Responses of tomato fruit water balance and xylem hydraulic property of pedicel and calyx to water deficit and salinity stress

Abstract

Moderate water scarcity and high soil salinization always limit fruit growth but largely promote fruit quality. Water is transported to the fruit via xylem and phloem and lost via transpiration, which is important for fruit growth and quality parameters. So far, few studies have quantified the relationship between fruit water balance and fruit growth and quality under water deficit and salinity stress (especially combined water deficit and salinity stress). In this study, tomato (Solanum lycopersicum) plants were grown under full irrigation (CK), water deficit (WD), salinity stress (SS), and combined water deficit and salinity stress (WS). We measured fruit size and quality parameters, investigated fruit water balance using a subtraction method, evaluated xylem hydraulic resistance of pedicel and calyx applying a hydraulic head, and assessed the xylem functionality by fruit rehydration and dye-tracing experiments. Results showed that WD, SS, and WS decreased plant water status and leaf gas exchange and fruit growth, but increased fruit total soluble solids. These effects were more pronounced in WS than both WD and SS. The inhibition of fruit growth by three treatments was caused by the jointly reduced xylem and phloem flow. Although xylem hydraulic resistance of pedicel and calyx at 35 d after anthesis was not significantly affected under WD and SS, it greatly increased by 112% under WS, which coincided with a significant reduction of daily growth rate and xylem flow computed on a per fruit basis by 83% and 87% under WS. These results also suggested that the xylem tissues were still functional regardless of the applied stress and fruit growth period. Phloem flow computed on a per fruit basis was decreased under all treatments, resulting in a reduction in fruit dry matter quantity, partly due to the decreased photosynthates. Phloem flow and fruit transpiration computed on a per fruit fresh weight basis were not affected due to the synergistic reduction of flow and fruit size by all treatments. This study provides a new understanding on the water relations of plant-fruit system under water deficit and salinity stress.