YIELD AND PARTITIONING IN CROPS OF CONTRASTING CULTIVARS OF WINTER-WHEAT IN RESPONSE TO CO2 AND TEMPERATURE IN-FIELD STUDIES USING TEMPERATURE-GRADIENT TUNNELS

Diverse cultivars of winter wheat (Triticum aestivum L.) were grown in the field in 1993/94 and 1994/95 at Reading UK in temperature gradien t tunnels at normal atmospheric (c. 370) or elevated CO2 concentration (c. 700 mu mol CO2 mol(-1) air). In 1993/94, grain yield of cv. Avalo n was insensitive to mean temperature (between 8.8 and 10.9 degrees C) , while elevated CO2 increased yield by 1.3 t ha(-1) (12.6%). In all o ther cultivars, warming reduced grain yield and CO2 increased grain yi eld. In 1993/94, in cvs Galahad and Mercia the effects of CO2 and temp erature on yield were additive. However, for cv. Hereward in both year s and for cv. Soissons in 1994/95, there were negative interactions be tween the effects of CO2 and temperature on yield: the maximum benefit of doubling CO2 to grain yield, 4.5 and 2.7 t ha(-1) (65 and 29%) res pectively, occurred at cooler temperatures; there was no benefit from doubling CO2 (i.e. 0%) once the temperature had increased above the se asonal mean by 2.2-2.6 degrees C in cv. Hereward and by 1.3 degrees C in cv. Soissons. The beneficial effect of doubling CO2 on grain yield in cvs Galahad, Hereward, Mercia and Soissons was negated by an increa se in mean seasonal temperature of only 0.7-2.0 degrees C. Warming dec reased root dry mass at anthesis in 1994/95 while it increased at elev ated CO2 (49 and 186%, coolest and warmest regime, respectively). Carb on partitioned to roots declined progressively with warming, while at elevated CO2 there was an average of 56% increase in allocation to roo ts. The relative impacts of both CO2 and temperature were greater on r oot dry mass than on either grain yield or total above-ground biomass, while the effects on grain and biomass yield varied considerably betw een cultivars, suggesting that the impact of rising CO2 and temperatur e are likely to be dependent on cultivar.