Litchi is a highly perishable fruit. Ripe litchi fruit loses quality quickly as they hang on tree, giving a very short hanging life and thus harvest period. This study attempted to explore the roles of cytokinin in regulating fruit ripening and senescence of litchi and examine the possibility of applying cytokinin in “on-tree storage” of the fruit. Exogenous cytokinin, forchlorfenuron (CPPU), was applied at 20 mg L−1 7 weeks after full bloom on litchi (Litchi chinensis cv. Feizixiao) fruit clusters. Color parameters, chlorophylls, anthocyanins, fruit and fruit part weights, total soluble solutes (TSSs), soluble sugars, organic acids, non-anthocyanin flavonoids, ethanol, and also CPPU residue in fruit were traced. CPPU residue was higher but decreased faster in the pericarp than in the aril, where it maintained < 10 μg kg−1. CPPU had no significant effect on fruit weight but tended to increase pericarp weight. The treatment suppressed chlorophyll loss and anthocyanin accumulation in the pericarp, increased non-anthocyanin flavonoids in the aril, but had no significant effects on non-anthocyanin flavonoids in the pericarp and total sugar and organic acids in the aril. As the commercially ripe fruit hanged on tree, TSSs, total sugar, and sucrose decreased with ethanol and acetic acid accumulation in the aril. CPPU significantly suppressed the loss of sucrose and total sugar and the accumulation of ethanol and acetic acid in the aril and inhibited malondialdehyde accumulation in the pericarp of the overripe fruit. Soluble invertase, alcohol dehydrogenase, and pyruvate decarboxylase (PDC) activity and gene expression in the aril were downregulated by CPPU. The results suggest that cytokinin partially suppresses the ripening process in litchi and is effective to slow quality loss in the overripe fruit on tree.

This study aimed to provide a reference for the practical use of forchlorfenuron (CPPU) in the improvement of fruit set and quality (especially size) in litchi (Litchi chinensis Sonn.). CPPU at 5 mg/L was sprayed to fruit clusters of ‘Feizixiao’ at 2, 4, 6 and 8 weeks after female bloom (WAFB) in 2017 and at 2, 4, 6, 8, 9 or 10 WAFB in 2018, with spraying water as the control. The treatments were all effective at suppressing fruit abscission, resulting in higher fruit retention at harvest. Except for treatment at 2 WAFB, which caused stunted fruit and significantly reduced fruit weight, all the treatments had no significant influence on fruit weight and flesh (aril) weight, although CPPU tended to increase pericarp weight and strongly suppressed fruit coloration, and such effects were most significant in treatments at 4, 6 and 8 WAFB. Content of total soluble solids (TSS) in commercially ripe fruit (11WAFB) was not significantly influenced by CPPU applied at any of the stages. Ripe fruit continued to gain redness but lost TSS as it became overripe. CPPU suppressed redness gaining, the effect being stronger in later application. The treatments did not influence TSS loss, but treatments at 4, 6 and 8 WAFB significantly reduced the increase in fruit membrane leakage. In treatment at 6 WAFB, CPPU residue in the pericarp decreased sharply with time but relatively constant in the aril, where it maintained lower than 10 μg/kg, the maximal residue limitation in Australia and the United States. The results suggest that 5 mg/L CPPU is effective at improving fruit set and suppressing fruit pigmentation but has no significant effect on fruit size and TSS accumulation or maintenance in litchi.