GROWTH AND YIELD OF WINTER-WHEAT (TRITICUM-AESTIVUM) CROPS IN RESPONSE TO CO2 AND TEMPERATURE

Crops of winter wheat (Triticum aestivum L. cv. Hereward) were grown w ithin temperature gradient tunnels at a range of temperatures at eithe r c. 350 or 700 mu mol mol(-1) CO2 in 1991/92 and 1992/93 at Reading, UK. At terminal spikelet stage, leaf area was 45% greater at elevated CO2 in the first year due to more tillers, and was 30% greater in the second year due to larger leaf areas on the primary tillers. At harves t maturity, total crop biomass was negatively related to mean seasonal temperature within each year and CO2 treatment, due principally to sh orter crop durations at the warmer temperatures. Biomass was 6-31% gre ater at elevated compared with normal CO2 and was also affected by a p ositive interaction between temperature and CO2 in the first year only . Seed yield per unit area was greater at cooler temperatures and at e levated CO2 concentrations. A 7-44% greater seed dry weight at elevate d CO2 in the first year was due to more ears per unit area and heavier grains. In the following year, mean seed dry weight was increased by > 72% at elevated CO2, because grain numbers per ear did not decline w ith an increase in temperature at elevated CO2. Grain numbers were red uced by temperatures > 31 degrees C immediately before anthesis at nor mal atmospheric CO2 in 1992/93, and at both CO2 concentrations in 1991 /92. To quantify the impact of future climates of elevated CO2 concent rations and warmer temperatures on wheat yields, consideration of both interactions between CO2 and mean seasonal temperature, and possible effects of instantaneous temperatures on yield components at different CO2 concentrations are required. Nevertheless, the results obtained s uggest that the benefits to winter wheat grain yield from CO2 doubling are offset by an increase in mean seasonal temperature of only 1.0 de grees C to 1.8 degrees C in the UK.