Moderate water deficit improves tomato fruit quality via carotenoid and volatile enhancement

• Water deficiency (WD) optimization improves consumer satisfaction and water waste. • WD improved tomato color & aroma via regulating carotenoid and volatile metabolism. • Our classification model serves as a basis for consumer preferences in tomato quality. Despite considerable focus on water deficit (WD) irrigation for boosting water use efficiency in yield-driven production, research into precise water management strategies that effectively balance the trade-off between yield penalty and fruit quality enhancement remains limited. Using the tomato cultivar ‘Micro-Tom’, we investigated water deficit treatments [T1–T4: 80 %, 65 %, 55 %, and 45 % of maximum field moisture capacity (FMC)] against full irrigation (CK: 90 % maximum FMC) at the mature green stage on tomato fruit maturation and quality attributes. Compared to CK, WD irrigation significantly accelerated fruit color transition, with T2 treatment upregulating key carotenoid biosynthesis genes ( SlPDS, SlZEP ), enhancing lutein and lycopene accumulation. The lutein content in the T2 treatment was significantly increased by 46.43 % versus CK. Furthermore, WD treatments markedly altered the composition and abundance of volatile organic compounds (VOCs), particularly aldehydes. T2 specifically elevated characteristic tomato aroma compounds (e.g., (E)-2-heptenal, hexanal, phenylethyl alcohol) by activating genes in fatty acid- ( SlTomloxC, SlHPL ), amino acid- ( SlAADC1B, SlPAR1/2, SlSAMT ), and carotenoid-derived ( SlCD1A/B ) volatile pathways. Furthermore, T2 treatment notably increased the contents of key fruity aroma compounds, 1-penten-3-one and 2-isobutylthiazole, by 28.83 % and 43.45 %, respectively. Multivariate classification modeling using principal component analysis (PCA) and hierarchical cluster analysis (HCA) indicated that the T2 treatment exhibited a distinct accumulation pattern for carotenoids and volatile compounds compared to other irrigation levels, demonstrating the most favorable profile. These findings demonstrate the applicability of moderate WD irrigation for enhancing tomato fruit quality, providing a theoretical basis for future water-saving and high-quality tomato cultivation.