Submitted to: Plant Physiology Supplement
Publication Type: Abstract Only
Publication Acceptance Date: 8/2/1997
Publication Date: N/A
Citation: Interpretive Summary:
Technical Abstract: Alfalfa (Medicago sativa L.) is a deeply rooted perennial legume that may be able to remove NO3 from the subsoil beneath the rooting zone of annual crops. Field experiments have demonstrated that ineffectively nodulated alfalfa, which cannot fix N2 gas, absorbs NO3 more efficiently than alfalfa with effective nodules. The objective of this study was to determine whether NO3 uptake kinetics could explain the disparity between these alfalfa types. Plants of two effectively nodulated alfalfa germplasms, Saranac and Agate, and two ineffectively nodulated alfalfa germplasms, Ineffective Saranac and Ineffective Agate, were grown from seed for seven weeks in sand culture, shoots were harvested, and plants were transplanted into a hydroponic system. Thirty days after transplanting, water use and NO3 depletion of the nutrient solution containing 0.6 mg NO3-N L-1 were monitored over an 8-h period. The time course of NO3 disappearance from the enutrient solution was fit to a Lineweaver-Burke plot to determine the parameters of apparent Michaelis-Menten nitrate uptake kinetics. We found that the two ineffectively nodulated alfalfa germplasms had a higher affinity for NO3 uptake than the effective N2-fixing germplasms [Cmin (micromolar): 12.7 vs 18.5; Km (micromolar): 17.5 vs 22.2; Imax (microg N min-1 plant-1): 4.85 vs 1.95]. Additionally, the ineffective germplasms transpired 37% more water per unit shoot weight than the effective germplasms (2.60 vs 1.95 g H2O g shoot DM-1 h-1). These results clarify the underlying physiological differences that lead to higher NO3 uptake efficiency of ineffective alfalfa than N2-fixing alfalfa in the field.