Inhibition of N2 fixation in N-fertilized common bean (Phaseolus vulgaris L.) plants growing on the fields of farmers in the Eastern Cape of South Africa was measured using 15N natural abundance and tissue ureide analysis. The N-fertilized bean plants revealed greater soil N uptake, higher concentrations of nitrate in organs, low tissue ureide levels, and much lower percent relative ureide-N abundance when compared with unfertilized plants. In contrast, the unfertilized plants showed greater nodule fresh weight, higher N derived from fixation (e.g., 84.6, 90.4, and 97.1% at Lujecweni fields 2, 3, and 4, respectively), increased amount of N-fixed (e.g., 163.3, 161.3, and 140.3 kg ha−1 at Lujecweni fields 2, 3, and 4, respectively), greater ureide concentration in stems and petioles, higher % relative ureide-N abundance, and low soil N uptake. We also found that the percent N derived from fixation (%Ndfa) was very high for some bean plants receiving a double dose of N fertilizer [e.g., Lujecweni field 1 (51.8%) and Tikitiki field 1 (53.3%], and quite high for others receiving a single dose of N fertilizer [e.g., Tikitiki field 2 (50.1%), Mfabantu fields 1 and 2 (45.5 and 79.9%, respectively), and St. Luthberts field 1 (58.9%)]. Though not assessed in this study, it is likely that the rhizobia that effectively nodulated the N-fertilized bean plants and fixed considerable amounts of symbiotic N had constitutive and/or inducible nitrate reductase genes for reducing nitrate in nodules and bacteroids, hence their ability to form root nodules and derived high %Ndfa in bean with added N. While single- and double-dose N fertilizer applications increased plant growth and grain yield compared to unfertilized bean plants, the single-dose N fertilizer application produced much greater grain yield than the double dose. This indicates that farmers should stop using a double dose of N fertilizers on bean production, as it decreases yields and can potentially pollute the environment. This study has however shown that government supply of free N fertilizers to resource-poor farmers in South Africa increased bean yields for food/nutritional security.
and Pan1664R), Vigna subterranean (Bambara groundnut), Vigna unguiculata (cowpea) and Macrotyloma geocarpum (kersting groundnut). 1 ml culture of each isolate grown in yeast mannitol extract broth, grown to early log phase was used to inoculate 2 day old emerged seedlings planted on acid washed sand. Non-inoculated plants, with and without mineral N fertilizer (0.05% KNO 3 ) was included as controls. A commercial inoculant; Bradyrhizobium japonicum strain WB74 was used to inoculate the control soybean plants, whiles Bradyrhizobium sp. (Vigna) was used to inoculate the non-soybean control plants. The plants were harvested at 65 days after inoculation. The results indicate that, out of the ten isolates tested, 7 were able to induce ineffective nodules on cowpea (3-203 nodules per plant) and effective nodules on Bambara groundnut (35-171 nodules per plant), Kersting groundnut was nodulated by 6 isolates forming between 2 and 31 ineffective nodules per plant. Eight out of ten isolates induced effective nodules on TGx soybean, whiles the non-TGx soybean variety PAN1664R was highly promiscuous nodulating effectively with 9 out of the 10 isolates. The isolates exhibit variable effectiveness on plant growth and nitrogen fixation on the legume host tested. Isolates TUTRSRH3A, TUTRLRB, TUTM19373A and TUTMCJ7B were highly promiscuous, nodulating all the test crops, and exhibited high N 2 -fixing potential comparable to the commercial inoculant. These strains could be improved further for inoculant development.
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