PHOTOSYNTHESIS OF A C3 GRASS AND A C4 GRASS UNDER ELEVATED CO2

The net photosynthetic rate (P(N)), intercellular CO2 concentration (C (i)), transpiration rate (E), stomatal resistance (r(s)), and water po tential (PSI(W)) of a C3 grass (Kentucky bluegrass, Poa pratensis L.) and a C4 grass (big bluestem, Andropogon gerardii Vitman) growing in t he spring in a tallgrass prairie under two levels of CO2 (ambient and twice ambient) were compared. Elevated CO2 (HC) increased P(N) of Kent ucky bluegrass (C3) by 47.0 % but did not affect P(N) of big bluestem (C4). HC increased C(i) of both grasses by about the same amount (is-a pproximately-equal-to cm3 m-3), but reduced E (and parallelly increase d r(s)) of big bluestem more than those of Kentucky bluegrass. HC incr eased PSI(W) of both grasses by about 30 %. Kentucky bluegrass had a l ower PSI(W) than big bluestem, but HC increased PSI(W) of Kentucky blu egrass to values more similar to those of big bluestem under ambient C O2 (LC). Hence a high PSI(W), resulting from HC, was necessary for a h igh P(N).