Differences in effective accumulated temperature and leaf physiological indices of early-ripening characteristics among bayberry varieties and their correlation analysis

【Objective】Chinese bayberry (Myrica rubra) is a distinctive fruit indigenous to southern China, renowned for its unique flavor and high economic and nutritional value. However, the market supply period for this fruit is notably brief and concentrated, posing significant challenges in meeting consumer demands. Therefore, gaining insights into the mechanisms that contribute to early maturity is essential for the development of early-maturing varieties. As an early-maturing variant derived from a bud mutation of the Biqizhong cultivar, the Zaojia cultivar shows considerable potential in the Chinese bayberry market by effectively filling the supply gap and fostering the diversified growth of the bayberry industry. This study aims to analyze and compare the differences in total effective temperature and leaf physiological indices among three distinct Chinese bayberry cultivars—Zaojia, Biqizhong, and Dongkui. Furthermore, the research will investigate whether the early flowering and ripening observed in Zaojia are linked to these physiological and thermal differences.【Methods】The study was conducted from July 2022 to June 2023, using three vigorously growing Chinese bayberry cultivars (Zaojia, Biqizhong, and Dongkui) cultivated in a greenhouse in Lanxi, Jinhua, Zhejiang. Research focused on two critical stages: flower bud morphological differentiation and fruit development, with five trees sampled per cultivar and three years of replicates. Daily microclimate data from 2022 to 2024 were recorded using a temperature-humidity monitor. The effective accumulated temperature (K) was calculated using the formula K=(T−T0)N, where T is the daily average temperature, T0 is the biological zero (determined via the least squares method and linear regression based on the 10-day pre-bud-swelling average temperature), and N represents the growth duration in days. For leaf physiological indices analysis, leaves (the 3rd to 8th fully matured from shoot apices, three branches per direction) were sampled, blanched at 105 ℃, dried at 70 ℃ to constant weight, and ground. Soluble sugars and sucrose were extracted with ethanol and quantified using the anthrone-sulfuric acid method at 620 nm, while starch content was measured by hydrolyzing residues with perchloric acid, followed by the same colorimetric method. Nitrogen content was determined followed by digesting leaves, neutralizing the solution, reacting with sodium tartrate and Nash reagent, and measuring via the Nash colorimetric method at 420 nm. Pearson’s correlation coefficients were calculated using SPSS to analyze the relationships between flower bud differentiation stages and physiological indices, with heat maps generated by Origin for visualization.【Results】The results indicated that the effective accumulated temperature required for flowering and full maturity of Zaojia was lower than that of Biqizhong and Dongkui. In terms of leaf physiological, Zaojia exhibited a higher soluble sugar content during the flower bud differentiation stage, with a moderate coefficient of variation (CV), while Dongkui showed the highest CV. During the fruit development stage, the soluble sugar content decreased across all varieties, with CV reaching 36.5%-38.1%, indicating rapid consumption. The CV for sucrose content in Zaojia during the flower bud differentiation and fruit development stages were 34.4% and 24.2%, respectively, showing significant fluctuations and suggesting dynamic accumulation and consumption processes. The CV of starch content in Zaojia and Biqizhong during the flower bud differentiation stage were 38% and 44.9%, respectively, with Zaojia exhibiting the highest CV during fruit development, indicating notable changes in starch metabolism. The CV of nitrogen content ranged from 16.2%-30% during the flower bud differentiation stage, while those for Zaojia and Dongkui during the fruit development stage were 16.3% and 27.2%, respectively, indicating a rapid nitrogen turnover. Correlation analysis revealed that flower bud differentiation in Zaojia had a significant positive correlation with soluble sugar (Pearson correlation coefficient r=0.83, P＜ 0.05) and a significant negative correlation with nitrogen content (r=-0.91, P＜0.01), highlighting the regulatory role of carbon-nitrogen metabolism balance in flowering. In Dongkui, soluble sugar was positively correlated with flower bud differentiation (r=0.82, P＜0.05), and sucrose was positively correlated with starch (r=0.88, P＜0.01), indicating the synergistic effect of carbohydrate metabolism. Conversely, the correlations among various indices in Biqizhong were weak, suggesting limited interactive regulation during the differentiation stage.【Conclusion】This study reveals that the Zaojia cultivar of Chinese bayberry achieves early flowering and ripening possibly through two primary mechanisms. First, it requires a lower effective accumulated temperature for critical phenological stages, allowing it to complete its development more quickly than the Biqizhong and Dongkui varieties. Second, Zaojia exhibits a higher accumulation of soluble sugars in its leaves during flower bud differentiation, coupled with a more rapid turnover of sucrose and starch, which provides essential energy and signaling for early flowering. Additionally, a negative correlation between nitrogen and soluble sugars in Zaojia suggests a metabolic shift that favors reproductive growth over vegetative growth. These findings underscore the combined influence of thermal accumulation and carbon-nitrogen balance in regulating phe-nology, offering valuable insights for cultivar breeding and the management of production periods in Chinese bayberry. In summary, the differences in effective cumulative temperature and leaf physiological traits between Zaojia and other varieties likely account for its early flowering and ripening.