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Diurnal Relationship Between Evapotranspiration and Leaf Water Potential of Field-Grown Soybeans

Research is needed to describe the effect of management practices on plant water stress and water use efficiency. This study investigated the diurnal relationship between evapotranspiration (ET) and leaf water potential (₁) for field grown soybeans [Glycine max (L.) Merr.] as affected by row spacing and irrigation on an Ida silt loam [fine, silty, mixed (calcareous) mesic family of Typic Udorthents]. ET and ₁ were determined in detail on 2 days in the 1979 growing cycle, one just preceding canopy closure (11 July) and the second at the midpod fill stage (1 August), ₁ was measured by using a pressure chamber technique, and ET was measured by using a portable chamber. On both days, the relationship between ₁ and ET was similar for both row-spacing treatments. The nonirrigated treatments, however, demonstrated a slight hysteresis in the ₁-ET relationship, whereas the irrigated treatments did not. The ET at a given ₁ was much greater on 1 August than on 11 July, probably because of an increase in soil evaporation caused by a recent rainfall and an increase in transpiration due to the increased leaf area index. The ₁-ET relationship seemed to be affected more by irrigation than by row spacing. The simplified Ohm's Law analogy does not seem to adequately describe the relationship of plant water flux to resistance and potential difference under field conditions.

Key Words: Plant water relations • Water use • Row spacing • Resistance to water transport • Soil water stress

Received for publication October 8, 1981.