Analysis of the reasons why 3, 5, 6-TPA promotes the enlargement and maturation of litchi fruits

【Objective】3,5,6-trichloro-2-pyridinyloxyacetic acid (3,5,6-TPA) has been proven to have a positive effect on the enlargement and maturation of litchi fruits. However, previous studies have been limited to phenotypic observations, lacking in research on the mechanisms by which it regulates fruit size and maturation. This study used the high-quality late-maturing litchi variety Xianjinfeng as the experimental material to explore the regulatory mechanisms by whole-tree spraying of 40 mg · L- 1 3,5,6- TPA on litchi fruit growth and development, aiming to provide a scientific basis for its rational application in litchi production.【Methods】Sixteen- year- old Xianjinfeng litchi trees grafted onto Feizixiao rootstocks were used as experimental materials. Six trees with relatively consistent vigor were selected, with three trees sprayed with 40 mg · L- 1 3,5,6-TPA and the other three sprayed with water as controls. The spraying was conducted five weeks after flowering (when the young fruit was about 1 g), until the leaf surface was dripping wet, with a volume of 8 L per tree. Fifteen branches were randomly selected from each treatment for tagging and fruit drop rate statistics. If fruit splitting occurred, the fruits were removed after counting and 20 fruits were randomly selected from each treatment for tagging to measure their transverse and longitudinal diameters for dynamic fruit development analysis. Eight mature leaves of the same age and position were selected from the external canopy of each tree and tagged, and their net photosynthetic rate was measured using a CIRAS- 3 portable photosynthesis system (PP system, USA) every three days between 9:00—11:00 a.m., before and after treatment, until photosynthetic damage was restored. Fruit samples were collected once a week, with 30 fruits per sample. The pericarp, pulp, and seeds were separated, frozen in liquid nitrogen, and stored in a -80 ℃ freezer for the determination and analysis of major physiological indices of the fruit. The fruit quality was finally measured at harvest.【Results】Spraying with 3,5,6-TPA significantly enlarged the Xianjinfeng litchi fruit. The average fruit weight of the treated group reached 25.0 g, while that of the control was only 22.9 g, representing a 9.2% increase in the treated group compared to the control. The proportion of first-grade fruit in the treated group increased by 137.5% compared to the control, while the proportion of substandard fruit decreased by 128.3%. The treatment also advanced fruit maturation by one week but did not alter the final coloration of the pericarp after maturation. It had no significant effect on the fruit soluble solids, titratable acid, solid-acid ratio, edible rate, or rate of sunken seeds. Spraying with 3,5,6-TPA inhibited fruit drop in the first four weeks after treatment. However, some fruits on the panicles stopped growing and the pericarp turned yellow. These developmentally arrested fruits began to drop from the third week, and by harvest, the cumulative fruit drop rate in the treated group was higher than that in the control, although the difference was not significant. Spraying with 3,5,6-TPA temporarily induced photosynthetic damage in litchi leaves within 10 days after treatment, which then returned to normal. Spraying with 3,5,6-TPA significantly increased the sucrose content in the aril 2-3 weeks after treatment, as well as the contents of fructose, glucose, and total sugar, with no significant differences at harvest. Spraying with 3,5,6-TPA altered the activity of key enzymes in aril sucrose metabolism, significantly increasing acid invertase activity in the third week after treatment, neutral invertase activity in the second week, with no significant difference in sucrose synthase activity, and increased sucrose phosphate synthase activity before the fourth week, which then fell below the control. Foliar application of 3,5,6-TPA advanced the peak of endogenous IAA during fruit ripening and accelerated the subsequent decline by 1 week relative to the control, while it increased the ABA content in various tissues of the fruit. The potential reasons for the promotion of fruit enlargement by 3,5,6-TPA treatment include: (1) Briefly inhibiting the net photosynthetic rate of litchi leaves, causing some fruits to be abortted in the early developmental stage and then drop, which had a thinning effect; (2) Increasing the endogenous IAA content in the pericarp, aril, and seeds during the early stages of fruit development; (3) Enhancing the activities of cell wall invertase, acid invertase, neutral invertase, and sucrose phosphate synthase, thereby promoting sugar accumulation and fruit development. The potential reasons for the accelerated fruit maturation include: (1) Increasing the endogenous ABA content in the pericarp, aril, and seeds during fruit development; (2) Promoting the early decline of endogenous IAA content in the pericarp and aril during fruit maturation.【Conclusion】Compared with the control, 3,5,6-TPA can temporarily inhibit the net photosynthetic rate of litchi leaves without significantly affecting the intrinsic quality of Xianjinfeng litchi. It can also alter the activity of related metabolic enzymes in the aril, promote sugar accumulation in the early stages of fruit development, and regulate the endogenous IAA and ABA contents in various fruit tissues, thereby effectively increasing fruit size and the rate of high-quality fruit, and promoting early fruit maturation.