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SUMMARYCrops of winter wheat (Triticum aestivum L. cv. Hereward) were grown within temperature gradient tunnels at a range of temperatures at either c. 350 or 700 μmol mol−1 CO2 in 1991/92 and 1992/93 at Reading, UK. At terminal spikelet stage, leaf area was 45% greater at elevated CO2 in the first year due to more tillers, and was 30% greater in the second year due to larger leaf areas on the primary tillers. At harvest maturity, total crop biomass was negatively related to mean seasonal temperature within each year and CO2 treatment, due principally to shorter crop durations at the warmer temperatures. Biomass was 6–31% greater at elevated compared with normal CO2 and was also affected by a positive interaction between temperature and CO2 in the first year only. Seed yield per unit area was greater at cooler temperatures and at elevated CO2 concentrations. A 7–44% greater seed dry weight at elevated CO2 in the first year was due to more ears per unit area and heavier grains. In the following year, mean seed dry weight was increased by > 72% at elevated CO2, because grain numbers per ear did not decline with an increase in temperature at elevated CO2. Grain numbers were reduced by temperatures > 31 °C immediately before anthesis at normal atmospheric CO2 in 1992/93, and at both CO2 concentrations in 1991/92. To quantify the impact of future climates of elevated CO2 concentrations and warmer temperatures on wheat yields, consideration of both interactions between CO2 and mean seasonal temperature, and possible effects of instantaneous temperatures on yield components at different CO2 concentrations are required. Nevertheless, the results obtained suggest that the benefits to winter wheat grain yield from CO2 doubling are offset by an increase in mean seasonal temperature of only 1·0 °C to 1·8 °C in the UK.

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Roots of annual crop plants are a major sink for carbon particularly during early, vegetative growth when up to one-half of all assimilated carbon may be translocated belowground. Flowering marks a particularly important change in resource allocation, especially in determinate species, with considerably less allocation to roots and, depending on environmental conditions, there may be insufficient for maintenance. Studies with 14C indicate the rapid transfer belowground of assimilates with typically 50% translocated in young cereal plants of which 50% is respired; exudation/rhizodeposition is generally <5% of the fixed carbon. Root:total plant mass decreases through the season and is affected by soil and atmospheric conditions. Limited water availability increased the allocation of 13C to roots of wheat grown in columns so that at booting 0.38 of shoot C (ignoring shoot respiration) was belowground compared to 0.31 in well-watered plants. Elevated CO2 (700 #mol CO2 mol-l air) increased the proportion of root:total mass by 55% compared with normal concentration, while increasing the air temperature by a mean of 3 °C decreased the proportion from 0.093 in the cool treatment to 0.055 in the warm treatment.

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