Abstract
Leaf conductances and transpiration rates from potted plants were studied on two pairs of barley lines selected for high or low stomatal frequency on the flag leaf. Although there were large differences in stomatal frequency on the flag leaves, there was no evidence that the low frequency lines had the lower conductances at equal leaf water potentials. This may have been due to the changes in the size of the stomata which compensated for differences in stomatal frequency. Although there were no differences in stomatal conductance (expressed on a unit leaf area basis) the rate of water use per plant was up to 50% faster for the low frcquency lines than for the high, particularly after emergence of the sixth leaf. This was caused by a larger green leaf area at this stage which was in turn due to larger individual leaves, more tillers, and a slower senescence of the older leaves. These observations are discussed in relation to the possibility of selecting for stomatal characteristics as a means of breeding varieties able to tolerate drought.

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