CHARACTERIZATION OF PLANT GROWTH PROMOTING RHIZOBACTERIA AND THEIR EFFECTS ON GROWTH AND YIELD OF FIELD PEA (Pisum sativum L.) IN EAST WOLLEGA AND HORRO GUDURU WOLLEGA ZONES, WESTERN ETHIOPIA

Abstract

Crop yield in most parts of western Ethiopia, where the study area is located, is generally low due to low soil fertility, particularly as a result of nitrogen (N) and phosphorus (P) deficiency, and soil acidity. Although field pea is capable of fixing atmospheric nitrogen biologically in association with root nodule bacteria, the effectiveness of the bacteria under different host plants and environmental factors has not been evaluated in the study area. This study was initiated to characterize and evaluate the symbiotic effectiveness of field pea rhizobia from East Wollega and Horro Guduru Wollega in western Ethiopia using standard methods under laboratory and greenhouse conditions. The results showed that all the 32 isolates exhibited typical colony characteristics of fast growing rhizobia. About 50.4 and 40.6% of the isolates were characterized by large mucoid and watery colony texture, respectively. Most of the isolates were able to grow at pH levels ranging from 4.5 to 9.0 and salt (NaCl) concentrations of 1-4%, and displayed tolerance to a wide range of temperature (10-45°C) under laboratory conditions. They were also capable of growing on a number of carbon and nitrogen sources. They also showed significant variations in resisting different types of antibiotics and heavy metals. The data also showed that 94.4% of the isolates performed best in symbiotic nitrogen fixation from which 34.4% were highly effective and 62% were effective. The inoculated plants also showed differences in nitrogen content, which ranged from 1.72 to 2.93%.. The highly effective isolates (RHU22, RHU18, RHU35, RHU3 and RHU29) and the effective isolates (RHU10 andRHU13) were nutritionally versatile and ecologically competitive and thus can be recommended for further test on several pea varieties under field conditions to enhance field pea production. In addition, phosphorus (P) is plant nutrient whose deficiency severely restricts plant growth and yields due to fixation problem. The problem of fixed P can be solved partly by phosphate solubilizing bacteria and the study was carried out to characterize and evaluate the phosphate solublizing and other plant growth promoting properties of rhizobacteria from field pea (Pisum sativum L.) collected in East Wollega and Horro Guduru Wollega, Ethiopia using Pikovskaya methods in laboratory condition. The 11 isolates were further evaluated on Pikovskaya medium containing Ca3(PO4)2, AlPO4 and FePO4 sources. Morphological, physiological and preliminary biochemical tests were used for identification of the bacteria. Eleven of the isolates that were positive for phosphate solubilization were identified as belonging to six genera. Pseudomonas spp. and Bacillus spp. were the dominant bacteria species among the isolates. Isolate PsHU2 and PsHU6 showed highest P-solubilization efficiency of 233% in TCP, 133% in AlPO4, 75% and 50% in FePO4 respectively, whereas PsHU4 showed P-solubilization efficiency of 200% in TCP and 50% FePO4 in solid media. The isolates showed significant differences in their soluble P and pH changes in the supernatant of all liquid medium containing Ca3(PO4)2, AlPO4 and FePO4 over the entire incubation period. Generally, based on in vitro tests, the isolates PsHU2, PsHU6, and PsHU4 were the best P-solubilizers in both alkaline and acidic environments and possess multiple plant growth promoting properties. An experiment was also carried out to evaluate the effects of single and co-inoculation of Rhizobium and Pseudomonas on growth and yield of field pea under field condition of Jimma-rare and Holeta in Ethiopia. A 16 total of eleven treatment combination composed from three Rhizobium and two Pseudomonas isolates with control and each replicated three times and arranged in Randomized Complete Block Design. Seeds of “Burkitu” improved local variety of Pisum sativum were sown after uniformly coated with inoculants. Single and dual inoculations with Rhizobium and Pseudomonas spp. significantly (P<0.05) increased all parameters investigated as compared with uninoculated treatments. The highest mean nodule number per plant, nodule dry weight and shoot dry weight were 64.33 (RHU35PsHU6), 191.5mg (RHU35PsHU2) and 11.55 g/plant (RHU35PsHU2), respectively. However, the highest mean pod number per plant, seed per pod and grain yield were 22.03 by RHU35PsHU2, 5.41 (RHU35) and 2225.0 kgha-1(RHU35PsHU6), respectively. The highest mean plant nitrogen and plant phosphorus in straw were 4.42% (RHU35PsHU6) and 1721.67mgkg-1(RHU35PsHU6), respectively. It is concluded that isolates RHU22, RHU18, RHU35, RHU3, and RHU29 were nutritionally versatile, ecologically competitive, and highly symbiotically effective. Isolates PsHU6 and PsHU2 were the most efficient in their overall performance on the different inorganic phosphate sources. The dual RHU35PsHU6, RHU35PsHU2, RMPsHU2 and single RHU35 were identified as potential inoculants. This implies that it could enhance growth and yield of field pea in soils of N and P deficiency.