Interactive Regimes of Reduced Irrigation and Salt Stress Depressed Tomato Water Use Efficiency at Leaf and Plant Scales by Affecting Leaf Physiology and Stem Sap Flow

Interactive effects of reduced irrigation and salt stress on leaf physiological parameters, biomass accumulation, and water use efficiency (WUE) of tomato plants at leaf and whole plant scales were investigated in a field experiment during 2016 and a greenhouse experiment during 2017. Experiment utilized two irrigation regimes (full, 2/3 of full irrigation) and four soil salt regimes (0, 0.3, 0.6, 0.9% in 2016 season; and 0, 0.2, 0.3, 0.4% in 2017 season). Three salts, sodium chloride, magnesium sulfate, and calcium sulfate (mass ratio of 2:2:1), were homogeneously mixed with soil prior to packing into containers (0.024 m(3)). Li-COR 6400 was used to measure tomato leaf physiological parameters. Instantaneous water use efficiency (WUE(ins), μmol mmol(−1)) and intrinsic water use efficiency (WUE(int), μmol mol(−1)) were determined at leaf scale, yield water use efficiency (WUE(Y), g L(−1)), and dry biomass water use efficiency (WUE(DM), g L(−1)) were determined at whole plant scale. Plants irrigated with 2/3 of full irrigation with zero soil-salt treatment had higher dry biomass and yield per plant, resulting in the highest WUE(DM) and WUE(Y) at whole plant scale. Increasing soil salinity decreased dry biomass and yield, leading to greater decreases in whole plant WUE(DM) and WUE(Y) under both irrigation treatments. At full irrigation, no decreases in stomatal conductance (g(s), mol m(−2) s(−1)) and slight increase in photosynthetic rate (P(n), μmol m(−2) s(−1)) led to higher WUE(int) at leaf scale during both years. Under full and reduced irrigation, increasing soil salt content decreased P(n) and transpiration rate (T(r), mmol m(−2) s(−1)) and led to reductions in WUE(ins) at the leaf scale. However, compared to full irrigation, reduced irrigation improved WUE(ins) with a significant decline in T(r) in no salt and 0.3% soil-salt treatments during both years. For soil salt content of 0.6%, stomatal limitation due to salt stress resulted in higher WUE(int), but soil salt content of 0.9% decreased WUE(int) due to non-stomatal limitation. Soil salt content significantly decreased sap flow, with the maximum variation of daily sap flow per plant of 7.96–31.37 g/h in 2016 and 12.52–36.02 g h(−1) in 2017. Sap flow rate was linearly related to air temperature (T(a), °C), solar radiation (R(s), W m(−2)), and vapor pressure deficit (VPD, kPa). These results advance knowledge on tomato response to abiotic stresses and could improve management of tomato production in water- and salt-stressed areas.