Pg. 20-25 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 oann.) were investigated. In 12-year-old ‘Tai So’ trees growing at Burgershall in subtropical South Africa (lat. 25 °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 °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 tenyear-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 was 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 was 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.

Summary:

• The study investigated the roles of current CO₂ assimilation (photosynthesis) and stored carbohydrates (reserves) on fruit retention in lychee (Litchi chinensis) using girdling (cincturing) experiments on branches with varying leaf and fruit numbers.
• Experiments were conducted on different cultivars and ages of lychee trees in subtropical South Africa and Australia.
• Removing all leaves from branches greatly reduced fruit retention, indicating stored reserves in stems alone were insufficient to support fruit growth to maturity.
• Fruit retention and yield were strongly dependent on the number of leaves per fruit, with 3 to 6 leaves per fruit being optimal for maximum fruit retention.
• Branches with more leaves showed higher net CO₂ assimilation rates, supporting more fruit and higher yields.
• Cincturing branches to isolate fruit increased fruit retention and total yield by redirecting assimilates towards fruit growth, with up to a 35% yield increase observed in whole-tree cincturing.
• Starch generally accumulated in stems when fruit were removed, suggesting starch reserves are normally mobilized to support fruit development.
• Fruit retention was low (less than 5%) on branches without leaves, but reached 50-60% on branches with six leaves per fruit.
• Fruit thinning increased relative fruit retention, while leaf thinning drastically reduced fruit retention.
• The study concluded lychee fruit is mainly dependent on current CO₂ assimilation from leaves rather than stored carbohydrate reserves for successful fruit growth.
• The findings emphasize the importance of maintaining healthy foliage for photosynthesis during fruit development to maximize lychee yields.