PHOSPHORUS REQUIREMENT AND SOURCES OF NITROGEN IN 3 SOYBEAN (GLYCINE-MAX) GENOTYPES, BRAGG, NTS-382 AND CHIPPEWA

The effect of different levels of soil phosphorus on nodulation, N2 fi xation and growth of supernodulating soybean mutant, nts 382, its pare nt, Bragg, and soybean cultivar, Chippewa, were compared in a greenhou se study. The P rates were, 0, 30, 60 and 90 mg kg-1 soil (P0, P1, P2 and P3, respectively). The reduced shoot growth of nts 382 (25% at P0 and 17% at P3) could not be attributed to P deficiency, as nts 382 ach ieved near maximum yield at a lower P rate (P1) than Bragg (P2). The P response of Chippewa that produced the lowest shoot yield, was simila r to that of Bragg. Root growth of nts 382 was more retarded than shoo t growth (on average 50% that of Bragg), and vet absorbed a similar am ount of total P as Bragg. Also, Chippewa that had the lowest root dry matter accumulated the highest P in the shoots. The data indicate that plant growth was influenced by differences in P use efficiency rather than differences in total P. In contrast to plant growth, nts 382 acc umulated 26% more N than Bragg, and Chippewa accumulated a similar amo unt of N as Bragg. Thus again, Bragg used N most efficiently for growt h. Soil N uptake was not governed by root biomass, and Chippewa absorb ed more N from soil than both Bragg and nts 382 which absorbed similar amounts of soil N. Nodulation in nts 382 was significantly enhanced. being 528 and 892% greater than Bragg at P0 and P3, respectively, and nts 382 was the only cultivar whose nodulation was consistently enhanc ed by P addition. The percentage of N derived from fixation (% Ndfa) w as not affected by supernodulation, but because of a greater accumulat ion of total N in nts 382, amount of N fixed (Ndfa) was on average 30% higher in nts 382 than Bragg. Also, while % Ndfa in either nts 382 or Bragg was not influenced by P addition, total N and consequently, Ndf a increased. However, for Chippewa which fixed the lowest N, both % Nd fa and Ndfa were increased by P addition. We conclude that although su pernodulation had an adverse effect on shoot and root growth, these re ductions could not be attributed to a reduced uptake of the two major plant nutrients, N and P, neither was there evidence that the mutation had caused a greater demand for these nutrients, other than for the g reater requirement of P for nodulation.