Objective To clarify the physiological regulation mechanism of fruit drop and germplasm genetic characteristics of Xanthoceras sorbifolium, this study explored the correlation between fruit drop rate and nutrients as well as endogenous hormones, screened excellent clones with low fruit drop rate, and analyzed germplasm genetic diversity, so as to provide a basis for germplasm innovation and high-yield breeding of X. sorbifolium.

Method A total of 22 X. sorbifolium clones were used as experimental materials. Observations on fruit drop rate and new shoot growth, and determinations of carbohydrate and endogenous hormone contents in leaves and fruits were conducted during the first fruit drop peak. The correlation and accumulation patterns between each index and fruit drop rate were analyzed. To resolve the intrinsic genetic basis of phenotypic differences among the 22 clones, 20 pairs of SSR primers were adopted to analyze the genetic diversity of tested germplasms.

Result (1) The fruit drop rate of the 22 clones ranged from 39.54% to 81.02%, which could be classified into three groups: low, moderate and high fruit drop rate. Clones 121, 171 and 102 with low fruit drop rate were screened out. The fruit drop rate was extremely significantly positively correlated with new shoot growth, revealing that nutrient competition between shoots and fruits is an important cause of young fruit abscission. (2) For carbohydrates, soluble sugar content followed the order of normal fruit > leaf > abscised fruit, and starch content followed leaf > normal fruit > abscised fruit. The contents of soluble sugar and starch in fruits of low fruit-drop clones were significantly higher than those of high fruit-drop clones, demonstrating higher translocation efficiency of photosynthates to fruits. (3) The contents of IAA, GA₃ and ZT in normal and abscised fruits of low fruit-drop clones were significantly higher than those of high fruit-drop clones, while ABA and JA contents were significantly lower. This indicates that differences in hormone balance among clones constitute an important intrinsic physiological basis for fruit drop differentiation. (4) The average PIC value of the 20 pairs of SSR primers was 0.60, and the tested germplasms possessed abundant genetic diversity, which provides a favorable genetic foundation for screening low fruit-drop elite clones. Nevertheless, the matching degree between fruit drop phenotypic clustering and SSR molecular clustering was low, because fruit drop is a quantitative trait susceptible to environmental regulation.

Conclusion Clones 121, 171 and 102 with low fruit drop rate were identified in this study. Nutrient allocation and endogenous hormone balance act as the key physiological basis regulating the fruit drop level of X. sorbifolium clones. The 22 tested germplasms have rich genetic diversity and excellent breeding potential. This study preliminarily elucidated the regulatory mechanisms underlying differences in fruit drop from the two dimensions of physiological traits and genetic background, providing theoretical support for the selection of superior varieties and cultivation management of X. sorbifolium.