Soybean Leaflet Movements as an Indicator of Crop Water Stress1

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Abstract
Vertical orientation of soybean [Glycine max (L.) Merr] leaflets during the onset of water stress, and the usefulness of this phenomenon as an indicator of crop water stress, were investigated in relation to leaf water potential (Ψ), stomatal diffusive resistance (ra) and soil water status in a field study. Observations were made during the reproductive stage on plants in a field with a Rhodic Paleustalf soil. Terminal leaflet movements were influenced by plant water stress and were not influenced by their azimuth direction. Increases in the angle of the terminal trifoliolate leaflet to the soil coincided with the earliest notable changes in Ψ and ra and thus provided a clear indication of the onset of crop water stress. Leaflet inversion began when Ψ was approximately ‐1.4 MPand was associated with a greater than 60% depletion of plant available water. The possible mechanism of leaflet inversion is related to changes in osmotic and pressure potentials and potassium concentration across the pulvinus.