Asian Journal of Plant Sciences

Year: 2002 | Volume: 1 | Issue: 1 | Page No.: 12-14
DOI: 10.3923/ajps.2002.12.14

Effectiveness of Rhizobium Under Salinity Stress

Nazir Hussain , Fakhar Mujeeb, M. Tahir, G.D. Khan, N.M. Hassan and Abdul Bari

Abstract: This research work was conducted to evaluate the efficacy of Rhizobium trifolii in enhancing the salt tolerance of Trifolium alexandrinum (Berseem or Egyptian clover), protein value of fodder and nitrogen fixation in the soil. Three salinity levels (8,12, 16 dS m^-1) were developed in a loamy soil in pots. A similar set of inoculated pots with Rhizobium trifolii was also arranged. Berseem crop was grown in both of the sets. The data indicated that total green and dry matter yield as well as root dry weight of berseem decreased consecutively with increase in salinity level while Rhizobium inoculation created a positive effect and these parameters remained significantly higher than the similar levels without inoculation. Nodulation and nodule dry weight was promoted markedly by inoculation and depressed significantly with consistent increase in salinity. Nitrogen percentage of berseem shoot increased with inoculation and decreased significantly at 8 dS m^-1 but further increase in salinity had a little effect. Percent nitrogen of soil and nitrogen fixation in soil was enhanced significantly by inoculation and decreased significantly with increase in salinity. Inoculation was helpful to keep the protein content higher. Soil ECe and pH were lowered.

Keywords: berseem, rhizobium inoculation and salinity strees

Introduction

A legume provides a nutritious body building food for man and animals of the world. Air is 80% N_2, thus, there are around 6400 kg of nitrogen above every hectare of land and water. This free air nitrogen can be fixed through symbiotic or associated nitrogen fixation processes. Fixation of 100-150 kg N₂/ha/year are common by root nodulation bacteria of legumes (Rhizobium spp.) in temperate climate (Alexander, 1976). Since nitrogenous fertilizer is the most commonly expensive due to high-energy consumption in its conversion, the special ability of leguminous crops to work symbiotically with rhizobia to fix nitrogen and produce protein is becoming increasingly important in world agriculture. The agronomist interest in rhizobia is due to the N₂-fixing ability of these bacteria. Rhizobia are soil bacteria characterized by their unique ability to interact with root hairs of legumes and induce effective nitrogen fixing nodules. However, soils do not become productive immediately but gradually as the favorable microorganisms are given time to multiply.

Changes in microbial population of soil are continuous and very important. The alkali salts, which lower the production of crops, similarly affect activities of desirable soil microorganisms. Saline and alkali soils constitute an unfavorable habitat for the growth and multiplication of rhizobia. Bernstein (1962) observed almost complete failure of nodule development on soybean when ECe of rhizosphere exceeded 7 dS m^-1. Nevertheless, it is not uncommon to expect the presence of many Rhizobium species in salt affected soils because legumes like sesbania and Molalities have been found to nodulate and play important role in reclamation. Forage legumes, such as Persian clover or shaftal (Trifolium resupinatum L.) and Egyptian clover or berseem (Trifolium alexandrinum L.) have been recommended for growing in sodic soils (Chillar and Bhumbla, 1972; Kumar et al., 1981; Sharma et al., 1983). Burton (1967) reported that Rhizobia are either totally absent or are scanty in salt affected soils. More recent work of Asthma et al. (2000) proved that number of primary roots, root proliferation and root uptake was enhanced even in salt stressed condition when Rhizobium inoculation and 2,4-D was applied to wheat.

Saline soils are generally deficient in nutrients and microbial activities and population is low. Adding Rhizobium bacteria in the soil can enhance it. Grain and forage legumes vary in their nutritional requirements and are better adopted to soil low in nitrogen provided proper rhizobial cultures are used for seed inoculation. Generally leguminous crops are salt sensitive and show the host-strain specifically also within the same cross inoculation group. Little literature is available related to soil salinity. Studies on the sensitivity of Rhizobium for its nodulation and symbiotic behavior in salt affected soils are not well documented. So this investigation was conducted to explore all such possibilities.

Materials and Methods

The study was carried out in pots at Salinity Research Institute, Pindi Bhattian. Pots were filled with 12 kg air dried, ground loam soil which was passed through 2 mm sieve (ECe = 0.97dS m^-1, pHs = 8.4, saturation percentage = 30 and soil nitrogen = 0.03%). Three salinity levels (8, 12, 16 dS m^-1) were developed with four salts; NaCl, Na₂SO₄, CaCl₂.2H₂O and MgSO₄.7H₂O in Ca²⁺ Mg²⁺ratio of 2:1 and SO_4¯²: Cl^-1 ratio of 1:1. Sodium adsorption ratio was fixed at 10 (m mol L^-1) ^½. Two treatments of inoculation (inoculated and un-inoculated) were kept at each salinity level. Seed of Egyptian clover or berseem (Trifolium alexandrinum L.) was inoculated with Rhizobium trifolii. Population of Rhizobial trifolii material was 5 x 10⁹ cells of bacterial per ml of inoculum. A uniform dose of 20-75-0 Kg of NPK ha^-1 was given to all pots. Five plants of Berseem were maintained in each pot and these pots were irrigated as and when required. The pots were kept open at the bottom in order to allow the leaching of salts, if any. Three cuttings were obtained. Soil and plant samples were analyzed for nitrogen as per method of Jackson (1960). Soil EC and pH were determined after the last cutting. The data were processed statistically applying split plot design described by Steel and Torrie (1986).

