Controlled Water Deficit at the Mature Green Stage Alters Tomato Fruit Sugar Composition Without Yield Reduction

Water deficit (WD) irrigation has garnered considerable attention for its potential to enhance crop water use efficiency. However, limited research has been conducted on its ability to improve tomato fruit sweetness-related traits without significantly affecting yield. In this study, we investigated the effects of varying WD levels [T1–T4: 80%, 65%, 55%, and 45% of field capacity (FC)] compared with full irrigation (CK: 90% FC) on tomato fruits from the mature green to red-ripe stages, aiming to evaluate yield, textural attributes, and sugar composition. Notably, the fruit yield per plant under T2 treatment was not significantly different from that under CK. Compared with CK, T2 significantly reduced fruit water content by 2.39% while markedly increasing individual fruit dry weight by 13.61%. At 44 days after flowering, fruit firmness under T2 showed no substantial difference from CK, whereas adhesiveness was markedly elevated by 33.85%. Furthermore, T2 substantially boosted the activities of key Calvin cycle enzymes (ribulose-1,5-bisphosphate carboxylase/oxygenase, glyceraldehyde-3-phosphate dehydrogenase, fructose-1,6-bisphosphatase, fructose-1,6-bisphosphate aldolase, and transketolase) in tomato leaves, thereby increasing the photosynthetic rate and consequently elevating the total sugar content in tomato fruits. Additionally, T2 stimulated the activities of sucrose-hydrolyzing enzymes (acid invertase and neutral invertase) in fruits, leading to increased fructose and glucose accumulation at the red-ripening stage, with respective increases of 69.60% and 34.67% relative to CK. Multivariate classification based on principal component analysis and hierarchical cluster analysis revealed that T2 and T3 were distinctly separated from other treatments in terms of yield, texture parameters, and sugar profiles. These findings provide a valuable strategy for applying WD to improve fruit sugar composition without compromising yield.