Coordinated regulation of tomato sugar accumulation by relative humidity and field capacity through source–sink–transport balance

To clarify the regulatory mechanism underlying the interaction between air relative humidity (RH) and soil moisture (field capacity, FC) on sugar accumulation in tomato fruits, this study used two tomato cultivars, 'Xiuzhen' and 'Jinhong 208', as materials. Four RH levels (40%, 60%, 80%, and 95%) and two FC levels (40% and 80% FC) were established in artificial climate chambers to systematically analyze the relationships among photosynthesis, transpiration, vascular transport, sugar metabolism enzyme activity, and sugar accumulation. The results showed that low soil moisture (40% FC) reduced the photosynthetic rate but significantly increased sugar metabolism enzyme activity, thereby promoting sugar accumulation. In contrast, high soil moisture (80% FC) was associated with higher photosynthetic rates but lower sugar-metabolizing enzyme activity. High air humidity (95% RH) markedly associated with lower photosynthesis, stomatal conductance, and sugar metabolism enzyme activity, accompanied by a significant decrease (<i>P</i> < 0.01) in glucose, fructose, and sucrose contents; conversely, low humidity promoted sugar accumulation. Both 'Xiuzhen' and 'Jinhong 208' achieved the highest soluble sugar content under 40% RH and 40% FC conditions. Gray relational analysis revealed that FC had a stronger influence on photosynthesis, while RH had a more pronounced effect on sugar metabolism. Under sufficient water supply, air humidity management played a greater role in regulating fruit quality. Path analysis indicated that RH exerted significant negative effects on sugar accumulation via reduced transpiration (path coefficient = -0.590), sugar metabolism enzyme activity (-0.358), and phloem transport (-0.424). In contrast, FC promoted phloem transport (0.680) but somewhat associated with lower enzyme activity (-0.500). Humidity is closely related to sugar accumulation, but weakly related to yield and its components. This study reveals the specific strategies of sugar regulation among different tomato cultivars and provides insights for humidity and irrigation optimization in greenhouse tomato production.