Low vapour pressure deficit reduces the beneficial effect of elevated CO₂ on growth of N₂‐fixing alfalfa plants

Abstract

Plant responses to elevated CO 2 can be modified by many environmental factors, but very little attention has been paid to the interaction between CO 2 and changes in vapour pressure deficit (VPD). Thirty‐day‐old alfalfa plants ( Medicago sativa L. cv. Aragón), which were inoculated with Sinorhizobium meliloti 102F78 strain, were grown for 1 month in controlled environment chambers at 25/15°C, 14 h photoperiod, and 600 µmol m −2 s −1 photosynthetic photon flux (PPF), using a factorial combination of CO 2 concentration (400 µmol mol −1 or 700 µmol mol −1 ) and vapour pressure deficit (0.48 kPa or 1.74 kPa, which corresponded to relative humidities of 85% and 45% at 25°C, respectively). Elevated CO 2 strongly stimulated plant growth under high VPD conditions, but this beneficial effect was not observed under low VPD. Under low VPD, elevated CO 2 also did not enhance plant photosynthesis, and plant water stress was greatest for plants grown at elevated CO 2 and low VPD. Moreover, plants grown under elevated CO 2 and low VPD had a lower leaf soluble protein and photosynthetic activity (photosynthetic rate and carboxylation efficiency) than plants grown under elevated CO 2 and high VPD. Elevated CO 2 significantly increased leaf adaxial and abaxial temperatures. Because the effects of elevated CO 2 were dependent on vapour pressure deficit, VPD needs to be controlled in experiments studying the effect of elevated CO 2 as well as considered in the extrapolations of results to a warmer, high‐CO 2 world.