Water Relations of Wheat Leaves in the Field1

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Abstract
Stomatal closure at low leaf turgor and water potentials has been demonstrated in a number of crop plants but as far as we are aware, the magnitudes of these critical potentials have not been established for temperate cereals; nor have their variation from one leaf to the next been studied. We have examined the relationships between turgor potential (P), water potential (Ψ), osmotic potential (π), and stomatal closure in leaves at different positions on the stem of mature wheat plants (Triticum aestivum L. ‘Heron’) in the field.Thermocouple psychrometry was used to determine P, Ψ, π, and the resistances of leaves to mass‐flow were used as an indication of stomatal aperture.Stomatal closure was induced at about the same P, 8 bars, in leaves at all positions on the stem, but because of a different relationship between and Ψ for each leaf, the corresponding critical water potentials varied from —7 bars for leaves at the bottom of the stem, to —14 bars for the next highest leaves, to —19 bars for top leaves. The different relationships between P and Ψ resulted from a systematic decrease in π with leaf position from bottom to top of the stem.There appear to be important differences in leaf water relations between this wheat variety and some other crops. For instance, the critical P of 8 bars is higher than has been reported for maize, sorghum, and tobacco, but π is generally lower. The agronomic significance of these species differences is discussed.

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