Coupled mechanisms of water deficit and soil salinity affecting tomato fruit growth

Water deficit and soil salinity lead to lower yield by limiting fruit growth. Investigating the process mechanisms by which water and salt stress limit fruit growth is important to fully exploit the biological and agronomic water saving potential of tomato in saline soils. We set up two irrigation regimes (W0, 90% of field capacity as the upper limit; W3, 60% of field capacity as the upper limit) and two soil salinity regimes (S0, no salt added; S3, salt was added to the soil at 3 g Kg−1 dry soil) to investigate the effects of water deficit and salt stress on tomato fruit carbon input rate, water input rate, volume expansion rate, water potential difference between plant stem water potential and fruit water potential (water potential gradient), osmotic potential, cell turgor pressure, hexose content and sucrose content of tomato fruits at different periods and their interaction. The correlation among the indicators was analyzed as well as the process mechanism leading to the reduction of fruit growth rate. The result showed that cell turgor pressure under water deficit and soil salinity was the key factors affecting fruit expansion. Under water deficit and soil salinity, reducing osmotic potential was the main strategy for plants to maintain cell turgor pressure, with hexoses and inorganic ions contributing more than 60% to osmotic potential in tomato fruit. For rapidly expanding fruit, water deficit did not significantly increase soluble sugars content or osmotic potential contributed by soluble sugars, and fruit cell turgor pressure was substantially reduced. The addition of 0.3% salt to the soil significantly increased the osmotic potential contributed by inorganic ions in the fruit during the rapid expansion period, resulting in a significant decrease in the osmotic potential of the fruit and improving the ability to maintain cell turgor pressure under water deficit. These results can provide useful guidance for regulated deficit irrigation of tomato in salinized areas.