PLANTARUM 68: 375-3S2, Copenlugen Growth comparisons of a supernodulating soybean (Glycine max) mutant and its wild-type parent David A. Day, Hans Lambers, Jan Bateman, Bernard J. Camdl and Peter M. Gresshoff Day, D. A., Lambers, H., Bateman, J., Carroll, B. J. and Gresshoff, P. M. 1986. Growth comparisons of a supernodulating soybean (Glycirte max) mutant and its wildtype parent. - Physiol. Plantarum 68: 375-382. The growth of a superaodulating, nitrate-tolerant soybean [Glycine max (L.) Merr.) mutant nB382 (nitrate-tolerant symbiosis) was compared to that of its wild-type par- ent, cy. Bragg, over the first 50 days after sowing. Plants were grown either inocu- lated in the absence of an external nitrogen source or uninoculated in the presence of 5 mM KNOj. For both treatments, nte382 growth up to 13 days after planting was faster than that of cv. Bragg. Thereafter, supemodulation of inoculated n£s382 oc- curred and growth of cv. Bragg was faster; shoot and root dry weight increments and leaf area were greater in cv. Bragg, but the N content of ntt382 was higher. Relative growth and net assimilation rates were lower in n«382, which had faster shoot and root respiration rates. Shoot growth of uninoculated plants was similar for both mu- tant and wild-type but roots of nc382 were slightly smaller than those of cv. Bragg. Total plant N content was similar in uninoculated cv. Bragg and na382 but the latter had a higher leaf N content. Early lateral root fonnation (prior to nodule emergence) was greater in n»382 regardless of whether rhizobia or KNO3 were present. We con- clude that nl!382 has some inherent differences from its parent but that supemod- ulation significantly retards plant growth. Additional key words - Nitrogen fixation, nodule number, planl growth. D. A. Day (reprint requests), J. Bateman, B. J. Carroll and P. M. Gresshoff, Botany Dept, Australian National Univ., Canberra A. C. T. 2061, Australia; H. Lambers, Dept of Plant Ecology, Univ. of Utrecht, Lange Nieuwstraat 106, 3512 PN, Utrecht The Netherlands. Introduction ^°^ (Gresshoff et ai, 1985, Delves et al. 1986). These plants offer an opportunity to elucidate the mechanisms Symbiotic development in legumes is tightly regulated, by which symbiotic development is controlled in leg- with nodule number, development and activity being umes, and to investigate the interaction between nodule governed by both internal (autoregulatory) and envi- formation, nitrogen fixation and plant growth. In the ronmental factors. In wild-type soybean and other leg- present study, the growth characteristics of one super- umes, external nitrogen sources such as soil nitrate in- nodulating mutant, HK382, are compared to those of the hibit nodule fonnation (Carroll and Gresshoff 1983, wild-type cultivar cv, Bragg, from which the mutant was Herridge 1982). Recently our laboratory has isolated a derived (Carroll et al. 1985a). number of soybean mutants that produce nodule num- bers far in excess of wild-type cultivars and that con- Abbreviations - nts, nitrate-tolerant symbiosis, tinue to nodulate prolificaily in the presence of an exter- nal nitrogen source (Carroll et al. 1985a,b). These mu- mi .^ • 1 J »|,-J tants have been termed supernodulating and nitrate- tolerant symbiotic {nts) mutants and appear to have an Soybean \Glydne rrwtx (L.) Merr,] cv, Bragg and its rxts altered autoregulatory system, governed by shoot fac- derivative, mutant line 382 were used. The isolation and Received 2 May, 1986; revised 3 July, 1986 Physiol. Pkoitanun 6S, 1986 375