolic or recycling pathways.