Canopy development and tillering of field-grown crops of two contrasting cultivars of winter wheat (Triticum aestivum) in response to CO2 and temperature

Elevated CO₂ (691 cf. 371 /miol CO₂ mol^-1 air) and warmer temperatures (over the range 1.0^UC below to 1.6^oC above ambient) increased light interception by crops of two contrasting cultivars (Hereward and Soissons) of winter wheat (Triticum aestivum L.) during winter growth in the field. The fractional interception of light by the canopy increased more rapidly initially in Soissons than in Hereward, but Hereward showed a much greater response to CO₂ (35% increase in Hereward but only 7% in Soissons) at 500^oCd after sowing. By terminal spikelet formation, in contrast, fractional interception was greater in Hereward than in Soissons, while the effect of CO₂ was the same in both cultivars (9%). Thus, although differences in the relative response of canopy development to CO₂ were detected between cultivars initially, differences were negligible during later development. The greater interception of light by the canopy in elevated CO₂, at any one temperature, resulted from increased tillering. The number of tillers plant“‘ at terminal spikelet was a linear function of main stem dry mass at this developmental stage but with a greater response in elevated CO₂, viz 2.3 and 3.8 tillers g^-1 main stem dry mass at 371 and 691 /μmol CO₂ mol^-1 air, respectively; these relations were unaffected by cultivar.