Objective This study aimed to quantify the dynamic changes in soluble sugar content and photosynthetic physiological characteristics of Platycladus orientalis seedlings during progressive drought and subsequent rewatering, and to elucidate the correlations between soluble sugars (and their components) and photosynthetic traits under different water conditions. The findings are expected to provide scientific basis and theoretical support for scientifically planning greening tree species in Beijing under drought climate and for post-drought vegetation restoration and reconstruction.

Method Based on the physiological wilting point identified in preliminary experiments, P. orientalis seedlings in the Beijing area were subjected to progressive drought (TD, sampled on days 0, 2, 6, 10, 15, and 18), post-drought rewatering (TW, sampled on days 3, 5, 10, 13, 17, and 25), and control (CK, maintained at 80%−90% field capacity). Soil volumetric water content and leaf water potential were synchronously monitored to quantify stress intensity at each sampling node. At critical nodes, photosynthetic and chlorophyll fluorescence parameters were measured, and soluble sugars and their components were determined in various organs. A recovery index was established to quantify post-drought restoration, and Pearson correlation analysis was used to reveal the dynamic shift in sugar-photosynthesis relationships under different water conditions.

Result (1) Photosynthetic physiological indices showed significant stage-responsive characteristics during the drought and rewatering phases. During the drought phase, photosynthetic indices of P. orientalis peaked at mid-drought (TD₆−TD₁₀) and then declined. During the rewatering phase, intercellular CO₂ concentration (C_i), transpiration rate (T_r), and chlorophyll fluorescence parameters recovered first, followed by net photosynthetic rate (A_n) and stomatal conductance (G_s). (2) Organ-specific allocation of soluble sugars and their components exhibited marked drought-responsive differences. During the drought phase, leaf soluble sugar and its component contents increased (total sugars increased by 247.7%, glucose by 203.6%, and fructose by 264.9%), while those in twigs and stems decreased, indicating that photosynthates were preferentially allocated to leaves. Underground organs showed an initial increase followed by a decrease in soluble sugar and component contents. Regarding sugar components, the proportion of sucrose was relatively low in both aboveground and belowground organs, whereas glucose and fructose proportions were high, suggesting that sucrose synthesis and transport were inhibited and that osmotic adjustment mainly relied on glucose and fructose. During the rewatering phase, except for sucrose, sugar contents in aboveground and underground organs recovered to CK levels by day 3. (3) The correlations between soluble sugars and photosynthetic physiological characteristics underwent directional shifts with changing water conditions. During the drought phase, leaf sucrose was significantly negatively correlated with A_n and G_s (P < 0.05); except for sucrose, soluble sugars and their components in fine roots were positively correlated with G_s, indicating that root sugar changes were coordinated with stomatal conductance. During the rewatering phase, leaf sucrose content shifted to a positive correlation with G_s, with sucrose recovery accompanying stomatal conductance restoration. Soluble total sugars and glucose in leaves, twigs, and coarse roots, as well as fructose in twigs, stems, and coarse roots, were significantly negatively correlated with maximum photochemical efficiency (F_v/F_m) (P < 0.05), with sugar changes accompanying photosystem repair.

Conclusion P. orientalis copes with water stress through stage-specific adjustments in photosynthetic physiology and reorganization of soluble sugar allocation among organs. During the drought phase, photosynthetic carbon is preferentially concentrated in leaves, with glucose and fructose dominating osmotic adjustment; during the rewatering phase, recovery of photosynthetic and chlorophyll fluorescence parameters proceeds at different rates, while stomatal conductance and sucrose metabolism recover in tandem. This study reveals the physiological adaptation mechanisms of P. orientalis under water stress, providing a theoretical basis for water management and post-drought vegetation restoration of Platycladus forests in Beijing.