Rates of Photosynthesis and Transpiration and Diffusive Resistance of Six Grasses Grown Under Controlled Conditions1

Photosynthesis (Pn) and transpiration (E), and the components of diffusion resistance to CO2 and water vapor transport were measured in western wheatgrass (Agropyron smithii Rydb.), crested wheatgrass (A. desertorum [Fisch.] Schult.), pubescent wheatgrass (A. intermedium var. trichophorum [Link] Haloc.), Russian wildrye (Elymus junceus Fisch.), orchardgrass (Dactylis glomerata L.), and reed canarygrass (Phalaris arundinacea L.) to determine the rates of these processes and the role of stomates in controlling Pn and E. Plants were grown in a glasshouse and Pn and E measured at tillering and heading growth stages at three ambient CO2 concentrations and four light levels. At 1,160 μEm−2s‐−1 and 300 μl/liter CO2, Pn at tillering ranged from 10.2 to 26.2 mg CO2 dm−2hour−1 for reed canarygrass and western wheatgrass, respectively. E was lowest in reed canarygrass (1.1 g H2O dm−2hour−1) and highest in western wheatgrass (3.6 g H2O dm−2hour−1). Pn increased and E decreased for all species as ambient CO2 increased from 100 to 500 μl/liter CO2. Somate sensitivity to CO2 concentration and light level was evident in pubescent wheatgrass and orchardgrass but other species exhibited only slight changes. Minimum stomatal diffusion resistance to water vapor transport (rs) varied considerably among species. Partitioning of the components of resistance to CO2 and water vapor transport indicated that, at 1,160 μEm−2s−1 and 300 μ1/1 CO2, Pn of pubescent wheatgrass was primarily limited by stomatal apertures controlling diffusion of CO2 into the leaf (r's), whereas Pn of western and crested wheatgrasses, Russian wildrye, reed canarygrass, and possibly orchardgrass was limited more by the chemical process and/or internal resistances to CO2 diffusion in the liquid phase than by stomatal resistance. Ratios of E/Pn and rs/(r's+rm) (rm is mesophyll resistance), indicate that water use efficiency was highest in pubescent wheatgrass and lowest in western wheatgrass.