ELEVATED CO2 EFFECTS ON WATER-USE AND GROWTH OF MAIZE IN WET AND DRYING SOIL

It is unclear from the literature as to whether growth of C-4 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 atmospher ic CO2 concentrations of 362 or 717 mu L L(-1) under continuously wet or drying soil regimes. The aims were to evaluate the comparative grow th promotion from elevated CO2 in a C-4 plant under the two contrastin g water regimes and the causes of any such promotion, and also how wat er-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 signifi cantly affected by CO2 level. Leaf area was not influenced significant ly, so daily water use per plant was 25% lower and WUE was increased e ntirely 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 h igher soil water content at high CO2. Plant growth showed a marked res ponse, 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 i ndicating that C-4 photosynthesis responded to elevated CO2 during dro ught. 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 ex amples where maize growth responded to elevated CO2 may have involved (possibly unrecognised) minor water deficits.