Genetic Diversity of Indigenous Brad'yrhizobium Strains Nodulating Dual-Purpose Soyabean (Glycine Max L. Merr.) Genotypes and Their Potential to Fix Nitrogen in Kenya

Abstract

Dual-purpose / promIscuouS soyabean [Glycine max (L.) (Merrill)], have recently been introduced in Kenya. The objectives of this study were to determine the natural nodulation of promiscuous soyabean varieties under different inputs, assess the genetic diversity, effectiveness and competitiveness of indigenous Bradyrhizobium strains and determine the soyabean N credit to maize of managed fallows. Varieties were grown under phosphorus, phosphorus + lime and nitrogen + phosphorus + lime in two highland sites and under DAP (18:46:0) in three lowland sites. Genetic diversity was assayed using the PCRRFLP markers by amplifying the 16S-23S rDNA IGS region and sequencing the 16S rRNA gene. Indigenous strains were compared with those deposited in the GenBank through BLAST. Selected strains were tested for effectiveness and competitiveness in sand and soil respectively. Competitive strains were inoculated on released promiscuous soyabean varieties and maize grown at three N rates (0, 30, 60 kg ha-') to estimate soyabean N credit to maize in a split plot design field trial replicated three times at two sites. Data from greenhouse and field studies were analyzed using ANOVA and means separated using LSD and DMRT at p<0.05. In presence of P, promiscuous varieties showed improvements in nodulation (102 to 280 nodules per 0.5 m) than the local variety (3 to 167 nodules per 0.5m). In absence of P, none of the varieties performed better than the local control. PCR-RFLP analysis distinguished 18 intergenic spacer groups (I-XVIII) in the highlands and eight (A-H) IGS groups in the lowlands sites. The IGS groups were specific to sites and treatments but not varieties. All indigenous strains belonged to Bradyrhizobium genus. Bradyrhizobium japonicum, Bradyrhizobium elkanii and Bradyrhizobium spp related strains were predominant in highland and lowland sites accounting for 37.5%, 30.0% and 25.0% respectively, while Bradyrhizobium yuanmigense related strains accounted for 7.5%. Seventy percent of the tested indigenous strains were more effective than USDA 110. TSBF 531 was the most competitive indigenous strain. Maize grain yield following promiscuous soyabean (SB 19) inoculated with TSBF 531 increased by 38.30% over that following maize, 20.60% over that following local soyabean, 18.50% over that following fallow and by 12.96% over that following 1: 1 mixture of TSBF 531 and TSBF 442 strains. Phosphorus should be applied when cultivating introduced promiscuous soyabeans in Kenya. Indigenous elite Bradyrhizobium strain, TSBF 531, is effective and competitive and can be used in soyabean inoculants. It is recommended that maize be grown in sequence with promiscuous soyabeans inoculated with TSBF 531 in order to achieve economic maize grain yields.