Carbohydrate Budget of Pecan Trees and Its Impact on Nut Growth Regulation

Nonstructural carbohydrates (NSCs), including soluble sugars and starch, are the primary energy reserves for pecan (Carya illinoinensis) trees. Insufficient NSC availability during fruit development can reduce nut yield and quality; however, the role of carbohydrate allocation across different stem ages and fruit developmental stages remains poorly understood. This study examined carbohydrate dynamics in current-season, one-year-old, and two-year-old stems and their relationship with pecan fruit growth through three years. Carbohydrate content in both wood and bark was analyzed using the Anthrone reagent with an optimized microplate method. Fruit size was assessed at each sampling time using machine learning models developed in our previous research. Soluble sugar concentrations were highest in current-season fruiting stems, whereas starch levels were greatest in two-year-old stems, followed by one-year-old stems, with the lowest concentrations in current-season stems. These findings suggest that current-season stems primarily supply soluble sugars for nut development, while older stems function as starch reservoirs. NSC concentrations declined during the nut dough stage, coinciding with oil deposition, but increased again near nut maturity. Notably, higher carbohydrate reserves at the onset of a high-yield year, followed by depleted reserves in the subsequent low-yield years, indicate that stem carbohydrate status at the beginning of the growing season may be a key determinant of annual nut production. These findings provide insight into the role of NSC allocation in pecan trees and highlight the importance of carbohydrate reserves in regulating alternate bearing cycles.