Organic fertilization enhances carbon–water coordination and water use efficiency of greenhouse tomato under deficit irrigation

Water scarcity poses a major challenge to sustainable agricultural production. Organic fertilization may mitigate the negative effects of water deficit by improving soil conditions and sustaining plant physiological activity; however, its physiological benefits under deficit irrigation in greenhouse tomato remain insufficiently quantified. This study evaluated the effects of fertilizer type and irrigation level on tomato growth, transpiration, gas exchange, yield, and yield-based water use efficiency across spring–summer and autumn–winter growing cycles. A factorial experiment included three fertilization treatments, namely no fertilizer (CK), chemical fertilizer (CF), and organic fertilizer (OF), under full irrigation (FI) and deficit irrigation (DI, 70% of FI). The results showed that deficit irrigation reduced daily transpiration, gas exchange, and yield; however, OF alleviated these reductions. Under DI, OF maintained greater plant height, stem diameter, and leaf area index than CF, approaching the level observed under fully irrigated CF. Although DI reduced daily and cumulative water use, OF sustained relatively high transpiration, net photosynthetic rate, and stomatal conductance, while stabilizing intercellular CO₂ concentration and delaying the decline in stomatal conductance under increasing leaf temperature. Consequently, DI combined with OF reduced yield losses to 3.9% and 8.8% relative to fully irrigated CF in the two seasons, while increasing yield-based water use efficiency by 16.4–22.1%. These findings indicate that organic fertilization enhances physiological resilience and promotes carbon–water coordination under moderate water deficit, thereby sustaining greenhouse tomato productivity and improving yield-based water use efficiency. Integrating organic fertilization with deficit irrigation may represent a promising strategy for improving water use efficiency in semi-arid greenhouse systems.