Root phenotyping of soybean (Glycine Max (L.) Merr.)related to nodulation and biological nitrogen fixation

Abstract

ABSTRACT Soybean is an important leguminous crop, mostly used for soybean cake and soybean products for human consumption. The increase in cost of fertilizers has led farmers to opt for biological nitrogen fixation. This is a process whereby atmospheric nitrogen is converted to organic form by symbiotic microorganisms in the soil. Therefore, there has been a need to identify the best cultivar X rhizobium interaction. Nitrogen application at planting may reduce N2-fixation on soybean and yield, however results from previous researchers have been inconclusive. This trial was conducted in a glasshouse to assess the effect of Bradyrhizobium japonicum and Sinorhizobium fredii strains on nodulation efficiency of soybean cultivars. This will also help in understanding if the different cultivars perform the same or the interaction is cultivar specific. The first phase, was a germination trial with five soybean cultivars and three cultivars with the highest germination percentage were chosen. After that root length, root angle using a protractor, and root density was measured. PHB 95Y 20R had significantly high (P≤ 0.05) root length, root angle and root density followed by PHB 94Y 80R, there was no significant difference between Williams 82, PAN 1521R and PHB 96T 06R. In the second phase the three cultivars PHB 94Y 80R, PHB 95Y 20R and PAN 1521R, were planted in a glasshouse, each cultivar was inoculated with the two strains including an uninoculated control. Nodule parameters namely, nodulation rate, nodule number per plant, nodule volume per plant, and nodule dry weight were measured. Ureide assays were also conducted to determine the amount of nitrogen in the nodules. Cultivar PAN 1521R had the highest efficiency. After that one cultivar was planted with starter Nitrogen source KNO3 at four rates 0mM, 5mM, 10mM and 15mM. To find the effect of starter nitrogen on nodulation as researcher have inconclusive results on whether it inhibits root growth and activity thus affects nodulation. Plants inoculated with S. fredii exhibited inhibition of nodulation when N rate was more than 10 mM KNO3 and in B. japonicum 5 mM KNO3 inhibited nodulation. Keywords: Nodulation, Bradyrhizobium, Sinorhizobium and soybean.