Effects of current CO2 assimilation and stored reserves on lychee fruit growth

The roles of current CO2 assimilation and stored carbohydrates on fruit retention in lychee (Litchi chinensis Sonn.) were investigated. In 12 year old 'Tai So' trees growing at Burgershall in subtropical South Africa (lat. 25 degrees S), terminal branches were cinctured (girdled) 0.5 or 1.0 m from the fruit cluster about 2-4 weeks after anthesis in October to isolate the fruit from the rest of the tree. Each branch had 0, 5, 10, 20 or 30 leaves, and 0, 5, 10, 20 or 30 fruit. In a separate experiment, branches were cinctured 0.5 or 1.5 m from the fruit cluster in October, while uncinctured branches acted as controls. At Nambour in subtropical Australia (lat. 27 degrees S), branches of ten year old 'Souey Tung' were cinctured in October after fruit set about 1.2 m from the fruit cluster, while other branches were cinctured and thinned to five leaves or five fruit per fruit cluster or left uncinctured and unthinned. In other experiments, seven year old trees of cv. Wai Chee and ten year old trees of cv. Kwai May Pink were cinctured on the trunk in November or left uncinctured. The number of fruit retained per panicle, net CO2 assimilation, yield and concentrations of starch in the leaves and stem were determined. In South Africa, the greatest number of fruit per panicle at harvest (8.6) occurred with 30 leaves and 30 fruit at the start of sampling, but a different response was given when the number of fruit retained was expressed as a proportion of that soon after fruit set. Relative fruit retention was below 5% in branches with no leaves and 50-60% in branches with six leaves per fruit. In Australia, about a quarter of the fruit were retained at harvest in control and cinctured branches compared with more than two-thirds after fruit thinning and only one-tenth after leaf thinning. Starch generally accumulated in the stems after fruit were removed, whereas CO2 assimilation was greatest after leaf removal and least with fruit removal. There was double the relative fruit retention with cinctures at 0.5 m (25%) compared with controls (14%), and three times as many with cinctures at 1.5 m (38%), and a 35% increase in yield when whole trees were cinctured. These results suggest that lychee fruit appear to be mainly dependent on current CO2 assimilation. Cincturing increased yields presumably by redirecting assimilates to the developing crop.