Water Physiological Characteristics and Tonoplast Transporter Expression Profiles During Granulation in ‘Newhall’ Navel Oranges

Granulation with drying juice sacs is a common and seriously quality-affecting physiological disorder in citrus fruit. However, the intrinsic connection between it and the physiological regulation of fruit water, sugars and acids is still poorly understood. This study systematically quantified dynamic changes in key water status and sugar-acid transport parameters across granulation development to uncover how imbalanced water and sugar-acid metabolism and disrupted cellular hydration trigger granulation. In this study, ‘Newhall’ navel oranges (Citrus sinensis Osbeck) were used as the experimental material. The dynamic changes of key water and sugar/acid transport physiological indicators during the granulation process were systematically determined, aiming to clarify the physiological mechanism of water and sugar/acid metabolism imbalance and cell hydration state disorder during the occurrence of granulation. Both granulation rate and index increased significantly over time, reaching 84.98% and 18.33 by day 200, respectively. Granulated fruits exhibited accelerated water loss, a marked decrease in pulp water potential, and a progressively widening water potential gradient between peel and pulp. Physiological analysis revealed a significant decline in organic osmolytes (sugars, organic acids, and soluble proteins) in both tissues, alongside a redistribution of mineral elements (K, Ca, Mg) from peel to pulp during granulation. Gene expression profiling showed that granulation progression was associated with tissue-specific upregulation of several tonoplast intrinsic protein (CsTIP) genes (e.g., CsTIP1;1, CsTIP1;2, CsTIP2;1) and specific vacuolar membrane transporters for sugars and acids. Correlation analysis indicated strong relationships among water status, osmolyte dynamics, and vacuolar transporter gene expression. These findings demonstrate that the expanding peel–pulp water potential gradient drives osmotic disturbance and alters vacuolar transport activity, which collectively induce juice sac desiccation and granulation. These insights provide a physiological and molecular basis for developing water-management strategies to mitigate this quality disorder in citrus.