Submitted to: International Journal of Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/10/1995
Publication Date: N/A
Interpretive Summary: Warm-season plants dominate vegetation on many dry rangelands throughout the world. Leaf-level processes were measured on the grasses maize and little bluestem and shrub four-wing saltbush, all warm-season plants, to determine how greatly plant growth per unit of water consumed, water use efficiency, might increase as the concentration of carbon dioxide in air rises. Water use efficiency of little bluestem increased relatively faster than did carbon dioxide over low concentrations representative of the past. Water use efficiency was greater in little bluestem than in four-wing saltbush at high carbon dioxide concentrations expected in the future, but increased relatively slower than did carbon dioxide over these high concentrations. By contrast, water use efficiency of maize increased by the same fractional amount as did carbon dioxide regardless of the concentration range considered. The differences in water use efficiency at a given carbon dioxide concentration and in the response of water use efficiency to higher carbon dioxide levels that occurred among these plants suggest that abundances of different warm-season plants on dry rangelands may have changed through the past and may continue to change as the concentration of carbon dioxide rises further.
Technical Abstract: Leaf gas exchange and delta-13C values were measured on C4 species grown from near glacial to current CO2 concentrations (200 to 350 ppm) and from 350 ppm to possible future levels (700, 1000 ppm) to determine effects of rising CO2 on intrinsic water use efficiency (CO2 assimilation, A/stomatal conductance to water, g) of C4 plants. Studied were species differing in life form or life history, the perennial grass Schizachyrium scoparium (little bluestem), perennial shrub Atriplex canescens (four-wing saltbush), and annual grass Zea mays (maize). Effects of nitrogen fertilization on the response of A/g and plant water use efficiency (WUE, production/ transpiration) to CO2 were examined for perennials. The ratio of leaf intercellular (ci) to external CO2 concentration (ca) declined, and A/g increased relatively more than did CO2 from 200 to 350 ppm in S. scoparium and, in one of two years, in Z. mays. Discrimination against 13C did not vary with CO2 in Z. mays, but increased slightly from 210 to 330 ppm in S. scoparium, reflecting a greater relative response of A/g to CO2 than occurs in most C3 species. Nitrogen fertilization increased biomass, but did not alter gas exchange of perennials. Leaf A/g and WUE both were substantially higher (78% to 150%) in S. scoparium than A. canescens. WUE was not, however, significantly affected by CO2 in either species. There were large differences in A/g and WUE at a given CO2 and in the response of A/g to CO2 among the species examined. Because much of the positive response of C4 plants to CO2 derives from higher WUE, these differences could influence the relative productivities of C4 species or life forms.