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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 C₃ grass, tall fescue (Festuca arundinacea Schreb.), and some anatomical characteristics similar to those of C₄ grasses. Since C₄ 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 C₄ 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 (k₁) and mesophyll conductance (k_m) in all four species. In P. maximum (C₄), k_m was greater than k₁, contributing to the greater water use efficiency for this species than was the case for tall fescue in which k₁ 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 CO₂ concentrations. At low CO₂, 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 C₃ and C₄ species.

Key Words: Stomatal conductance • Mesophyll conductance • Water stress C₃ species • C₄ species • Panicum schenckii • Panicum milioides • Panicum maximum • tall fescue

² Professor of agronomy and research technician, respectively.

Received for publication October 4, 1978.