Isotopic Discriminations During the Accumulation of Nitrogen by Soybeans

Abstract

Soybeans were grown in a glasshouse in sand-vermiculite medium supplied daily with a mineral nutrient solution essentially free of combined N or containing 5 mM nitrate of known ¹⁵N abundance. The natural abundance of ¹⁵N in parts of plants and in nitrogen remaining in the medium was determined from 15 days after planting until fruiting.

In nodulated plants completely dependent on N₂ fixation for growth, the δ¹⁵N of plant nitrogen was uniformly negative at 56 days (overall mean: -0.90± 0.17) after adjustment for the effect of seed nitrogen. The δ¹⁵N of root nodules increased with time (max. 9.6‰), as that of shoots declined (min. - 1.3 ‰). The δ¹⁵N of every mainstem trifoliolate leaf and of the first (unifoliolate) leaf declined from initially positive values (0.5 to 2 ‰) to about - 2‰ with similar time courses, irrespective of the time of initiation. There were no significant losses of N from the plants during growth. There were differences between the δ¹⁵N of the total N of root-bleeding xylem sap and of sap extracted by vacuum treatment of stems. These were due to differences between the proportions of ureide-N and amino-N and between the δ¹⁵N values of these components.

When nodulated plants were supplied daily with 5 mM nitrate (δ¹⁵N = 7.68‰) between 21 and 35 days, N₂ fixation was reduced to 63% of N assimilated but growth and accumulation of nitrogen were affected little. Following removal of nitrate, there were changes in growth which led to enhanced nodulation and N₂ fixation. The δ¹⁵N of the total N of trifoliolate leaves which were initiated or expanded before or during the period of nitrate treatment remained positive; those expanded or initiated after the treatment became negative in δ¹⁵N, as in the corresponding leaves of untreated nodulated plants. The δ¹⁵N of nodules was unaffected by the nitrate treatment. In plants (non-nod. Clark '63) supplied continuously with nitrate, the δ¹⁵N of the total N of entire plants rose quickly from values for seeds, but to values significantly higher than in the nitrate.

These results are discussed in relation to the effects on the use of ¹⁵N natural abundance data for estimating utilisation of atmospheric N₂ by nodulated plants.