Growth responses of C4 grasses of contrasting origin to elevated CO2

Nine grass species representing three independent origins of the C-4 photosynthetic pathway were grown at ambient (350 ppm) and elevated (700 ppm) CO2 and were harvested after flowering. Setaria and Arundinella are both members of the subfamily Panicoideae, and represent a single origin of the pathway. Aristida and Stipagrostis are sister genera in the subfamily Aristidoideae (formerly classified in subfamily Arundinoideae), and represent a second origin. Sporobolus, a member of the subfamily Chloridoideae, represents the third. By investigating two genera each within Panicoideae and Aristidoideae, we test the hypothesis that genera sharing the same origin of C-4 respond similarly. To explore variation among congeneric species, five species of Setaria were also examined to test the hypothesis that congeneric species have similar responses. Plant height and numbers of tillers, branches and inflorescences were measured, both over time and at final harvest. Biomass of roots, shoots, and inflorescences was also measured. Members of the Aristidoideae were generally significantly larger in elevated CO2, as indicated 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 outweighed any effect of CO2 alone. Sporobolus, though equally distantly related to Panicoideae and Aristidoideae, had a CO2 response similar to that of some panicoid species. Even within the genus Setaria, some species were significantly smaller at elevated than at ambient CO2, whereas others were larger. This may reflect diversity in internal regulation rather than acclimation or changes in source-sink allocation of carbon. The variation complicates any prediction of responses of C-4 plants to future atmospheric change. Comparison of closely related species, however, may well lead to intriguing new insights into how regulatory pathways of CO2 assimilation are modified during evolution. (C) 1999 Annals of Botany Company.