How water-saving irrigation enhances woody fruit quality via metabolic regulation

Although water-saving irrigation increases water efficiency, it frequently reduces output, making it critical to compensate for this loss with improved fruit quality. However, fruit quality under water-stressed conditions remains highly unpredictable, as irrigation systems are usually developed without regard for regulatory mechanisms. Woody crops, due to their increased adaptation potential, can reduce output losses under optimized irrigation while improving fruit quality; knowing the regulation mechanism of fruit quality is critical. This review systematically reveals the multifaceted mechanisms by water-saving irrigation regulates fruit quality: moderate water stress activates enzymes such as sorbitol dehydrogenase and sucrose synthase, facilitating sugar transport into fruits and significantly increasing total soluble solids; organic acid levels exhibit species-specific variation, with total acidity generally decreasing in most fruits; fruit firmness is modulated by cell wall polymer degradation associated with pectin methylation and hemicellulose acetylation; enhanced fruit coloration results from the synergistic effect of increased phenylalanine ammonia-lyase and flavonoid glycosyltransferase activity and improved canopy light penetration, which collectively promote anthocyanin synthesis; antioxidant capacity is elevated through increased superoxide dismutase and catalase activities. At the molecular level, abscisic acid serves as the central signaling molecule: upon upregulation of NCED, it phosphorylates downstream transcription factors via the PYL-PP2C-SnRK2 module, synergistically activating sugar transport and anthocyanin synthesis genes, thereby forming a cascade regulatory chain of "water stress-ABA signaling-sugar transport-secondary metabolism". These provide specific targets for improving fruit quality in woody crops under water-saving irrigation and offer direct theoretical guidance to address the challenge of unpredictable fruit quality.