Photosynthesis of Grass Species Differing in CO2 Fixation Pathways. I. Water‐use Efficiency1

Panicum milioides Nees ex. Trin. and P. schenckii Hack, have lower photorespiration rates than the C 3 grass, tall fescue ( Festuca arundinacea Schreb.), and some anatomical characteristics similar to those of C 4 grasses. Since C 4 grass leaves have little detectable photorespiration, but greater apparent photosynthesis (AP) and greater water use efficiency (apparent photosynthesis/ transpiration; AP/T), comparisons were made of the photosynthetic responses to water stress in the C 4 grass, guineagrass ( Panicum maximum Jacq.), tall fescue, P. milioides , and P. schenckii . The latter two species have similar leaf anatomical, photosynthetic, and photorespiratory characteristics. Decreases in AP under water stress were associated with decreases in stomatal plus boundary layer conductance (k 1 ) and mesophyll conductance (k m ) in all four species. In P. maximum (C 4 ), k m was greater than k 1 , contributing to the greater water use efficiency for this species than was the case for tall fescue in which k 1 was greater than k m . P. schenckii appeared to have higher AP/T than tall fescue, but lower than P. maximum . P. milioides had AP/T values between those for P, schenckii and tall fescue at near atmospheric CO 2 concentrations. At low CO 2 , both P. milioides and P. schenckii had AP/T values higher than tall fescue and lower than P. maximum . Thus it appears that the reduced photorespiration in P. milioides and P. schenckii causes them to have water use efficiencies intermediate between C 3 and C 4 species.