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Total Resistance to Water Flow in Field Soybeans: II. Limiting Soil Moisture¹

Experiments were conducted to determine the total (soil plus plant) resistance to water flow through field grown soybeans [Glycine max (L.) Merr.] subjected to two drying cycles during pod addition and early pod fill (4 to 16 May) and mid-to-late pod fill (30 May to 7 June). Root length density distributions with depth were measured during each drying cycle and soil water potential distributions were measured daily. The experiment was.conducted in the field on Arredondo fine sand (hypothermic, coated Typic Quartzipsamments). Transpiration flux and leaf water potential measured hourly every 2 days during each drying cycle were used with daily measurements of soil water potential to calculate total resistance. Midday transpiration rates decreased relative to transpiration rates of irrigated plots starting 8 and 5 days after irrigation for the first and second drying cycles, respectively, when average soil water potential was –0.04 MPa. The increase in total resistance which corresponds to the decrease in transpiration flux was attributed to increased soil resistance. However, calculated soil resistances based on experimental results were four to six orders of magnitude higher than theoretical calculations of the hulk soil resistances based on the Gardner (1960) model. Calculations of water potential at the root surface during drying conditions indicate that unsaturated hydraulic conductivity of soil adjacent to the root may be several orders of magnitude lower than that of the bulk soil and could explain the observed reduction in the transpiration flux.

Key Words: Transpiration • Leaf-water-potential • Soil-resistance • Drying-cycle

² Formerly, visiting research scientist, Dep. of Soil Sci., Univ. of Fla.; Current address, Agricultural Engr. Dep., The Technion, Haifa, Israel; Professor, Dep. of Agricultural Engineering, Univ. of Florida, Gainesville, FL; Associate Professor, Dep. of Agronomy, Univ. of Florida, Gainesville, FL; Professor, Dep. of Soil Science, Univ. of Florida, Gainesville, FL.

Received for publication January 5, 1981.

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