Rubber biosynthesis activity is a key factor associated with seasonal fluctuations in rubber molecular weight of rubber trees

Seasonal fluctuation in rubber molecular weight is a key factor affecting the quality stability and performance of natural rubber (NR), but the underlying physiological mechanisms remain unclear. Over an entire tapping year in Hainan, China, we monitored latex physiological parameters (sucrose, thiols, and inorganic phosphorus), rubber particle size (RPS), rubber biosynthesis activity (RBA), and rubber molecular weight (Mn, Mw, and Mv) to elucidate the physiological basis of season-associated variation in rubber molecular weight. Latex physiological parameters, RPS, RBA, and rubber molecular weight all varied significantly across monthly and seasonal scales. Seasonal factors were the dominant driver of variation in latex physiology, while exerting moderate to modest effects on rubber molecular weight, RBA, and RPS. RBA was negatively associated with sucrose and RPS, while showing positive relationships with thiols and inorganic phosphorus. Furthermore, smaller rubber particles had both higher RBA and higher rubber molecular weight than larger ones, and this negative correlation between RPS and rubber molecular weight was consistently observed across the tapping season. RBA was also positively correlated with all three rubber molecular weight indices, with stronger associations observed for Mw and Mv than for Mn. The effects of latex physiological parameters on rubber molecular weight were predominantly associated with RBA, supporting RBA as a potential mediating factor in this pathway. Collectively, these findings suggest RBA may function as a potential biological hub linking latex metabolism with rubber molecular weight. Season-associated shifts may perturb carbon metabolism, redox balance, and energy status of laticifers, thereby potentially modulating RBA and contributing to rubber molecular weight fluctuations, although further multi-year and multi-clone investigations are needed to confirm this pathway. These insights provide a theoretical foundation for breeding rubber cultivars with stable molecular weight and for developing improved agronomic and processing strategies to enhance NR quality consistency.