Relationship between flowering rate and fruit set, and the dates of“white millete”appearance and last autumn shoot flush maturation in litchi

Abstract: 【Objective】Litchi (Litchi chinensis Sonn.) is an important tropical fruit crop cultivated in more than 20 countries where it plays a crucial role in local economies. The low yielding and the biennial bearing are problems worldwide in litchi production. Shoot control can influence the phenology of last autumn flush maturation and“white millete”appearance, and thereby it is an effective management to improve blossoming and fruit setting in litchi. However there has no consensus on the application time of shoot control. To explore the optimum last autumn flush maturation date and the“white millete”appearance date in Feizixiao, Huaizhi, Guiwei and Nuomici litchi, the experiment was carried out to provide biological guidance for the management of shoot control and flower promotion in litchi. 【Methods】A total of 522 sets of data including phenology, flowering rate and fruit production of Guiwei and other varieties were obtained from 2009 to 2021 in the SCAU Main Campus Teaching & Research Base. The last autumn flush maturation date was split into three periods: before October 15, from October 16 to October 31 and after November 1. The“white millete”appearance date was split into three periods: before January 15, from January 16 to January 31, and after February 1. The floweringrate was split into: High rank (flowering rate ≥ 75% ), Medium rank (25%≤flowering rate＜75% ) and Low rank (flowering rate＜25%) to analyze the relationship between flowering rate and the date when late autumn flush matured. The number of fruit was split into: High rank (number of fruit≥upper quartile), Medium rank (median≤number of fruit＜upper quartile) and Low rank (number of fruit＜median) to analyze the relationship between fruit production and the date of“white millete”appearance. The box plots were applied to determine the outliers of the survey data. Data distribution was analyzed using the violin plot. T- test was used to determine whether there were differences between the data groups. The changes in the number of litchi plants from last autumn flush maturation,“white millete” appearance, and flowering to fruit ripening was tracked with Sankey diagram. Significant difference analysis and Pearson correlation were performed using R-project (V3.6.1).【Results】A total of 522 datasets were obtained from four varieties, including 81 sets from Feizixiao, 116 sets from Huaizhi, 202 sets from Guiwei, and 123 sets from Nuomici. The average value of the last autumn flush maturation date, the“white millete”appearance date, the flowering rate and the number of fruit for four varieties were October 22 to October 27, January 13 to January 20, 57% to 75% and 93 to 372, respectively. Among the four varieties, the flowering rate and fruit production were the highest when the last autumn flush matured between October 16 and October 31 and the“white millete”appearance before January 15. The flowering rate was over 75% , and concentrated at around 90% . The flowering rate decreased to less than 60% and the data were highly volatile when the late autumn flush matured after November 1 and initialed after January 16. For Feizixiao, the distribution of flowering rate was dispersed and lower than 20% when it matured before October 15 or after November 1. For Huaizhi and Guiwei, the flowering rate was about 50% when it matured after November 1. The fruit production also showed a similar pattern. The last autumn flush of plants with high rank of flowering rate matured between October 16 and October 31 with proportion of 45%. The“white millete”of plants with high rank of number of fruit appeared before January 15 with proportion of 74%. The correlation between the number of fruit and flowering rate was 0.54 (p＜0.001). The correlation between the number of fruit and the date of“white millete”appearance was - 0.51 (p＜0.001). The correlation between the flowering rate and the date of “white millete”appearance was -0.59 (p＜0.001). The correlation between the flowering rate and the date of last autumn flush maturation was -0.20 (p＜0.001). These results indicated that there was significant correlation between the fruit production, flowering rate and the last autumn flush maturation date and the“white millete”appearance date.【Conclusion】Overall, in Guangzhou, the best phenology for Feizixiao, Huaizhi, Guiwei and Nuomici varieties would be mature from October 16 to October 31 for the last autumn shoots, and initial before January 15 for the“white millete”appearance. The regulation such as girdling, precise control with growth regulator, timely irrigation, and spraying with foliar fertilizer in these phonological stages to improve flowering was a foundation for high and stable yield of litchi. This study provided a reference for shoot control and flower promotion in litchi.