The responses of net photosynthesis to vapour pressure deficit and CO₂ concentration in Spartina townsendii (sensu lato), a C₄ species from a cool temperate climate

As the initial part of a detailed study of photosynthetic CO₂ assimilation in the temperate C₄ grass, Spartina townsendii, the responses of net photosynthesis to the leaf-air vapour pressure deficit (VPD) and to CO₂ concentration are examined. Water vapour and CO₂ exchange for single attached leaves were measured under controlled-environment conditions in an open gas-exchange system. The responses of net photosynthesis, stomatal resistance (r_s), and residual resistance (r_r) to vapour pressure deficit(VPD) and CO₂ concentration under a range of light and temperature conditions are reported. Net photosynthesis was insensitive to increase in the VPD up to 1.0 kPa, but beyond this value net photosynthesis decreased with further increase in VPD as a result of an increase in r_s. The residual resistance was not affected by VPD under any of the conditions examined. Net photosynthesis responded linearly to increase in the CO₂ concentration in the external air (C_a up to the normal atmospheric concentration where there was a sharp change in the response, net photosynthesis being independent of any further increase in C_a. Differences between the response curves observed here and in other studies are discussed and the possible reasons for these differences are considered.