ENERGY CONTENT, CONSTRUCTION COST AND PHYTOMASS ACCUMULATION OF GLYCINE-MAX (L) MERR AND SORGHUM-BICOLOR (L) MOENCH GROWN IN ELEVATED CO2 IN THE FIELD

Grain sorghum [Sorghum] bicolor (L.) Moench, a C-4 crop] and soybean [Glycine max (L.) Merr. cv. Stonewall, a C-3 crop] plants were grown in ambient (c. 360 mu l l(-1)) and twice-ambient (c. 720 mu l l(-1)) CO2 levels in open-top chambers in soil without root constriction. Plant dry mass, energy content, composition and construction cost (i.e. amount of carbohydrate required to synthesize a unit of plant dry mass) were assessed at the end of the growing season. Elevated CO2 (a) increased phytomass accumulation (kg per plant) in both species, (b) had little affect on energy concentration (MJ kg(-1) plant) but caused large increases in the amount of plant energy per ground area (MJ m(-2) ground), and (c) did not alter specific growth cost (kg carbohydrate kg(-1) plant growth) but greatly increased growth cost per ground area (kg carbohydrate m(-2) ground) because growth was enhanced. For soybean, twice-ambient CO2 resulted in a 50 % increase in the amount of nitrogen and energy in grain (seed plus pod) per ground area. This response to elevated CO2 has important implications for agricultural productivity during the next century because the rate of human population growth is exceeding the rate of increase of land used for agriculture so that future food demands can only be met by greater production per ground area.