Developmental and tillering responses of winter wheat (Triticum aestivum) crops to CO2 and temperature

Winter wheat (Triticum aestivum L., cv. Hereward) was grown in the held within four double-walled polyethylene-covered tunnels along which near-linear temperature gradients were imposed at normal atmospheric or at an elevated CO2 concentration (c. 700 mu mol mol(-1) CO2) in 1991/92 and in a further experiment in 1992/93. Development was more rapid the warmer the temperature. In 1991/92 an increase in mean seasonal temperature of 3.5 degrees C reduced the duration from sowing to harvest maturity (the stage when grain moisture content reduced naturally to 15-18%) by c. 38 days, and reduced the duration from the double ridge stage to harvest maturity by c. 34 days. A similar difference resulted from only 1.6 degrees C warming in 1992/93. Although the range of mean seasonal temperatures differed between years, the relation between temperature and rate of development from sowing to harvest maturity was common to both years (base temperature, -0.8 degrees C; thermal time 2410 degrees C d). Carbon dioxide concentration had no effect on this relation or on that between temperature and the rate of development from solving to the double ridge stage and from the double ridge stage to harvest maturity. Carbon dioxide enrichment increased tillering substantially in 1991/92; there were 200 more shoots m(-2) at terminal spikelet formation in crops grown at elevated compared to normal CO2 (additional shoots were principally coleoptile tillers and those developing after tiller 2) and this difference was reduced to 100 shoots m(-2) approaching harvest maturity (additional shoots remaining were those developing after tiller 2). In contrast, no effect of CO2 enrichment on tillering was detected at any stage of development in 1992/93. The number of tillers per plant at terminal spikelet formation was a linear function of main stem dry weight at this developmental stage; this relationship was not affected by year or CO2. As CO2 enrichment increased main stem dry weight in the first year only, when main stem dry weights at normal CO2 were only one half of those values determined in the following year, it is concluded that any benefit of increase in CO2 concentration to tillering in winter wheat may be greatest in those crop production environments where main stem dry weights at terminal spikelet are least and vice versa.