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 hel d within four double-walled polyethylene-covered tunnels along which n ear-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 te mperature 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 mea n seasonal temperatures differed between years, the relation between t emperature 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 fr om solving to the double ridge stage and from the double ridge stage t o harvest maturity. Carbon dioxide enrichment increased tillering subs tantially in 1991/92; there were 200 more shoots m(-2) at terminal spi kelet formation in crops grown at elevated compared to normal CO2 (add itional shoots were principally coleoptile tillers and those developin g after tiller 2) and this difference was reduced to 100 shoots m(-2) approaching harvest maturity (additional shoots remaining were those d eveloping after tiller 2). In contrast, no effect of CO2 enrichment on tillering was detected at any stage of development in 1992/93. The nu mber of tillers per plant at terminal spikelet formation was a linear function of main stem dry weight at this developmental stage; this rel ationship was not affected by year or CO2. As CO2 enrichment increased main stem dry weight in the first year only, when main stem dry weigh ts 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 co ncentration to tillering in winter wheat may be greatest in those crop production environments where main stem dry weights at terminal spike let are least and vice versa.