Pg. 11-16 Litchi Fruit Growth Depends on Current CO₂ Assimilation rather than on Stored Reserves

The roles of current CO₂ assimilation and stored carbohydrates on fruit retention in 12-year-old Mauritius litchi trees growing at Burgershall in subtropical South Africa lat 25°07’S) were investigated over two seasons. Terminal branches were cinctured (girdled) 0,5 or 1,0 m from the fruit cluster on October 6 or 25, about two to four weeks after anthesis (day 0), 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 to give a range in leaf: fruit ratio from 0,2:1 to 6:1.

The number of fruit retained per panicle and net CO₂ assimilation were measured at intervals during the season, and concentrations of starch in the leaves and branches were determined after harvest. In a separate experiment, branches were cinctured 0,5 or 1,5 m from the fruit cluster on October 6 (day 0) and the number and mass of fruit recorded. Encinctured branches acted as controls.

In the leaf and fruit removal experiments, there was a decline in the number of fruit per panicle over time in all treatments, with the number of fruit at harvest related to the number of fruit at the start of sampling or soon after. The greatest number of fruit in both years (8,5 or 9,0 per panicle) occurred with 30 leaves and 30 fruit at the start of sampling. However, a different response occurred when the number of fruit retained was expressed as a proportion of the number soon after fruit-set. In year 1, relative fruit retention 26 days before harvest increased from 1,3% with no leaves:30 fruit (ratio 0:1) to 72% with 30 leaves: 5 fruit (ratio of 6:1). In contrast, fruit retention at harvest was below 2% with 0,2 leaves per fruit or less, and 12 to 30% with 0, 7 to 6 leaves per fruit. In year 2, fruit retention increased with increasing leaf:fruit ratio up to a maximum of 3 leaves per fruit at all sampling times. At harvest, 60 to 68% of fruit were retained with a leaf:fruit ratio of 3: 1 or 6: 1 compared with only 3% in branches with no leaves and 30 fruit at the start of sampling.

Starch accumulated in the stems of the branches with 30 leaves and 0 to 10 fruit (4,0 to 10, 1% compared with 0,8 to 4,8% in the other treatments). Starch was much lower in the leaves and was only slightly increased in branches with no fruit (2, 7 to 4,5% compared with 0,5 to 3,7% in the other treatments).

In year 1, net CO₂ assimilation decreased as the number of leaves supporting a fruit increased when samples were collected 10 days before harvest. In contrast, there was no response at harvest. In year 2, with more frequent sampling, CO2 assimilation increased from day 2 to day 23 and then decreased to day 49, about four weeks before harvest. CO₂ assimilation was also higher with low leaf:fruit ratios up until day 69 (a week before harvest). Net CO₂ ranged from 1,9 μmo/ m2s·1 with a leaf:fruit ratio of 6:1 at harvest, to 18,4 μmo/ m·2s·1 with a leaf:fruit ratio of 0,2:1 at day 23, about half way through fruit development.

Fruit-set was virtually eliminated in branches without leaves, suggesting that stored reserves alone cannot support the crop. Fruit-set was no better in large branches than in small branches when the number of leaves was the same. In contrast, larger branches where all the leaves were retained had greater fruit-set than small branches with fewer leaves. Developing fruit are mainly dependent on current CO2 assimilation.

Litchi growers should monitor inputs of water and nitrogen, which are likely to have a major influence on the ability of the canopy to fix CO₂ needed for fruit growth. In the cincturing experiment, there was double the number of fruit with cinctures at 0,5 m (7,9 per panicle) compared with controls (3, 7 per panicle), and three times as many with cinctures at 1,5 m (11,2 per panicle). Total fruit mass was 752, 74 g and 225 g, respectively These responses were not associated with differences in the leaf or stem starch at harvest which were below 2% in all treatments. Further experiments are required to evaluate the role of cincturing as a tool for increasing litchi fruit-set.