Effects of the interaction between vapor-pressure deficit and potassium on the photosynthesis system of tomato seedlings under low temperature

Both vapour pressure deficit (VPD) and potassium (K) have been widely studied in regulating leaf photosynthesis and plant growth. A close correlation between VPD and plant nutrient absorption has also been observed for diverse plant species under various environmental conditions. However, the interactive effect of VPD and K on plant photosynthesis remains unclear, especially at low temperatures. Here, we investigated the role of VPD in plant K uptake and the effects of VPD x K on plant photosynthesis system under chilling stress. Plants were subjected to different levels of K nutrient solution supply (2, 4, 8 mmol/L) with contrasting VPD levels (high vs. low). Compared to high VPD, low VPD increased plant transpiration, but there was no significant difference in leaf K content between different VPD treatments; K accumulation under low VPD increased significantly, which can be partly attributed to enhanced biomass production. Meanwhile, an increase in stomatal density and size occurred under low VPD conditions. Stomatal conductance and mesophyll conductance tended to increase with K level under low VPD condition, which was beneficial to the diffusion of carbon dioxide and thus increased photosynthesis. In terms of photosynthetic performance, low VPD and elevated K increased the utilization of light energy and reduced heat dissipation, which specifically manifested as up-regulated photosynthetic pigment content and chlorophyll fluorescence parameters and down-regulated Car/Chl ratio, NPQ, and ROS content. In summary, reducing VPD and adding K promoted photosynthetic performance and can alleviate the lowtemperature stress of tomato plants.