- N J R Roets
- ARC-Institute for Tropical and Subtropical Crops Private Bag X11108, Nelspruit 1100, South Africa
- 2004
Application of plant growth substances on litchi fruit resulted in improved fruit retention and fruit size. Trials were undertaken in the Malelane and Nelspruit areas to determine the effect of synthetic auxins and cytokinins on fruit set and retention of ‘HLH Mauritius’. In Malelane, the synthetic auxins Corasil E® (Dichlorprop) at a concentration of 100 and 125 ppm and Maxim® (TPAJ at 50 ppm were applied when fruit weighed approximately 2 g. Applications were done on trees and branches, but on branches 75 and 150 ppm Corasil E® treatments, as well as 0.4% Kelpak® were also included. With the branch treatments fruit retention was significantly improved by the 100 and 125 ppm Corasil E® and 50 ppm Maxim® treatments. The 125 ppm Corasil E® treatment, however, gave significant smaller fruit. The other treatments had no significant effect on fruit size. However, on the full tree applications none of the treatments had any significant effect on fruit retention, fruit size and production. In the Nelspruit trial the synthetic cytokinin, CPPU, at a concentration of 20 ppm was applied two and three weeks after full female bloom. In another treatment a 5 ppm NAA application two weeks after full female bloom, were followed by a 20 ppm CPPU application three weeks after full female bloom. Fruit retention was slightly, but not significantly, lower in all the treatments. Fruit size was improved in all treatments although only the 20 ppm CPPU treatment three weeks after full female bloom gave significant larger fruit.
Summary:
- Objective: Evaluate how synthetic auxins and cytokinins affect fruit set, retention, and size in HLH Mauritius litchi, under Malelane and Nelspruit conditions.
- Key issue in litchi production: Two major periods of fruit drop reduce potential yield; early drop occurs within two weeks after full female bloom (FFB), and a second drop around 55 days after FFB, with low embryo/endosperm reserves contributing to abscission.
- Treatments tested in Malelane (tree-wide applications): Corasil E® at 100 and 125 ppm, and Maxim® (TPA) at 50 ppm, applied when fruit ~2 g and embryo heart-shaped. Branch applications included 75, 100, 125, 150 ppm Corasil E®, 50 ppm Maxim®, and 0.4% Kelpak.
- Results in Malelane:
- Tree-wide applications: No significant improvements in fruit retention, size, or overall production for any treatment.
- Branch applications: Significant improvement in fruit retention with 100 and 125 ppm Corasil E® and 50 ppm Maxim®; however, 125 ppm Corasil E® reduced fruit size. Other treatments showed no significant size effects.
- Treatments tested in Nelspruit (fruits on branches, not whole trees): CPPU at 20 ppm (two timings: 2 weeks and 3 weeks after FFB), NAA at 5 ppm (2 weeks after FFB) followed by CPPU at 20 ppm (3 weeks after FFB).
- Results in Nelspruit:
- Fruit retention: Slightly lower with all treatments, not statistically significant.
- Fruit size: Increased in all treatments; only the 20 ppm CPPU applied 3 weeks after FFB produced a statistically significant increase in fruit size.
- Abbreviations used: CPPU (N-(2-chloro-4-pyridyl)-N’-phenylurea), NAA (Naphthalene acetic acid), TPA (Maxim®; 3,5,6-trichloro-2-pyridyl-oxyacetic acid); FFB (Full female bloom).
- Background on hormonal roles:
- Endogenous indoleacetic acid (IM) rises early to aid fruit retention; abscisic acid is associated with abscission and peaks during the two drop periods.
- Synthetic auxins (e.g., TPA, 2,4-D) have shown promise in various crops for reducing fruit drop or increasing size, with litchi studies indicating potential retention gains and variable effects on size.
- Synthetic cytokinins (e.g., CPPU) can stimulate cell division and size in some fruits but may negatively affect retention in litchi when applied after FFB; effects on size can be positive in other crops.
- Comparative takeaway:
- Branch-focused applications of certain auxins (100–125 ppm Corasil E®) and Maxim® can improve fruit retention but may compromise size at higher doses (e.g., 125 ppm Corasil E®).
- CPPU can enlarge fruit size when applied in specific post-FFB timings, but retention effects are inconsistent and may be negative in litchi.
- Overall conclusion: Plant growth substances can partially mitigate fruit drop and influence fruit size in litchi, but results are highly dependent on product, concentration, application timing, and whether treatments are applied to trees or branches; optimizing for both retention and size requires careful selection of compound, dose, and timing, and single-year results warrant further testing.