Growth responses of C-4 grasses of contrasting origin to elevated CO2

Nine grass species representing three independent origins of the C-4 photos ynthetic pathway were grown at ambient (350 ppm) and elevated (700 ppm) CO2 and were harvested after flowering. Setaria and Arundinella are both membe rs of the subfamily Panicoideae, and represent a single origin of the pathw ay. Aristida and Stipagrostis are sister genera in the subfamily Aristidoid eae (formerly classified in subfamily Arundinoideae), and represent a secon d origin. Sporobolus, a member of the subfamily Chloridoideae, represents t he third. By investigating two genera each within Panicoideae and Aristidoi deae, we test the hypothesis that genera sharing the same origin of C-4 res pond similarly. To explore variation among congeneric species, five species of Setaria were also examined to test the hypothesis that congeneric speci es have similar responses. Plant height and numbers of tillers, branches an d inflorescences were measured, both over time and at final harvest. Biomas s of roots, shoots, and inflorescences was also measured. Members of the Ar istidoideae were generally significantly larger in elevated CO2, as indicat ed by measurements of biomass and plant height, whereas representatives of the Panicoideae varied considerably in their response. The two subfamilies differed significantly in their responses to elevated CO2 and this effect o utweighed any effect of CO2 alone. Sporobolus, though equally distantly rel ated to Panicoideae and Aristidoideae, had a CO2 response similar to that o f some panicoid species. Even within the genus Setaria, some species were s ignificantly smaller at elevated than at ambient CO2, whereas others were l arger. This may reflect diversity in internal regulation rather than acclim ation or changes in source-sink allocation of carbon. The variation complic ates any prediction of responses of C-4 plants to future atmospheric change . Comparison of closely related species, however, may well lead to intrigui ng new insights into how regulatory pathways of CO2 assimilation are modifi ed during evolution. (C) 1999 Annals of Botany Company.