Relative Evaluation of Water Stress Indicators for Soybeans1

Abstract The understanding of the relationships between changes in crop foliage to variations in the amount and status of soil water is at best incomplete. Since plant growth is directly affected by deficits of water in plants, and only indirectly by soil‐water deficits and atmospheric stresses, the importance of using plant measurements, rather than soil indices, is warranted and has led to this investigation. Field studies were conducted during 1976 on Ida silt loam [fine, silty, mixed (calcareous) mesic family of Typic Udorthents] at the Western Iowa Experimental Farm, Castana, Iowa, to evaluate three plant measurements (vis., stomatal conductance, leaf‐water potential, and leaf area) as water stress indicators for soybeans [Glycine max (L.) Merr.]. Stomatal conductance and leaf water potential were measured at 2‐hour intervals from 0600 to 2000. Stomatal resistance was measured with a diffusion porometer, with stomatal conductance taken as the reciprocal of leaf resistance. Leaf‐water potential was measured using a pressure chamber. Leaf area was measured with an electronic leaf‐area integrator. Daily means of stomatal conductance and leaf‐water potential measured several times during the growing season were closely related to changes in soil‐water potential. “Rate of leaf‐area expansion” which is defined as the change in average leaf area (leaf area/number of leaves) per plant over a period of time, also showed a close correspondence with soil‐water potential. Relative growth rates of soybeans showed a negative correlation with stomatal conductance, leaf‐water potential, and rate of leaf.area expansion. The three plant measurements should prove useful in explaining water‐deficit effects quantitatively under field conditions.