Abstract

Soil moisture plays a crucial regulatory role in determining the size and quality of citrus fruits. This study analyzed the fruit size and quality of citrus under different soil moisture conditions, including severe drought (SD), moderate drought (MD), and control (CK). Under drought stress, the fruit weight, longitudinal diameter, and transverse diameter of citrus fruits were significantly lower than those of the control. The content of soluble solids and titratable acid in fruits under drought stress was significantly higher than that of the control. Transcriptome sequencing revealed that compared with CK, there were 1186 differentially expressed genes in MD, including 414 up-regulated genes and 772 down-regulated genes; and 2315 differentially expressed genes in SD, including 1143 up-regulated genes and 1172 down-regulated genes. The differentially expressed genes were significantly enriched in cellular processes, metabolic processes, and plant hormone signal transduction. The down-regulated expression of auxin and gibberellin biosynthesis genes (YUC10, GA20OX1, GA2OX1, and GA20OX2) and signal transduction-related genes (AUX/IAA13, SAUR32, GH3.1, and ARRs), and the up-regulated expression of cytokinin decomposition gene (CKX5) may be associated with reduced fruit size under drought conditions. The up-regulated expression of citric acid synthesis genes (PEPC2 and PEPCK1) and vacuolar transporters (PH1, PH4, PH8, and VHA-c3) may be associated with pronounced accumulation of citric acid in citrus under drought stress. In conclusion, water control regulated fruit size and acidity by modulating phytohormone metabolism and signaling, along with the synthesis and transport of citric acid.

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