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This article in CS

  1. Vol. 20 No. 2, p. 213-219
     
    Received: Sept 6, 1979


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doi:10.2135/cropsci1980.0011183X002000020016x

Effects of Potassium Fertilization on Nitrogen Fixation and Nodule Enzymes of Nitrogen Metabolism in Alfalfa1

  1. Stanley H. Duke,
  2. Michael Collins and
  3. Ronald M. Soberalske2

Abstract

Abstract

Studies with four-year-old ‘Vernal’ alfalfa (Medicago saliva L.) fertilized yearly from planting with 673 kg/ha K as K2SO4 or KC1 and 448 kg/ha K as K2SO4 were made to determine the effects of K on nodulation, acetylene reduction, and nodule enzyme activities. The high rates of K fertilization increased nodule number and acetylene reduction rates over control plants, although the greatest increase in nodule mass and acetylene reduction rates was with the 673 kg/ha K as K2SO4 treatment. This indicates that Cl and/or S may alter or mask the effects of K fertilization. Increases in acetylene reduction rates on a plant basis with K fertilization were due to increases in nodule mass and not increases in activity per unit weight of nodule tissue. With all treatments a positive linear correlation (P <0.01, r = 0.854) was found between nodule number and acetylene reduction rate. The greatest increase in shoot weight per plant over control plants was with the 673 kg/ha K as KC1 treatment. Positive linear correlations were found between rates of acetylene reduction and the activities of enzymes involved in ammonia assimilation (NAD-glutamate dehydrogenase, glutamine synthetase, NADP-glutamate dehydrogenase), amino acid interconversions (glutamate oxaloacetate transaminase), C supply (NADP-isocitrate dehydrogenase) and energy transduction (glucose-6-phosphate dehydrogenase, malate dehydrogenase). Due to lower nodule protein concentrations and higher nodule masses in the K treatments, specific activities and, in general, activities on a per plant basis of enzymes assayed from K treatments were much higher than those of the control treatment. Malate dehydrogenase and NADP-glutamate dehydrogenase specific activities were significantly higher than control plants for both 673 kg/ha K treatments. There were few differences between treatments for enzyme data calculated on a gram fresh-weight basis. Yields of the two 673 kg/ha K treatments were nearly equal at third cut and were significantly higher than those of control plants. These data suggest that K fertilization increases alfalfa yields by increasing nodulation and N2-fixation.

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