Water use efficiency of two soybean cultivars differing in leaflet orientation

Attaining economic soybean (Glycine max L. Merr) yields in areas or seasons of limited water supply is a topic of great interest. The first step in realizing this goal is to exploit physiological processes that would be of benefit under such conditions. Heliotropic leaf movement is a characteristic that enables soybean plants to respond to, and perhaps avoid environmental stress. This study was conducted to (i) compare the transpiration rates and water-use-efficiencies of Dare and Essex soybean cultivars chosen to represent extremes in leaflet orientation; and (ii) evaluate the qualitative versus the quantitative genetic nature of transpiration rates of the F₁ hybrid of the two cultivars. Transpiration rates of the two soybean cultivars were measured under greenhouse and field conditions. In the greenhouse, plants of each cultivar were placed in a plexiglass chamber with constant temperature and light intensity, and the net CO₂ assimilation rate and transpiration rate were recorded every 30 min over a 3 h period. In the field study transpiration rates of each cultivar were measured at the R4 and R6 growth stages via sap flow measurements over an 8 h time period. Leaf temperatures were recorded on upper and lower surfaces of leaves of each cultivar exhibiting different heliotropic orientations. Potassium concentrations were measured in leaf and pulvini tissue from samples collected during paraheliotropic and diaheliotropic orientations. In the greenhouse at the R1 growth stage, Essex had a lower rate of transpiration than Dare under stress and non-stress conditions. However, at the R4 growth stage under non-stress moisture conditions. Dare had a lower rate of transpiration than Essex, but under stress, there was no difference between the cultivars. Under field conditions, Essex had a lower rate of transpiration than Dare at both the R4 and the R6 stages of growth. Essex maintained a cooler canopy than did Dare (29.1 versus 31.1 °C). Essex had higher levels of K in leaves and pulvini than did Dare, which could help explain why Essex has greater heliotropic movement. In the field Essex produced approximately the same amount of yield as Dare with less than half of the water. The water-use-efficiency of Essex was .00169 kg ℓ⁻¹ compared to .00078 kg ℓ⁻¹ for Dare.