EFFECTIVENESS OF ACID TOLERANT INDIGENOUS RHIZOBIA AND LIME APPLICATION ON THE PERFORMANCE OF GROUNDNUT IN WESTERN KENYA.

Abstract

Rhizobia inhabit root nodules where they reduce atmospheric nitrogen into a form available to plants. High productivity of grain legumes depends on effective symbiotic nitrogen fixation through successful legume inoculation with an efficient strain of rhizobia. The aim of the study was to identify efficient, acid tolerant rhizobia native to soils of western Kenya that could be used as groundnut (Arachis hypogeae L.) inoculants for enhanced crop performance. Rhizobia were isolated from nodules of groundnut collected from various localities of western Kenya. Initial characterization of the rhizobia was done using morphological, microbiological and biochemical features. The response of the isolates to varying pH (3.5, 4.5, 5.5 and 6.8) and aluminium (0.0, 50, 100, 130, 150 and 200 µM) was tested in a basal liquid media. Time taken by each isolate to show visible turbidity in the media was recorded in hours and used as a measure of tolerance to acidity and aluminium stress. Broth cultures of the isolates were used to inoculate groundnut (Red Valencia) to assess the ability of the isolates to nodulate groundnut and thereafter cross inoculation to three other varieties (ICGV 9991, ICGV 12991 and ICGVSM99568) was done in the greenhouse. Field evaluation of symbiotic effectiveness of the isolates was done using three isolates and one commercial strain as inoculants. The effect of two different limes (dolomitic lime and calcitic lime) on the yield of rhizobia inoculated groundnut was also assessed at Koyonzo and Ligala. Nodule number and dry weight, nut number per treatment, shoot and grain yield were determined. Recovery of inoculant strains from nodules of field grown plants was then done using resistance of the isolates to various antibiotics, to gauge the competitiveness of the strains. A total of 90 bacterial isolates were obtained from groundnut nodules. Based on their growth on Yeast Extract Mannitol Agar (YEMA) incorporated with Congo red, 15 isolates were found to be very fast growing, 52 fast growing while the remaining 23 isolates were moderate to slow growing. On YEMA incorporated with Bromothymol blue (BTB), 64 of the isolates produced acid while 26 were alkaline producers. Further characterization showed 84 of the isolates to be gram negative rod cells that reduced nitrate to nitrite, hydrolysed urea to ammonia and utilized citrate as a sole carbon source. Screening for tolerance to pH (3.5 - 6.8) showed that 76 isolates tolerated the lowest pH of 3.5. At aluminium concentration of 0-200 µM, 36 of the 76 acid tolerant isolates grew at 130 µM while only 5 strains namely, A6, Biofix, n3, V2 and W1 could withstand 200 µM Al. Among the 36 Al tolerant rhizobia, 3 were slow growing and 33 fast growing. Nodulation test found only 3 (A6, W1, and V2) of the 36 isolates to be the most promising in efficient nitrogen fixation. Significant improvement (p≤ 0.05) was observed for nodule number and shoot biomass for cross inoculated plants and this varied with the isolate. In the field experiment, the highest grain yield (1002 kg ha-1) was obtained in the short rain season at Ligala with strain V2 on plots with calcitic lime. At Koyonzo strain V2 again with calcitic lime gave the highest groundnut yield of 879 kg ha-1. There were significant difference for strain x lime, strain x lime x site and strain x lime x site x season. Strains V2 and A6 had no significant difference across sites and seasons. This study has identified effective indigenous rhizobia adapted to acid soils of western Kenya that can be used as groundnut inoculants for enhanced crop performance and grain yield. The findings reported here if adopted will contribute to enhancing groundnut production on acid soils of western Kenya and hence improve livelihoods. v