NITROGEN DEFICIT DURING SOYBEAN POD FILL AND INCREASED PLANT BIOMASS BY VIGOROUS N-2 FIXATION

The N concentration of a soybean [Glycine max. (L.) Merr.] seed freque ntly is greater than 60 mg g(-1). Because of high seed-N demand, a soy bean plant may become N-deficient during pod fill. To test the effect of N source during pod fill on seed N, plants were grown hydroponicall y so that N source could be controlled. From seedling stage to mid-R4, all plants were provided an excess of nitrate-N. From mid-R4 to harve st (R7), randomized sets of 20 plants were fed excess N in the form of any one of 4.0 mM KNO3, 2.0 mM NH4NO3, 2.0 mM KNO3 + 1.0 mM urea, 1.0 mM KNO3 + 1.5 mM urea, or 2.0 mM urea. The mean rate of N assimilatio n during pod fill for plants fed only nitrate was approximately 65% of the mean rates of plants fed the same amount, but different forms, of N during pod fill. Likewise, seed analysis showed that plants fed onl y nitrate during pod fill frequently produced smaller seeds that conta ined a lower seed-N concentration (experiment 1, 63.3 mg N g(-1) vs. 6 7.7-73.4 mg N g(-1); experiment 2, 61.6 mg N g(-1) vs. 63.0-67.2 mg N g(-1)) than those fed other forms of N. Thus, the total N content of s eeds produced by plants fed only nitrate during pod hh was approximate ly 77% of the mean values of plants fed the same amount, but different forms, of N during pod fill. On the other hand, nodulated plants prov ided a low level of nitrate or urea and fixing approximately 600 mg N- 2 plant(-1) frequently had a higher gain in harvested biomass (169%) t han did non-inoculated plants fed excess nitrate (139%). It is conclud ed that rapid N-2 fixation during pod filling enhances net photosynthe tic output of soybean. (C) 1998 Elsevier Science B.V.