Nodule Physiology of a Supernodulating Soybean (Glyine max) Mutant

The nodule physiology of a supernodulating, nitrate tolerant symbiosis soybean (Glycine max (L.) Merr.) mutant (nts382) was compared to that of its wild-type parent, cv. Bragg. Nodule number and mass were greater in nts382 than cv. Bragg and individual nodule mass, bacteroid and haem content, and acetylene reduction activity per g nodule were less. Acetylene reduction activity expressed per mg bacteroid protein was the same in the two genotypes. In median sections, the ratio of infected to total nodule area was smaller in nts382, infected cell size was smaller and there were fewer bacteroids per peribacteroid envelope. When inoculum dose was decreased from 109 to 103 viable cells per pot, nodule number on nts382 decreased approximately to that on cv. Bragg; nodule size, bacteroid and haem contents increased as did nodule acetylene reduction activity. Application of moderate levels of nitrate, which did not significantly affect symbiotic parameters of cv. Bragg or high inoculum nts382, stimulated nodule growth and nitrogenase activity of low inoculum nts382. A combination of nitrate and low inoculum levels enhanced nodule parameters of nts382 to the level usually seen with cv. Bragg. When supernodulated, plant dry weight of nts382 was less than that of Bragg; decreasing inoculum dose had no significant effect but nitrate application increased plant growth; nitrate plus low inoculum induced similar plant growth to that of cv. Bragg. Nodule carbohydrate content was similar in both genotypes but nodule and xylem sap ureide contents were higher in nts382. In general, nodules of supernodulated nts382 resembled under-developed cv. Bragg nodules; when supernodulation was avoided by using low inoculum doses, ,nts382 nodules resembled those of cv. Bragg. Nitrogen metabolism in the mutant seems to be disturbed, resulting in ureide accumulation.