Plasticity of physiological, anatomical and structural traits defines seedling growth during sustained drought

Understanding how trees acclimate to sustained drought by altering physiological, anatomical and structural traits is crucial for elucidating their acclimation to water scarcity. This study investigated the plasticity of Tilia amurensis Rupr. seedlings under moderate (50% of field moisture capacity) and severe (30%) drought over 2 years by assessing a range of anatomical, structural and physiological traits. Our results showed that drought reduced photosynthesis by 29.5% to 57.7%, leaf respiration by 37.3% to 45.7%, and ring width by 55.9% to 63.1%. Starch concentrations decreased with increasing drought severity, while soluble sugar concentrations increased. Additionally, cell number and total width decreased during enlargement and wall-thickening processes. Vessel density increased regardless of drought intensity, while hydraulic diameter increased under moderate drought but decreased under severe drought, suggesting that the safety-efficiency trade-off was not universal. The sieve-tube-to-vessel area ratio decreased under moderate drought but increased under severe drought. Drought-induced structural changes included increased Huber value, root mass fraction and root-shoot ratio, while leaf biomass fraction and specific leaf area decreased. These adjustments collectively enhanced the resistance of seedlings to sustained drought.