Field Studies of Photosynthetic and Growth Responses to Irradiance in Three Forest Understory Species of the C4 Grass Genus Muhlenbergia

Abstract

Three species of the C4 grass genus Muhlenbergia from the forest understory—M. frondosa, M. sobolifera, and M. schreberi—and one species from an open prairie, M. cuspidata, were transplanted to a sunny common garden and to a forest understory. After 4 mo of growth, all species in the common garden produced more biomass and greater leaf area and number than those in the understory. Muhlenbergia cuspidata died in the understory. Chlorophyll (chl) concentrations were higher, and a/b ratios were lower, in plants from the understory. All species were grown in the laboratory under two PPFD regimes: (1) 15 |xmol m~2 s _ l for 9 h plus two 10-min periods of 500 |xmol m~2 s _ 1 ; and (2) 15 fimol m~2 s _ l for 20 h. Total integrated PPFD, not peak photon flux density, appeared to determine all aspects of the photosynthetic responses of these plants. Characteristics of photosynthetic gas exchange in situ for M. frondosa and M. sobolifera were similar to laboratory findings. Although results indicated that the three understory species adjusted their chl characteristics and survived when grown under low levels of PPFD, they grew optimally under the higher PPFD.