Two pairs of awned and awnless near-isogenic lines of winter wheat were used in a field study in which canopy enclosure apparatus and carbon-14 dosing were employed to assess the contribution of the awns to photosynthesis and grain yield. Awns contributed an average of 12'2% to canopy gross photosynthesis but did not increase the net photosynthesis of the complete canopy. The presence of awns decreased photosynthesis in the remaining ear structures and in the flag and penultimate leaf laminae. Seven days after dosing during the phase of rapid grain filling, 80% of the carbon recovered was located in the grains. The awns intercepted 9% of the incident visible radiation when fully green, and senesced at similar rates as the ears and flag leaves.In a second experiment the effect of awns on grain yield and its components was investigated in crosses segregating for height and presence of awns. Awns did not increase grain yields in either experiment. It appears that for British conditions in the absence of severe drought there is little advantage to be gained at present by breeding awned varieties of wheat.

The net photosynthetic rates of ears and flag leaves of awned and awnless wheats were measured by infra-red gas analysis in a controlled environment experiment. At 107 W m--2 the rates of net photosynthesis of the ears of two awned Triticum aestivum lines were two to three times greater than those of their isogenic awnless counterparts. The net rates of photosynthesis of the flag leaves of all four lines were, however, similar. Net photosynthetic rates of ears of T . durum and T . turgidum, which have much larger awns, were considerably higher than those of T . aestivum. Between 70 and 90% of carbon-14 assimilated by the flag leaves and by the ears of T . aestivum was translocated to the grains, with no effect of awns in this respect.In a glasshouse experiment with detached shoots the contributions of the component organs to gross photosynthesis were determined by carbon-14labelling. Ears of awnless lines of T . aestivum contributed about 10 yo to the photosynthesis of the organs above the node of the penultimate leaf and awns increased this to about 18%. Fixation by awned ears of T . durum and T . turgidum was 21~29% and of this 78-86% was contributed by the awns.In both experiments the contribution of the flag leaf to shoot photosynthesis was greater in T . aestivum than in the other species.

The effect of temperature during grain set and grain growth on the contribution of awns to yield in wheat was examined in the Canberra phytotron. For eight pairs of awned and awnless near-isogenic lines, the mean advantage of the awned lines in grain dry weight per ear was greatest when plants were held at 21/16°C during grain development, less at 27/22°C and least at 15/10°C where grain weight per ear was highest. One pair, G16 (awned) and G18 (awnless), examined more closely, showed the same response with the yield advantage of the awned line tending to decline from 21/16°C to 33/28°C. There were no significant differences between G16 and G18 in photosynthetic rates of the flag leaves, but the presence of awns increased photosynthetic rates of the ears by up to 1.2 mg CO2 h-1 per ear. During the linear grain growth phase. the relative net contributions per hour by awns and flag leaf, neither of which were much affected by temperature, were about 1 : 10 in all treatments. It appeared probable that under well-watered conditions awns had an effect on yield only when assimilate supply was limiting. Awns did not always increase yield significantly in supra-optimal temperatures.

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