Regulatory effect of auxin inhibitors on flowering and fruit setting in litchi

[Objective] Litchi (Litchi chinensis Sonn.) is a hermaphroditic plant which possesses unisexual flowers formed by selective abortion of the floral sexual organs after meiosis. Too many flowers and low rate of females of panicles often lead to low fruit setting rate in litchi. This study aimed to develop new plant growth regulators that could be applied on litchi panicles to improve the yield and fruit quality. Exogenous auxin application on litchi panicles can decrease the rate of female flowers and reduce the amount of fruit set, so we wonder if auxin inhibitors have the opposite effects on flowering and fruit setting. [Methods] This study utilized litchi cultivar Feizixiao as the experimental material and applied naphthylacetic acid (NAA) along with varying concentrations of auxin inhibitors, namely 4-phenoxyphenylboronic acid (PPBo) and 2, 3, 5-triiodobenzoic acid (TIBA), by a spray on litchi flower spikes. The water treatment served as the control group. Astudy was performed to analyze the impacts of different treatments on the quantities of male and female flowers, the flowering duration of male and female flower, the quantity of fruit set, and the quality of litchi fruits. In terms of fruit quality, we tested the vertical diameter, transverse diameter, single fruit weight, peel weight, pericarp thickness, seed weight, edible rate and total soluble solid. In addition, the correlation between different flowering characters and fruit setting quantity of litchi were investigated. [Results] In the flowering aspect, the amount of female flowers and the rate of female flowers in litchi panicles were reduced by NAA treatment, as previously reported. Accordingly, the number of male flowers in litchi was decreased by PPBo and TIBA treatment at different concentrations, and TIBA treatment showed significantly difference compared with the control. However, the number of female flowers was not significantly altered between different auxin inhibitor treatments and the control group. The concentration of 100 mg·L-1 of TIBA (TIBA-2) had a notable impact on boosting the proportion of female flowers to 30.29%, but not other auxin inhibitor treatments. When compared with the control, all of the treatments had no impact on the duration of male and female flowering, as well as the overlap period between the males and females, however, the panicles of TIBA treatment had significantly longer flowering period of female flowers than the panicles of the NAA treatment. In terms of fruit setting quantity, both PPBo and TIBA treatments resulted in an increase in the average number of first and final fruit setting, which were calculated 1 and 7 weeks after female flowers withering, respectively. However, only the TIBA-2 treatment exhibited a considerable increase of the first and final fruit setting compared with the control, with an average of 59.17 and 15.00 per panicle, respectively. The correlation analysis of flowering traits and fruit setting quantity revealed significantly positive correlations between the initial fruit setting quantity and the ratio of female flowers as well as the number of female flowers, with the correlation coefficient of 0.70 and 0.51, respectively. On the contrary, the initial fruit setting quantity was negatively correlated with the number of male flowers and the total flowers, with significant correlation coefficients of-0.57 and-0.43, respectively. It is worth mentioning that the final and initial fruit setting amounts were correlated similarly with the flowering characteristics, however the correlation coefficients were larger in the initial fruit setting amount. Besides, the final and initial fruit setting amount showed a significantly positive correlation with the correlation coefficient of 0.73. From the perspective of fruit quality, there were no significant variations in fruit longitudinal diameter between the various treatments and the control. TIBA-2 had a considerably larger transverse diameter than the control, but there was no significant difference between the other treatments. Furthermore, the proper concentration of PPBo or TIBA could significantly boost the weight of individual fruits. The peel thickness of NAA and different concentrations of TIBA treatments was significantly lower than that of the control. There was no significant difference in the seed weight or total soluble solid (TSS) between the different treatments and controls. NAA and TIBA-2 treatment had the significantly higher edible rate than the control, reaching more than 80%, but the other treatment did not differ significantly from the control. [Conclusion] The auxin inhibitor TIBA treatments showed remarkable effects in reducing the number of male flowers, while increasing the proportion of female flowers and fruit setting quantity. Additionally, TIBA treatments reduced peel thickness and increased fruit weight and overall edibility. TIBA-2 with the concentration of 100 mg·L-1 was the most effective treatment that could enhance both the fruit quantity and fruit quality.