Yield Responses of Two Wheat Genotypes to Carbon Dioxide and Temperature in Field Studies Using Temperature Gradient Tunnels

Abstract

Clear, plastic-coated, temperature gradient tunnels (TGTs), 8 × 1.25 × 1-25 m were designed and built to examine how temperature and CO₂ affect the yield of wheat in the field. Each of the three modules of each TGT was maintained at a different temperature above the ambient temperature using solar heating during the day and electric heating at night. The maximum day-time increment above ambient for the warmest module was 5ºC and full-season averages were close to 2ºC. TGTs were paired, with air in one being enriched to 700 μL L^-1 CO₂, and in the other being maintained at ambient CO₂. Crops were planted in the TGTs at two sites in either summer (December) or winter (April and July) and they remained there until maturity. CO₂ enrichment increased the yield in summer plantings by up to 36%. In winter plantings, with mean temperatures between sowing and anthesis of around lVC, the responses to CO₂ were small averaging only 7% (range 1-12%). Though yield declined with increasing temperature in the TGTs in summer, there was a clear trend for an increasing response to CO₂ at these higher temperatures, i.e. yield declined less. In summer, there was no convincing evidence for a different relative response to CO₂ in two isolines which differed in maturity date, though the later line yielded more under the highest temperature regime (mean of 22-24ºC between sowing and anthesis). In winter there was a strong trend for the isoline requiring less vernalisation to respond more to CO₂. It is suggested that early progress towards flowering might predispose wheat to a greater CO₂ response. Overall, the data indicated that the positive response to CO₂ in grain yield is likely to increase at approximately 1.8% per 1°C in wheat crops that are not limited by water. Extrapolation indicated that the temperature at which there was no response to CO₂ was 5ºC. All yield responses reflected biomass responses as harvest index was unchanged by CO₂