EFFECTS OF CURRENT CO2 ASSIMILATION AND STORED RESERVES ON LYCHEE FRUIT-GROWTH

The roles of current CO2 assimilation and stored carbohydrates on frui t retention in lychee (Litchi chinensis Sonn.) were investigated. In 1 2 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 h ad 0, 5, 10, 20 or 30 leaves, and 0, 5, 10, 20 or 30 fruit. In a separ ate experiment, branches were cinctured 0.5 or 1.5 m from the fruit cl uster in October, while uncinctured branches acted as controls. At Nam bour in subtropical Australia (lat. 27 degrees S), branches of ten yea r old 'Souey Tung' were cinctured in October after fruit set about 1.2 m from the fruit cluster, while other branches were cinctured and thi nned to five leaves or five fruit per fruit cluster or left uncincture d 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 th e 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 a nd 30 fruit at the start of sampling, but a different response was giv en when the number of fruit retained was expressed as a proportion of that soon after fruit set. Relative fruit retention was below 5% in br anches 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 a fter fruit thinning and only one-tenth after leaf thinning. Starch gen erally accumulated in the stems after fruit were removed, whereas CO2 assimilation was greatest after leaf removal and least with fruit remo val. 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 yie lds presumably by redirecting assimilates to the developing crop.