Effects of Vapor Pressure Deficit and Potassium Supply on Root Morphology, Potassium Uptake, and Biomass Allocation of Tomato Seedlings

Atmospheric humidity, defined as the vapor pressure deficit (VPD), is an important factor affecting plant transpiration and nutritional status. Previous work has focused on the relationship between VPD and nitrogen metabolism; however, little is known about potassium (K) responses to VPD. Here, we investigated the effects of VPD and K supply on root morphology, potassium uptake, and plant growth with tomato (Solanum lycopersicum L., cv. Jinpeng). Experiments were conducted in greenhouses under low and high VPD conditions and featured three levels of K nutrient solution supply (2, 4, and 8 mmol/L). Characteristics of root morphology were significantly promoted by low VPD, including root length, root volume, root diameter, root surface area, number of fine roots, and proportion of fine roots, which combined were more conducive to the absorption of nutrients; low VPD reduced the transpiration rate, but the K accumulation by plants showed an increasing trend, indicating increased root morphology parameters could compensate for the impact of reduced transpiration-driven mass flow on K uptake. Meanwhile, higher biomass also promotes the accumulation of potassium. Photosynthesis is promoted due to increased potassium accumulation in the leaves, which in turn increases plant biomass, especially with moderate K supply. Furthermore, low VPD increases biomass allocation in leaves and reduces biomass allocation in stems but, in contrast, relatively more potassium is allocated to stems and roots but less to foliage. In summary, low VPD promotes K uptake by facilitating root system development in summer greenhouses. This result provides new knowledge for enhancing potassium utilization efficiency and reducing the use of chemical fertilizers.