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Water Use Efficiency in Soybean Pubescence Density Isolines— A Calculation Procedure for Estimating Daily Values'¹

Daily trends of water use efficiency (WUE) throughout a growing season are seldom, if ever, reported. In this paper a method for calculating daily WUE from growth analysis data coupled with weekly estimates of water use from soil moisture measurements are made for near-isogenic dense and normal pubescent lines of 'Harosoy' and 'Clark' soybean [Glycine mar (L.) Merr.]. Data were collected during a 2-yr field study at Mead, NE. The soil at this site is classified as a Typic Argiudoll. Evapotranspiration rates (ET) were lower in both dense pubescent isolines compared with the normal isolines, but differences were significant only for the Harosoy cultivar. Peak ET rates in 1980 were 4.7, 4.3, 5.8, and 4.8 mm day–¹ for the Clark normal, Clark dense, Harosoy normal, and Harosoy dense isolines, respectively. The highest WUE per unit ground area (WUE and per unit leaf area (WUE,) was observed in the dense pubescent Harosoy isoline. This was the result of a higher crop growth rate and a lower ET in this isoline. Average mid-season WUE,and WUE, values were, respectively, 1.4 and 0.25 g dry matter (kg H₂O)–¹ day–¹ greater for the dense Harosoy isoline than for the normal isoline. No effect of pubescent density on WUE in the Clark isoline was observed. The interaction of dense pubescence as indicated by results of our study illustrate the need to establish the effect of the dense pubescent gene in cultivars into which it is introduced.

Key Words: Glycine max (L.) Merr. • Evapotranspiration • Trichome • Isogenic lines • Morphology • Crop growth rate • Net assimilation rate • Photosynthesis

² Formerly assistant instructor-research (now soil scientist at the U.S. Water Conserv. Lab., Phoenix, AZ 85040); associate professor, Dep. of Agronomy; professor; and formerly visiting scientist (now agricultural engineer, INTA, Buenos Aires, Argentina), Center for Agric. Meteorol. and Climatol., Inst. of A 'c and Nat. Resour., Univ. of Nebraska, Lincoln, NE 68583-07g Portion of a dissertation submitted by the senior author in partial fulfillment of the requirements for the Ph.D. degree. This research was funded in part by the USDA-SEA-CSRS under grant no. 901-15-38.

Received for publication June 5, 1985.

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