Yield-quality relationships under deficit irrigation in greenhouse tomato: modulation by seaweed biostimulants.

Introduction: Water availability is a crucial factor for crop performance in greenhouse tomato production, affecting both yield development and fruit composition traits. This study examined how deficit irrigation alters the relationship between yield and quality in tomatoes and whether foliar application of seaweed-derived biostimulants impacts these responses. Methods: A greenhouse experiment was carried out under two different irrigation levels, characterized by soil moisture contents of approximately 23-27% under optimal watering and 12-15% under reduced watering, using treatments based on extracts of Laminaria digitata and Ascophyllum nodosum, applied separately and together. Yield components, temporal yield dynamics, and concentration-related compositional traits were evaluated, and integrative PCA and concentration-index analyses were performed. Results: Reduced irrigation altered the yield structure by increasing fruit number and decreasing average fruit weight, while total yield showed limited variation among treatments. At the same time, concentration-related traits, including dry matter, soluble solids, vitamin C, and titratable acidity, were consistently higher under reduced irrigation. Biostimulant effects were dependent on the irrigation regime. Under optimal irrigation, treatment effects were limited, whereas under reduced irrigation, some treatments were associated with higher yields, primarily due to differences in fruit number during the main production peak. However, these yield responses were not consistently aligned with changes in compositional traits. Temporal yield analysis further showed that treatment effects under reduced irrigation were expressed mainly through modifications in the timing and intensity of peak production rather than through uniform effects across the entire production cycle. Integrative PCA and concentration-index analysis indicated that compositional enhancement and yield performance were only partly associated under reduced irrigation, suggesting a partial decoupling between productivity and fruit quality responses. Discussion: Overall, the results show that the irrigation regime is the main factor influencing yield-quality relationships in greenhouse tomato, while biostimulant treatments add extra, context-dependent variability. These findings emphasize the importance of evaluating both yield components and compositional traits together when analyzing crop responses under limited water availability.