Organ-specific carbohydrate partitioning strategies in Pinus massoniana seedlings during drought approaching mortality

Abstract Carbohydrates (e.g., structural carbohydrates and non-structural carbohydrates) can enhance drought resistance of plant through various mechanisms such as osmotic regulation, cellular protection, xylem reinforcement and energy supply. Although organ-specific carbohydrate allocation under drought has been explored in some species, little is known about the allocation strategies of both structural and nonstructural carbohydrates and their key components across different organs during drought-induced tree mortality. Here, potted Pinus massoniana Lamb. seedlings were treated with 80-d progressive drought stress until mortality, with samples collected every 10 days. The concentrations of total soluble sugars (hereafter referred to as sugars), glucose, sucrose, fructose, starch, structural carbohydrates (cellulose), lignin, and key enzymes involved in carbon metabolism were quantified across various organs, including roots, stems (wood and bark), and needles. The results revealed that under progressive drought stress, not only starch but also sugars and their key components (i.e., sucrose, glucose, and fructose) were significantly elevated in all plant organs examined, compared with the control conditions. Similarly, the concentration of lignin and cellulose in the roots, as well as lignin in the stems, was markedly elevated compared to the control. These parameters only declined when the seedlings approached mortality (70-80 days). At the end, the needles retained elevated sucrose concentration, while bark and wood exhibited persistently heightened sugar concentration (particularly glucose). Root cellulose concentration and stem lignin concentration remained significantly higher than the control. In summary, different organs of P. massoniana employed distinct carbon investment strategies under extreme drought: needles prioritize osmotic regulatory substances over structural components; stems (e.g., wood and bark) maintain stable structural carbohydrate levels; and roots implement a dual-defense strategy through enhanced physical protection (lignin reinforcement) combined with increased osmotic regulatory substances to strengthen physiological functions.