Differential Effects of Increasing Vapor Pressure Deficit on Photosynthesis at Steady State and Fluctuating Light

Increasing photosynthetic efficiency in fluctuating light is an important strategy to improve plant growth. In nature, fluctuating light often occurs concomitantly with the change in vapor pressure deficit (VPD) in the daytime, but how VPD affects dynamic photosynthesis is little known. In this study, we measured photosynthetic performance in fluctuating light under different atmospheric VPD conditions in the daytime for mature leaves of greenhouse-grown Rosa hybrida “Orange Reeva”. A moderate increase of VPD significantly reduced photosynthetic efficiency in fluctuating light but hardly affected steady-state photosynthesis. With the increase of atmospheric VPD, stomatal conductance (g_s) and mesophyll conductance (g_m) in fluctuating light decreased simultaneously, but the maximum velocity of Rubisco carboxylation did not change. The significant effect of atmospheric VPD on g_s and g_m in fluctuating light was attributed to the change of relative humidity. Furthermore, photosynthesis in fluctuating light did not significantly differ between morning and afternoon under the same atmospheric VPD condition. These results indicate that atmospheric VPD is an overlooked environmental factor affecting photosynthetic efficiency under fluctuating light.