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
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.