Responses of Functional Traits in Spring Maize Leaves to Water Stress and Their Threshold Effects

To investigate the response patterns of functional traits to soil moisture during the jointing stage of spring maize and to identify key moisture thresholds and adaptive strategies, this study conducted a pot‐based water‐controlled experiment. Leaf morphological and physiological traits, water status and photosynthetic gas exchange parameters were measured under different soil relative humidity (RH) conditions. Linear regression, sigmoidal function and piecewise regression models were employed, combined with principal component analysis, cluster analysis and correlation analysis for a comprehensive assessment. The results showed that: (1) Specific leaf area (SLA) and chlorophyll content (SPAD) exhibited a linear decline with decreasing soil RH, with no significant threshold detected. Water status parameters (tissue water content and leaf water potential) displayed a two‐phase response, with synergistic thresholds concentrated in the 36%–47% RH range. Among photosynthetic parameters, stomatal conductance ( G s ), transpiration rate ( T r ) and net photosynthetic rate ( P n ) followed sigmoidal threshold responses, with T 10 values of 75.3%, 68.6% and 64.9% RH, respectively. In contrast, intercellular CO 2 concentration ( C i ), stomatal limitation value ( L s ) and water use efficiency (WUE) exhibited a three‐phase response; a critical transition from non‐stomatal to stomatal‐dominated photosynthetic limitation occurred in the 36%–47% RH range. (2) Cluster analysis divided the samples into two groups at a moisture cutoff of 41% RH. The low‐RH group exhibited differentiation between ‘water‐saving’ and ‘drought‐tolerance’ strategies, whereas traits in the high‐RH group showed convergence. Correlation analysis further revealed that under low‐RH conditions, maintaining higher tissue water content was associated with lower WUE and higher C i , indicative of non‐stomatal limitation; whereas under high‐RH conditions, photosynthetic activity was driven primarily by leaf water potential. This study identified multi‐stage thresholds of water stress in spring maize, revealing distinct adaptive strategies under drought stress and optimal moisture conditions. Maintaining soil RH above 47% can ensure the full expression of photosynthetic potential, whereas values below 41% warrant attention to the risk of drought‐induced impairment of plant functions. These findings provide a theoretical basis for water management of spring maize in arid and semi‐arid regions.