Dipankar Sen

Effect of high root temperature on Bradyrhizobium-peanut symbiosis

Three strains of Bradyrhizobium, 280A, 2209A and 32H1, that nodulated peanuts (Arachis hypogaea L.), were tested for their ability to grow and survive at elevated temperatures of up to 42°C in laboratory culture. Strain 32H1 was unable to grow at 37°C and was more sensitive to elevated temperatures than the other two strains. All three produced heat-shock proteins of molecular weights 17 kDa and 18 kDa. Two greenhouse experiments were conducted to determine the effect of high root temperature on nodulation, growth and nitrogen fixation of peanut. Two peanut varieties (Virginia cv NC7 and Spanish cv Pronto) were inoculated and exposed to root temperatures of 30°, 37° and 40°C. Nodulation and nitrogen fixation were strongly affected by root temperature but there was no variety × temperature interaction. At a constant 40°C root temperature no nodules were formed. Nodules were formed when roots were exposed to this temperature with diurnal cycling but no nitrogen fixation occurred. Highest plant dry weight, shoot nitrogen content and total nitrogen were observed at a constant root temperature of 30°C. Increasing root temperature to 37°C reduced average nitrogen content by 37% and total nitrogen by 49% but did not reduce nodulation. The symbiotic performance of the strains corresponded to their abilities to grow and survive at high temperature in culture.

A basis for different rates of N2-fixation by the same strains of Rhizobium in peanut and cowpea root nodules

Summary Same strains of Rhizobium that nodulate both peanut and cowpea roots are metamorphosed to large spherical bacteroids only in peanut where they show several times higher nitrogen-fixing activity than in cowpea. We investigated the basis for such differential activity. Bacteroids isolated from the two hosts showed equal rates of acetylene reduction and oxygen consumption but a much larger number of bacteroids were found per unit nodule volume of cowpea. This may produce an intranodular oxygen deficiency preventing higher expression of bacteroidal nitrogenase activity in cowpea nodules. This was supported by the pattern of response to increased ambient oxygen pressure.

Nitrogen fixing activity of rhizobial strain 32H1 in peanut and cowpea nodules

Abstract Rhizobial bacteroids undergo unique modifications in peanut root nodules. To determine the likely effect of such modifications on nitrogen fixation, peanuts and cowpeas were nodulated by the same strain of Rhizobium and nitrogenase activities (C 2 H 2 -reduction) were compared over 4 weeks. Peanuts were found to have one and a half to three times higher C 2 H 2 -reduction activity/unit nodule mass than cowpeas at different nodule ages. Nitrogen accumulated in the plant tops/unit weight of nodules on the roots was three times greater in peanuts. The bacteroid protein content/unit of nodule mass was very similar for the two species.

A comparison of nitrogen-fixing ability of peanut, cowpea and siratro plants nodulated by different strains of Rhizobium

SummaryAcetylene reduction activity and nitrogen accumulation in the plant top per unit nodule mass were compared among peanut, cowpea and siratro plants nodulated by six different strains of Rhizobium. Peanut was found to have several fold higher values than cowpea and siratro for both parameters for all strains of Rhizobium which nodulated it effectively, but the bacteroid content of the peanut nodules was similar to those of cowpea and siratro.

Diversity of Rhizobia isolated from various Hedysarum species

Cultural and physiological properties, serology, plasmid profiles and infective traits were determined for 23 strains of rhizobia isolated from various Hedysarum species: H. coronarium (common name: sulla) (16), H. carnosum (1), H. alpinum (3), H. mackenzii (2) and H. pallens (1) from Portugal, Spain, Tunisia, Alaska and Israel. Strains isolated from H. alpinum, H. mackenzii and H. pallens have slow growth rates on yeast-extract mannitol medium and were unable to nodulate H. coronarium plants, whereas the latter were effectively nodulated by all sixteen fast growing strains from sulla. Regardless of the country of origin all H. coronarium strains fell into one serogroup and were not serologically related with strains of other Hedysarum species. The RAPD (random amplified polymorphic DNA) fingerprinting method which was carried out on five H. coronarium and three H. alpinum strains allowed distinction to be made among serologically related rhizobia. No particular plasmid profile pattern was observed in relation to the host or geographical origin of the strains.

Cell wall (outer membrane) of bacteroids in nitrogen-fixing peanut nodules

Rhizobia in peanut nodules are transformed from rod-shaped cells to extremely large spherical bacteroids. The invading rhizobia shed their outer membranes soon after their release into the host cells. The outer membranes are seen to be peeled off and replaced by new outer membranes before rhizobia are differentiated into nitrogen-fixing bacteroids. Evidence is presented that peanut bacteroids, in spite of their spheroplast-or protoplast-like appearance, do possess both the inner and the outer membrane.

Nitrogen and Phosphorus for Growth of Oil-Degrading Microorganisms in Seawater

Seawater was supplemented with NH4 + and P to determine concentrations of N and P adequate for supporting exponential growth of bacteria utilizing crude oil, and to determine maximum rates of N and P uptake. Oil-degrading microorganisms were obtained by enrichment culture of indigenous oil-utilizing microorganisms in seawater. NH4 + at a concentration of 5.5 µM was limiting to growth of bacteria on crude oil. Exponential growth occurred at concentrations higher than 30 µM NH4 +. The P concentration of 0.13 µM was limiting to growth of bacteria on crude oil. Exponential growth occurred at 1.8 |J,M P. The maximum NH4+ consumption rate was 426 ± 30 |J,g NH4 + L−1 hr−1, and the maximum uptake rate of P was 48±4 µg P L−1 hr−1. Uptake of N and P with time showed zero-order kinetics, likely due to substrate solubility limitations. The uptake ratio of N:P was approximately 7:1 on a weight basis. Natural concentrations of N and P in marine and estuarine systems after hydrocarbon spillage initially may not limit oi...