Elevated CO₂ Effects on Water Use and Growth of Maize in Wet and Drying Soil

Abstract

It is unclear from the literature as to whether growth of C₄ species is responsive to elevated atmospheric CO₂ concentration. Reports vary between no response to strong response. To explore the origin of this discrepancy, spaced plants of maize (Zea mays) were grown at atmospheric CO₂ concentrations of 362 or 717 μL L^-1 under continuously wet or drying soil regimes. The aims were to evaluate the comparative growth promotion from elevated CO₂ in a C₄ plant under the two contrasting water regimes and the causes of any such promotion, and also how water-use efficiency (WUE) is influenced by high CO₂ under the two water regimes. In wet soil, transpiration rate was reduced on average by 29% at high CO₂, but neither total dry matter nor plant height was significantly affected by CO₂ level. Leaf area was not influenced significantly, so daily water use per plant was 25% lower and WUE was increased entirely due to reduced water use at high CO₂. In soil that was drying from field capacity, plants in high CO₂ used about 30% less water than those in ambient CO₂ while the soil was still wet. This resulted in higher soil water content at high CO₂. Plant growth showed a marked response, accumulating 35% more leaf area and 50% more dry matter. Young internodes elongated up to 170% more, giving taller plants. The growth enhancement was largely due to higher average net assimilation rate indicating that C₄ photosynthesis responded to elevated CO₂ during drought. In drying soil the increase in WUE was due to both increased dry matter and reduced water use, the contribution from each depending on the stage of soil drying. We hypothesise therefore that literature examples where maize growth responded to elevated CO₂ may have involved (possibly unrecognised) minor water deficits.