Aim
Water scarcity is a critical and growing constraint on agricultural productivity, especially in arid and semi-arid regions, thereby necessitating efficient irrigation strategies that balance water saving with acceptable yield. Tomato (Solanum lycopersicum L.) is a high-water-demanding yet economically important crop with considerable physiological plasticity that may allow adaptation under regulated deficit irrigation, provided that stress thresholds are properly understood. This study therefore aimed to (1) investigate the effects of different deficit irrigation levels on yield components and physiological traits of tomato under controlled greenhouse conditions, (2) determine the plant's drought tolerance threshold using key physiological indicators including leaf temperature, leaf water potential, stomatal conductance, and relative water content (RWC), (3) identify the most reliable physiological predictors of yield reduction under water limitation, and (4) propose an optimal and practical irrigation strategy for application in water-scarce environments.
Materials and Methods
The experiment was conducted in the greenhouse of the Department of Soil Science and Engineering, University of Tabriz, over a 5-month period from late winter to spring. The design was a completely randomized block design with four irrigation treatments based on field capacity (FC): full irrigation (FC control), mild deficit (0.75FC), moderate deficit (0.5FC), and severe deficit (0.3FC), with three replications. Tomato seedlings were planted in boxes (20×35×28 cm) containing 20 kg of silty clay loam soil (sand 5.50%, silt 58.40%, clay 36.10%) with initial soil properties including pH 7.12, ECe 2.54 dS/m, organic carbon 2.01%, CCE 20%, and field capacity moisture 34%. Two seedlings were established per box, and nutrients (N, P, K, Fe, Mn, Zn, Cu) were added based on soil testing. Soil moisture was monitored using electrical resistance sensors with a calibrated curve, and irrigation was applied when moisture reached the designated lower limits. Measured traits included leaf temperature (infrared thermometer), leaf water potential (pressure chamber), RWC (calculated from fresh, turgid, and dry weights), stomatal conductance (porometer), and yield components including fruit number, fruit weight, shoot wet weight, and root wet weight. Data were analyzed using ANOVA with Duncan's test for mean comparisons in SPSS version 21, and Pearson correlation coefficients were calculated.
Results
Irrigation level significantly affected all measured physiological traits—including leaf water potential, relative water content (RWC), leaf temperature, and stomatal conductance (P < 0.01)—as well as most growth and yield components, with root wet weight responding at a lower significance threshold (P < 0.05). Stomatal conductance remained near control levels under mild deficit (0.75FC) but dropped sharply under moderate (0.5FC) and severe (0.3FC) stress, indicating that only moderate-to-severe water limitation triggers strong stomatal closure, which reduces transpirational water loss but also restricts CO₂ diffusion and photosynthesis. Concurrently, leaf water potential became progressively more negative, falling from −3 bars under full irrigation to −14.66 bars under severe stress, while RWC declined from roughly 83% to 46%, revealing that the plant's capacity to maintain tissue hydration deteriorates rapidly once soil moisture falls below the mild-deficit threshold. Leaf temperature rose significantly with increasing water deficit, from 21.23°C under FC to 30.90°C under 0.3FC, consistent with reduced stomatal conductance and diminished transpirational cooling, thereby serving as a rapid, non-destructive indicator of stress intensity. In terms of vegetative growth, shoot wet weight peaked under mild deficit (192.61 g) and was lowest under severe stress (141.94 g), suggesting an adaptive balance between growth and water availability, whereas root wet weight decreased significantly only under the most severe treatment, from 234.44 g to 152.23 g. Reproductive performance was highly sensitive to stress: total fruit weight did not differ significantly between FC and 0.75FC but dropped by approximately 58–61% under moderate and severe deficits; fruit number, however, showed an even stronger response, declining by 63% and 77% under 0.5FC and 0.3FC, respectively, indicating that stressed plants prioritize assimilate allocation to already-established fruits rather than supporting new fruit initiation. Correlation analyses confirmed strong interrelationships among physiological responses and yield traits: leaf temperature was negatively correlated with leaf water potential (r = −0.97) and stomatal conductance (r = −0.91); fruit number showed the strongest negative correlations with leaf temperature (r = −0.91) and the water-potential stress index (r = −0.90), while fruit weight exhibited weaker correlations (−0.76 and −0.74, respectively). Notably, RWC demonstrated strong positive correlations with fruit number (r = 0.84), fruit weight (r = 0.67), and shoot wet weight (r = 0.86), making it the most consistent and reliable overall predictor of both vegetative growth and reproductive performance across all irrigation treatments.

Conclusion
In conclusion, deficit irrigation triggered a coordinated and progressive cascade of physiological responses in tomato plants, beginning with declines in leaf water potential and relative water content (RWC), followed by stomatal closure, reduced transpirational cooling, and a consequent rise in leaf temperature. Once the water deficit exceeded the mild level (i.e., below 0.75FC), these physiological disruptions became pronounced and were directly associated with substantial reductions in vegetative growth, fruit set, and total yield. Among all yield components, fruit number proved to be the most sensitive parameter, declining by up to 77% under severe stress, which likely reflects the particular vulnerability of flowering, pollination, and early fruit establishment to water shortage; in contrast, the relatively smaller reduction in individual fruit weight suggests that stressed plants adopt a conservative resource-allocation strategy, preferentially partitioning available assimilates and water toward sustaining already-formed fruits rather than supporting the initiation of new reproductive structures. Importantly, the 0.75FC treatment did not differ significantly from full irrigation (FC) in terms of fruit yield, fruit number, stomatal conductance, or several growth characteristics, establishing this level as the approximate drought-tolerance threshold under the conditions of this study. Among the physiological indicators evaluated, RWC emerged as the most robust and consistently reliable predictor of overall plant performance, given its strong positive correlations with fruit number (r = 0.84), fruit weight (r = 0.67), and shoot biomass (r = 0.86), while leaf temperature proved to be an especially valuable rapid, non-destructive diagnostic tool for real-time stress monitoring in greenhouse settings, owing to its tight negative correlations with leaf water potential (r = −0.97) and stomatal conductance (r = −0.91). Collectively, these findings support the recommendation of mild deficit irrigation at 0.75FC as a practical and water-saving strategy for greenhouse tomato production in arid and semi-arid regions, capable of reducing irrigation water use by roughly one-quarter while maintaining acceptable yields and fruit quality. However, it must be emphasized that before this strategy can be widely recommended to growers, further field-scale validation studies are required across a broader range of cultivars, soil types, and climatic conditions, along with long-term investigations into the cumulative effects of repeated mild deficit cycles on soil health, root dynamics, and fruit physicochemical properties, to ensure the generalizability and sustainability of this approach under diverse agricultural systems.