Reducing vapor pressure deficit improves calcium absorption by optimizing plant structure, stomatal morphology, and aquaporins in tomatoes

Abstract

High temperatures and arid air environments hinder calcium absorption in tomatoes (Solanum lycopersicum L.), decreasing the yield significantly. Changing atmospheric vapor pressure deficit (VPD) is an important measure to improve the passive water movement that pulls water and calcium out of the soil. However, it remains unclear how the changes in plant structure, stomatal morphology, plasma membrane intrinsic proteins (PIPs), and tonoplast intrinsic proteins (TIPs) drive calcium absorption and water transport under different VPDs. This study examined the role of plant structure, stomatal morphology, TIPs, and PIPs on the calcium absorption of two tomato cultivars (Jinpeng and Zhongza) to long-term growth under high and appropriate VPDs, besides analyzing the correlation between the indicators. The results showed that major vein density, net assimilation rate, and stomatal conductance were the main reasons hindering the leaf calcium absorption, while fruit calcium absorption had the highest correlation with palisade tissue thickness. Reducing the VPD reduced the water potential gradient at the leaf–air boundary, increased the cross-sectional area of the xylem duct in stems and roots, and increased the leaf vein density in the plant. Meanwhile, a reduced VPD increased net assimilation rate, stomatal conductance and size, and decreased the spongy tissue thickness. Over the long-term, calcium absorption in leaves and fruits, calcium accumulation in fruit peels, yield, and yield water use efficiency was greatly increased for the two tomato cultivars. Additionally, most SlTIPs and SlPIPs were significantly upregulated in both cultivars under a high VPD. These results suggest that plant structure, stomatal morphology, TIPs and PIPs efficiently regulate calcium absorption and water transport under different VPDs.