Korean Pines Demonstrate Cold Resilience through Non-Structural Carbohydrate Concentrations despite Light Deprivation during the Growing Season

Carbon is the fundamental element of plant life. Non-structural carbohydrates (NSC), synthesized using carbon dioxide through photosynthesis by plants, are essential for their growth and survival, which are also affected by light and temperature. However, few studies have investigated the effects of light conditions, season, and needle age together on field plants' carbohydrates. We measured total carbon, glucose, sucrose, fructose, and starch concentrations in current and 1-year-old needles of Korean pine (Pinus koraiensis Sieb. et Zucc) growing under two distinct light conditions (dense canopy and full light) from early summer to cold winter. Total carbon, glucose, fructose, SS (soluble sugar, sucrose + glucose + fructose), NSC (soluble sugar + starch), starch, and SS/NSC all significantly (p < 0.05) related to the DOY (day of the year, 2021). Total carbon reached the maximum in September when the fresh needles were mature. Glucose, NSC, SS, and SS/NSC reached the maximum at the last sampling time, which provided protection for the cells in cold winter. The season showed a bigger impact on total carbon and NSC (all parameters except total carbon) than light and needle age. Two different-aged needles under two light conditions all had similar patterns of variation in total carbon and NSC, but twigs showed a more significant (p < 0.05) difference in NSC concentrations between two light conditions on 25 October (DOY 249). Needles of Korean pines stored soluble sugars (mainly glucose) and consumed starch in winter. Moreover, we found that needles had more glucose, while twigs had more sucrose, which may be a result of their different functions and may be helpful for future spring growth. Although lacking light during the growing season, understory Korean pines still had enough cold tolerance, similar to full-light ones, which indicated that low light during the growing season has little effect on cold tolerance.