A comparative study of nitrogen fixation in selected cultivators of bush and climbing beans inoculated with different rhizobia strains
Gicharu, Gibson Kamau
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Nitrogen deficiency is a major factor limiting bean production in Africa and Latin America. This problem can be alleviated by the use of nitrogen fertilizers which on the other hand adversely affect the environment or by the use of environmental friendly biological nitrogen fixation technology. In this study, growth experiments were designed to investigate biological nitrogen fixation in determinate (bush) and indeterminate (climbing) groups of beans. Preliminary studies on plant height and nodulation of sixty determinate and sixty indeterminate cultivars were carried out to select six cultivars for the main field study. Selected cultivars were Bush beans-Ayenew, GLP 24, Ecab 0807 and climbing beans-Cargamanto, NG224-4 and G59/1-4. Results showed that the cultivars differed statistically (P < 0.05) in both plant height and number of nodules. Greenhouse and field studies were carried out to determine the most efficient bean cultivars and rhizobia strains in nitrogen fixation. Each cultivar was inoculated with the rhizobia strains CIAT 899, USDA 2674, USDA 2676, the three rhizobia strains combined and one was not inoculated. Results from greenhouse studies showed significant statistical differences in nodulation within bush bean cultivars. The bush bean cultivar GLP24 was poorly nodulated (40.0 nodules on average) as compared to Ayenew (52.0) and Ecab 0807 (58.0) nodules. There were no significant statistical differences in nodulation among the different treatments in climbing cultivars observed in the greenhouse experiments. However, climbing beans nodulated better than the bush cultivars with NG224-4 producing 74.0 nodules on average followed by Cargamanto with 76.0 nodules and G59/1-2 the highest nodule number at 110.0. It was also observed that bush beans inoculated with USDA 2674 strain of rhizobia in the greenhouse produced the highest nodule number but in the field, it was those inoculated with strain CIAT 899. Among the climbing cultivars in the greenhouse, there were no statistical differences in nodulation between the different inoculant treatments given but in the field climbing beans inoculated with rhizobia strain USDA 2676 formed the highest nodule number. Climbing beans nodulated better than bush beans possibly because of their larger photosynthetic area, more extensive root system and the fact that their life cycle was longer. Bush beans inoculated with USDA 2674 gave the highest total plant dry weight whereas among the climbing beans those inoculated with a mixture of the three rhizobia strains gave the highest total plant dry weight. Treatments within each of the cultivars in both climbing and bush beans showed differences in total plant dry weight yield in response to rhizobia strain used. Field experiments showed that plant dry weight differed statistically between cultivars. There were no significant differences in yields between the different treatments. There is therefore need for more detailed studies to especially identify the indigenous strains of rhizobia, which in this study were found to be as effective as the recommended strains.