The Responses of Net Photosynthesis to Vapour Pressure Deficit and CO2Concentration inSpartina townsendii (sensu lato), a C4Species from a Cool Temperate Climate

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Abstract
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 (rs), and residual resistance (rr) 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 rs. 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 (Ca up to the normal atmospheric concentration where there was a sharp change in the response, net photosynthesis being independent of any further increase in Ca. Differences between the response curves observed here and in other studies are discussed and the possible reasons for these differences are considered.