Carbohydrate Availability in Relation to Fruitlet Abscission in Citrus

Abscission of flowers and fruitlets in the Washington navel orange ( Citrus sinensis [L.] Osbeck) has been characterized in relation to carbohydrate availability. A main wave of flower abscission occurs shortly after anthesis while the carbohydrate reserves in the tree are high. Fruitlet abscission starts approx. 30 d after the commencement of flowering, while carbohydrates (mainly starch) are being accumulated in the leaves. Flower and early fruitlet abscission are not caused by carbohydrate shortage. During late fruitlet abscission sucrose concentration in the leaves falls to a low value demonstrating a limitation in supply and competition among the developing fruitlets for carbohydrates. Concentrations of sucrose and reducing sugars in the peel of the fruitlets also fall to low values, and a relationship could be demonstrated between these free sugar levels and abscission. Ringing increases carbohydrate supply to fruit and reduces late fruitlet abscission, but only has a marginal effect on the growth of the fruitlets, which seems less sensitive than abscission to carbohydrate shortage. The limitation of carbohydrate supply to the fruitlets occurs while starch levels in the leaves remain high. Slow mobilization of starch reserves may be one factor limiting set in Citrus .