Results and Discussion

Yield and Yield Components: Data (Table 1 and 2) indicated that berseem inoculation with Rhizobium increased the total green fodder and dry matter yield significantly at all salinity levels, which was maximum in inoculated control pots. Mean total green fodder and dry matter yield depressed significantly with increasing salinity levels. Minimum Berseem yield was observed at 16dS m^-1. Mean total green and dry matter yield of Berseem enhanced significantly by Rhizobium inoculation. The results are in conformity with Batra and Ghai (1987) who reported that Egyptian clover, Persian clover and Sengi showed a significant reduction in yield at ECe 16dS m^-1 compared with the normal soil while inoculation of seed with Rhizobium culture increased the green matter yield of these legumes.

In case of root dry weight (Table 3) treated pots with Rhizobium inoculum brought out significantly higher root dry weight of berseem at control and 8 dS^-1, but at 12 and 16dS m^-1 salinity levels the differences between inoculated and un-inoculated treatments remained similar statistically. Hence, the higher salinity levels over masked the beneficial effects of inoculation. When main effects of salinity and inoculation were considered, it was noted that salinity had depressing effect and rhizobial inoculation had boosting impact on growth and dry weight of root. Maximum green fodder during first cutting was obtained in inoculated pots with control salinity whereas minimum was recorded the fodder yield was higher in all the inoculated pots at the equivalent salinity levels.

When we compared means for different cuttings, it was noticed that green fodder yield was the highest (63.77g pot^-1) at second cutting followed by first and third cuttings with 60.5 and 44.53g pot^-1, respectively. The differences within cuttings were significant statistically. Similar trend was obtained for dry matter yield of different cuttings (Table 5). However, maximum mean dry matter was gained at third cutting followed by first cutting, which in turn, was at par with second cutting. Relative growth rate (RGR) is the increase of plant material per unit of material assimilated in a unit time. It determines the growth of plant and efficacy of plant enhancement on account of different treatments. The data (Table 6) exhibited that Berseem had the highest value of RGR in control with inoculation at all cuttings, while the lowest value was observed at ECe 16 dS m^-1 without inoculation in all cuttings. As a general trend the mean values of RGR showed an increase in the second and third cutting than first cutting. Relative growth rate increased on account of inoculation of Rhizobium at all the salinity levels. Hence, inoculation favored the relative growth rate of Berseem in saline condition. Saline media hindered the RGR and prevented the general growth of the legume at the higher salinity levels.

Nodulation: Data pertaining to nodule formation inferred that mean number of nodules decreases significantly with an increase in salinity level of soil and increased significantly by seed inoculation with Rhizobium (Table 7). Inoculation of seed increased the nodule formation significantly at control, 12 dS m^-1 and 16 dS m^-1 salinity levels, but at 8 dS m^-1 increase in number of nodules per pot is non-significant statistically. Batra and Ghai (1985) also concluded that Egyptian and Persian clovers could not nodulate at 8 and 12dS m^-1 respectively showing the host strain specificity within the same cross inoculation group. Nodules dry weight also reduced markedly due to higher levels of salinity (Table 8). Maximum mean nodule dry weight was obtained at control and minimum at 16 dS m^-1 ECe level. Interaction between salinity and Rhizobium levels was found to be significant. At all salinity levels, the nodule dry weights were enhanced significantly by inoculation of Rhizobium. Similarly Batra and Ghai, (1987) proved that number and weight of nodules was reduced at ECe 15 dS m^-1. Mean nodule dry weight increased significantly by seed inoculation.

Nitrogen : Data of evinced that mean nitrogen percentage in berseem shoot decreased significantly due to soil salinity than control, but differences between 8,12 and 16 dS m^¯1 ECe levels were nominal and non-significant statistically (Table 9). Mean figures revealed that shoot nitrogen percentage rose significantly by inoculation. No interaction between salinity and inoculation was observed.

Nitrogen percentage of soil was badly lessened due to the presence. It decreased significantly with salinity increase, being minimum at 16 dS m^-1 and maximum in control. The two middle levels of salinity (8 and 12 dS m^-1) had statistical equal nitrogen percentage. Inoculation had a significantly positive effect on soil nitrogen. Differences due to interaction between salinity and inoculation remained non-significant (Table 10). Mean nitrogen fixation in soil was suppressed with raising salinity and ascended significantly with inoculation of Rhizobium trifolii in control and different salinity levels (Table 11). No interaction between salinity and inoculation was observed.

Protein Contents: Protein percentage was influenced significantly by salinity and inoculation with Rhizobium (Table 12). All the there levels of ECe were statistically different as compared to control, but similar with one another when the main effect of salinity was considered. Plants with inoculation assimilated more protein than un-inoculated plants at all salinity levels.

Soil EC and pH: Data relating to electrical conductivity (ECe) indicated that salinity was reduced significantly by Rhizobium inoculation at all salinity levels. Mean figures showed that Rhizobium inoculation of seed deceased salinity level from 9.72 to 6.68 dS m^-1 (Table 13) Mean pHs values of soil after harvest of crop indicated that this parameter was similar at initial two levels of salinity and significantly higher than the next two levels which were in turn similar to each other. Effect of inoculation was found to be significantly positive (Table 14)

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