Effects of soil moisture on water transport, photosynthetic carbon gain and water use efficiency in tomato are influenced by evaporative demand

Although deficit irrigation has long been recognized as a water-saving practice, the beneficial effects on crop production and on water use efficiency under varying atmospheric evaporative demands has rarely been examined. In the present study, the coordinated effects of soil moisture and atmospheric vapor pressure deficit (VPD) on photosynthetic carbon gain versus water transport and water use efficiency in tomato were addressed. Experiments were designed with factorial combinations of two levels of VPD and two gradients of soil moisture. Low VPD (VPD < 1.0 kPa) compensated for the negative effect of soil water deficit on plant photosynthesis and productivity by reducing the resistance for CO2 transport. Moreover, low VPD moderated water stress by reducing the force driving passive water movement and preventing turgor loss, which sustained stomatal openness and reduced the resistance to CO2 uptake. In addition to stomatal conductance, the mesophyll conductance for CO2 transport from the substomatal cavities to the chloroplasts was increased synchronously in low-VPD-grown plants. The effect of water-use efficiency on the yields and plant biomass was substantially increased in the low-VPD treatment for both deficit irrigation and well-irrigated conditions. Water use efficiency was maximized in the combinatory treatment of deficit irrigation and low VPD. The present study demonstrated that the beneficial effect of deficit irrigation on tomato was amplified by decreasing VPD to the range of 0.5–1 kPa by decreasing the resistance for CO2 uptake from the atmosphere to the carboxylation site. Moreover, the adverse effects of deficit irrigation were pronounced with increased VPD when exceeding 1 kPa. Therefore, VPD plays significant roles in mediating the magnitudes of the beneficial or negative effects of deficit irrigation. The present study highlights the significance of the integrative regulation of soil and atmospheric moisture conditions, which provides novel insight for water-saving tomato production.