Although atmospheric vapour pressure deficit (VPD) has been widely recognized as the evaporative driving force for water transport, the potential to reduce plant water consumption and improve water productivity by regulating VPD is highly uncertain. To bridge this gap, water transport in combination with plant productivity was examined in tomato (Solanum lycopersicum L.) plants grown under contrasting VPD gradients. The driving force for water transport was substantially reduced in low-VPD treatment, which consequently decreased water loss rate and moderated plant water stress: leaf desiccation, hydraulic limitation and excessive negative water potential were prevented by maintaining water balance. Alleviation in water stress by reducing VPD sustained stomatal function and photosynthesis, with concomitant improvements in biomass and fruit production. From physiological perspectives, suppression of the driving force and water flow rate substantially reduced cumulative transpiration by 19.9%. In accordance with physiological principles, irrigation water use efficiency as criterions of biomass and fruit yield in low-VPD treatment was significantly increased by 36.8% and 39.1%, respectively. The reduction in irrigation was counterbalanced by input of fogging water to some extent. Net water saving can be increased by enabling greater planting densities and improving the evaporative efficiency of the mechanical system.

Background: Despite atmospheric vapour pressure deficit (VPD) was demonstrated as significant environmental factors affecting plant photosynthesis and productivity, the regulating mechanism under varying atmospheric evaporative demand was still unclarified. The contribution of stomatal, mesophyll resistance and biochemical limitation imposed on photosynthesis in tomato under varying evaporative demand was highlighted in the present study. Quantitative photosynthetic limitation analysis across a series of VPD was performed in well-watered tomato, by combining gas exchange and chlorophyll fluorescence. Results: Photosynthetic performance in tomato was gradually depressed with increasing in VPD. Under low VPD condition, stomatal and mesophyll conductance were sufficiently high for CO2 transport, which facilitated high chloroplast CO2 concentration for carbon fixation. Stomatal and mesophyll limitation accounted a low fraction, and photosynthetic potential was mostly constrained by biochemical limitation inside chloroplasts under low VPD condition. With increasing in VPD, plant water stress was gradually pronounced and triggered declines in stomatal and mesophyll conductance. Contribution of stomatal and mesophyll limitation on photosynthesis increased gradually with rise in VPD. Consequently, the low CO2 availability inside chloroplast substantially constrained photosynthesis under high VPD condition. Conclusion: Photosynthetic potential in tomato was mostly constrained by biochemical limitation inside chloroplasts under low VPD condition. CO2 diffusion limitation in series of stomatal and mesophyll resistance was the key rate-limiting step for photosynthesis under high VPD condition.

No abstract