Heavy Fruit Load Inhibits the Development of Citrus Summer Shoots Primarily Through Competing for Carbohydrates

Excessively and randomly producing summer shoots will lead to difficulty in citrus orchard management, specially in pest and disease control. Heavy fruit load can reduce the summer shoot number. However, the mechanism is still unclear. In this study, field investigation and de-fruiting treatment confirmed that heavy fruit load reduces the number of citrus summer shoots, which is zero when the yield surpasses 3.3 kg per 125 dm3 of tree canopy. Metabolite analysis indicated that fruits at the cell expansion stage attract more soluble sugars and de-fruiting significantly increase the content of sugars and the transcript levels of sink strength-related genes, CsSUS4/5/6 to over 3.0 fold in the axillary buds. Moreover, exogenous application of some sugar-related DAMs (differently accumulated metabolites) such as sucrose obviously promoted axillary bud outgrowth. Taken together, these results confirmed that heavy fruit load plays a role in inhibiting axillary bud outgrowth or shoot branching primarily through competing for soluble sugars, which provides the basis for the inhibition of summer shoots by increasing the fruit load in citrus orchard and for the improvement of pest and disease management effectiveness.