Nitrate and ammonium as nitrogen sources for lupins prior to nodulation

Two cultivars of Lupinus angustifolius L. were grown in a glasshouse in solutions containing NO₃^-, NH₄⁺ or NH₄NO₃ with a total nitrogen concentration of 2.8 M m^-3 in each treatment. One cultivar chosen (75A-258) was relatively tolerant to alkaline soils whereas the other (Yandee) was intolerant to alkalinity. Controlled experiments were used to assess the impact of cationic vs. anionic forms of nitrogen on the relative performance of these cultivars. Relative growth rates (dry weight basis) were not significantly different between the two cultivars when grown in the presence of NO₃^-, NH₄⁺ or NH₄NO₃. However, when NO₃^- was supplied, there was a modest decline in relative growth rates in both cultivars over time. When plants grown on the three sources of nitrogen for 9 days were subsequently supplied with ¹⁵NH₄NO₃ or NH₄¹⁵NO₃ for 30 h, NH₄⁺ uptake was generally twice as fast as NO₃^- uptake, even for plants grown in the presence of NO₃^-. Low rates of NO₃^- uptake accounted for the decrease in growth rates over time when plants were grown in the presence of NO₃^-. It is concluded that the more rapid growth of 75A-258 than Yandee in alkaline conditions was not due to preferential uptake of NH₄⁺ and acidification of the external medium. In support of this view, acidification of the root medium was not significantly different between cultivars when NH₄⁺ was the sole nitrogen source.