Fruit Set and Carbohydrate Status in Alternate and Nonalternate Bearing Citrus Cultivars

Abstract

A comparative study of fruit set of an alternate bearing cultivar of Citrus (‘Murcott’, a C. reticulata hybrid of unknown origin) and a nonalternating cultivar [ C. sinensis (L.) Osbeck ‘Shamouti’] was conducted. ‘Murcott’ set a higher percentage of flowers (13.6%) than ‘Shamouti’ (3.8%), because of a higher abscission rate in ‘Shamouti’ throughout the fruit set period. Prebloom girdling increased fruit set significantly in ‘Shamouti’ but had a smaller effect in ‘Murcott’. The nonstructural carbohydrate balance of ‘Murcott’ and ‘Shamouti’ fruiting branches during fruit set as affected by girdling also were investigated. ‘Murcott’ generally had a higher starch content in the fruiting twigs and a higher starch and soluble sugars content in the mature, source leaves. Girdling increased the starch content in mature leaves and twigs of both ‘Murcott’ and ‘Shamouti’. This increase was observed in ‘Murcott’ 2 weeks after girdling, but only 4 and 8 weeks after girdling in ‘Shamouti’ leaves and twigs, respectively. ‘Shamouti’ mature leaves and twigs also had increased soluble sugars content because of girdling. In expanding new leaves, ‘Shamouti’ had similar or higher amounts of nonstructural carbohydrates than did ‘Murcott’, but during maturation, ‘Murcott’ young leaves developed a higher sugar starch content than ‘Shamouti’ young leaves, similar to the situation in mature one-year-old leaves. These results are discussed in terms of source-sink relationships and fruit set.