Root system response of C-4 grass seedlings to CO2 and soil water

Aboveground growth of C(4)plants responds more strongly to atmospheric CO2 concentration when soil water is limiting rather than abundant. Whether the same is true of root growth and morphology, however, remains to be evaluat ed. We investigated interactive effects of CO2 and soil water on root growt h and morphology of two C-4 grasses. Seedlings of the dominant C-4 grasses from tallgrass prairie, Schizachyrium scoparium and Andropogon gerardii, we re grown for 8 weeks in an elongated, controlled environment chamber at CO2 concentrations of 368 (ambient) and 203 (subambient) mu mol mol(-1). Seedl ings were maintained at either high (ca. 90%) or low (ca. 50%) soil relativ e water holding capacity (RWC). Both root and shoot systems of C-4 grass se edlings responded similarly to CO2 enrichment irrespective of whether soil water was limiting or abundant. Root growth was affected primarily by incre ased RWC (40-51% increases) and secondarily by higher CO2 (15-27% increases ). The relative distribution of root surface area, number of root tips and length and volume of roots were significantly affected by CO2 enrichment wi th proportional increases of 55-61%, 39-52%, 50-55% and 53-58%, respectivel y, occurring in very fine (0-0.3 mm) roots. The indirect effect of CO2 enri chment on conservation of soil water in grasslands may be as important as d irect photosynthetic response effects in the CO2-induced enhancement of who le-plant growth in C-4 grasses.