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Plant Resistance to Water Flow in Field Soybeans: I. Non-Limiting Soil Moisture¹

Resistances to water flow from the soil to plant leaves determines the ability of plants to maintain favorable water conditions. Predictions of plant water relations and transpiration require estimates of resistances to water flow in the soil and in the plant. The purpose of this field experiment was to determine plant resistance to water flow under field conditions during two periods of soybean [Glycine mar (L.) Merr.] growth. The experiment was conducted on Arredondo fine sand (hypothermic, coated Typic Quartzipsamment). Root depth distributions were measured during each growth period and soil water potential distributions were measured daily. Every other day, during the periods: 4 to 16 May (Exp. I) and 30 May to 7 June (Exp. II), hourly measurements of leaf water potential and transpiration were used to calculate plant resistance using Ohm's law analogy. Daily plant resistance was calculated using midday leaf water potential and total integrated transpiration for each day. Calculated plant resistance increased with transpiration flux in a non-linear relationship. However, when plant resistance values for quasi-steady state conditions during midday hours were examined, they averaged 940 MPa h/cm² for the first experiment, independent of transpiration flux over a range of 0.02 to 0.05 cm/h. Plant resistance values increased with time and averaged 2,360 MPa h/cm² for quasi-steady state conditions in the second experiment. Plant resistance values calculated on a daily basis averaged 200 and 300 MPa-day/cm² for the first and second experiments, respectively, 3 to 5 times higher than plant resistance values based on hourly measurements. Under natural conditions, the plant is seldom at steady state and random or average measurements of leaf water potential do not represent the true driving force for water transport through this complex system.

Key Words: Transpiration • Leaf-water-potential • Root-density-distribution

² Professor, Dep. of Agric. Eng., Univ. of Florida, Gainesville, FL; formerly, visiting research Scientist, Dep. of Soil Sci., Univ. of Fla.; Present address, Dep. of Agric. Eng., The Technion, Haifa, Israel; associate professor, Dep. of Agronomy, Univ. of Florida, Gainesville, FL; Professor, Dep. of Soil Sci., Univ. of Florida, Gainesville, FL.

Received for publication January 5, 1981.

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