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Water-Use Efficiency and Its Relation to Crop Canopy Area, Stomatal Regulation, and Root Distribution¹

The differences in water-use efficiency between sorghum [Sorghum bicolor (L.) Moench] and soybean [Glycine max (L.) Merrill] were compared in terms of leaf area index (LAI), size of root system, canopy stomatal resistance (_c), net assimilation rate (NAR), and evapotranspiration (ET). The field water budget was measured by recording the rainfall, and irrigation water applied in relation to the ET as measured with two weighing lysimeters.

The evapotranspiration rates of the two crops began to diverge about August 15. This was the period when the soybean canopy began to close and the soybean LAI increased to about 1.5 times that of sorghum. NAR for sorghum during stalk elongation and heading (Aug. 15) was nearly four times that of soybeans during that time.

Divergence of the two evapotranspiration curves coincided with observed differences in canopy stomatal resistance. In August after a stress period, the _c for sorghum at 1 and 3 pm was nearly three times the _c for soybeans. The increase in _c for sorghum was concommitant with an air temperature increase of 3 C above the air temperature near soybeans.

Sorghum had approximately twice the weight of roots per volume of soil as soybeans. Even though atmospheric demand for H₂O was similar for the two crops, sorghum was able to close its stomata more than soybeans, and thus conserve soil water even with a larger root system and more water in the soil profile resulting from reduced evapotranspiration.

Water-use efficiency of sorghum (gm DM/kg H₂O) was approximately three times that of soybeans on dry matter or grain yield bases.

Key Words: Evapotranspiration • Net assimilation rate • Leaf area index • Root weights • Canopy stomatal resistance • Sorghum • Soybeans

² Associate Professor, Assistant Professor, Associate Professor, and Professor of Agronomy, Evapotranspiration Laboratory, Kansas State University, Manhattan 66502

Received for publication March 18, 1972.