High Plant Density of Cucumber (Cucumis sativus L.) Seedlings Mitigates Inhibition of Photosynthesis Resulting from High Vapor-pressure Deficit

Abstract

To evaluate the effects of plant density on gas exchanges under water stress resulting from high vapor-pressure deficit (VPD), we measured net photosynthetic rate (Pn), transpiration rate, and leaf conductance (gl) of cucumber (Cucumis sativus L.) seedlings before and after raising the VPD at different plant densities. Measurements were conducted continuously using a chamber and weighing method. Five, nine, or 12 seedlings with leaf area index (LAI) of 0.39, 0.73, and 1.10, respectively, were placed in the assimilation chamber. The average VPD in the chamber was raised from 1.1 to 3.7 kPa 30 min after the starting measurement. The Pn and gl decreased after raising the VPD above the plant community from 1.1 to 3.7 kPa. The VPD near the leaf surface (measured with 3-mm diameter humidity sensors) decreased with increasing LAI of the plant community, whereas average VPD in the whole chamber did not change with LAI. We noted significant negative correlations between the VPD near the leaf surface and Pn and gl. These results indicate that higher plant density mitigates the inhibition of photosynthesis resulting from high VPD by maintaining a lower VPD near the leaf surface with the development of a thicker boundary layer above the canopy.