Diverse cultivars of winter wheat (Triticum
aestivum L.) were grown in the field in 1993/94 and
1994/95 at Reading UK in temperature gradient tunnels
at normal atmospheric (c. 370) or elevated
CO2 concentration (c.
700 μmol CO2 mol−1
air). In 1993/94, grain yield of cv. Avalon was insensitive
to mean temperature (between 8·8 and 10·9°C),
while elevated CO2 increased yield by
1·3 t ha−1
(12·6%). In all other cultivars, warming reduced
grain yield and CO2 increased grain yield. In
1993/94, in cvs Galahad and Mercia the effects of
CO2 and temperature on yield were additive.
However, for cv. Hereward in both years and for cv.
Soissons in 1994/95, there were negative
interactions between 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%) respectively, occurred at cooler temperatures;
there was no benefit from doubling CO2 (i.e. 0%)
once the temperature had increased above the
seasonal mean by 2·2–2·6°C in cv.
Hereward and by 1·3°C in cv. Soissons. The
beneficial effect of doubling CO2 on grain yield
in cvs Galahad, Hereward, Mercia and Soissons was negated by an
increase in mean seasonal temperature of only
0·7–2·0°C. Warming
decreased root dry mass at anthesis in 1994/95 while
it increased at elevated CO2 (49 and 186%,
coolest and warmest regime, respectively). Carbon partitioned
to roots declined progressively with warming, while at
elevated CO2 there was an average of 56% increase
in allocation to roots. The relative impacts of both
CO2 and temperature were greater on root dry
mass than on either grain yield or total above-ground biomass,
while the effects on grain and biomass yield varied
considerably between cultivars, suggesting that the
impact of rising CO2 and temperature are
likely to be dependent on cultivar.