Will the exceptional productivity of Miscanthus x giganteus increase further under rising atmospheric CO₂?

Miscanthus x giganteus is a highly productive perennial rhizomatous C₄ grass originating from Japan. It has been widely promoted as a bioenergy crop on account of its high productivity and low input requirements. Furthermore, such plantations are expected to last for decades, raising the question of how production, composition and therefore supply to the end-user facility may change with global atmospheric change. Atmospheric [CO₂] is expected to rise from about 394μmolmol^-1 today to 550μmolmol^-1 by 2050. C₃ crops show large yield increases in response to rising [CO₂] through increased photosynthesis, while experiments on C₄ plants differ in their findings. Effects of elevated [CO₂] on photosynthesis and non-structural carbohydrate (NSC) in M. x giganteus have not been reported. The SoyFACE facility provided a unique opportunity to test the hypothesis that elevated [CO₂] under open-air conditions would alter the photosynthesis, productivity and composition of M. x giganteus in the field. The crop was planted in the spring of 2009 in 4 replicates of elevated (585μmolmol^-1) and of ambient [CO₂], and the plant material harvested following senescence in 2009 and 2010. Although significant increases in leaf CO₂ uptake due to elevated [CO₂] were observed during dry periods, there was no treatment effect on plant biomass. There was no significant treatment effect on leaf total NSC although they were higher on one occasion in the stems, primarily due to increased accumulation of hexoses. In both treatments, starch was the major stem NSC, whereas sucrose concentrations reached about 4-5% of total dry weight maturity, perhaps reflecting the close relationship of this plant with sugarcane. Starch content in roots and rhizomes increased along the season in both treatments, but was decreased in elevated [CO₂], which might affect regrowth during subsequent seasons.