Levels Of Combined Nitrogen Research Articles

Effects of Nitrate and Light Intensity on Photosynthesis and Nitrogen Fixation in Alfalfa Plants

Summary Alfalfa plants ( Medicago sativa , L. cv. Aragon) inoculated with Rhizobium meliloti weregrown with a N-free-nutrient solution in controlled environmental chambers. After 5 weeks plants were placed under 2 irradiance regimes (120 and 60 μE · m -2 · s -1 ) and different N concentrations were supplied to treatment plants as Ca(NO 3 ) 2 . Rates of acetylene-dependent ethylene production and CO 2 uptake were determined 2 days later. The addition of 1.75 and 3.5 mM N0 3 - to plants in each level of irradiance decreased both nitrogenase activity and CO 2 assimilation regardless of the light intensity. The addition of 7 and 14 mM NO 3 - resulted in an enhanced leaf photosynthesis. Simultaneously, the C 2 H 2 reduction rate by nodulated plants at 60 μE was lower than that of N 2 -dependent plants, while plants at 120 μE had similar acetylene reduction values. Higher levels of combined nitrogen, 28 and 56 mM NO 3 - , reduced nitrogenase activity of plants under both of the two light intensities and CO 2 uptake by plants at 60 μE. However, current photosynthesis was not affected in plants at 120 μE. These results are interpreted as showing that alfalfa plants can utilize nitrate to improve CO 2 assimilation without inhibiting N 2 fixation when plants are under conditions in which light is not saturating.

Studies on N2-fixing bacteria associated with the salt-tolerant grass,Leptochloa fusca (L.) Kunth

N2-fixing bacteria were isolated from the rhizosphere of naturally grown salt tolerant grass (Leptochloa fusca). A broad spectrum of diazotrophs was found to be associated with the roots ofL. fusca. the systematic position of the three isolates, NIAB-1, C-2 and Iso-2 was determined by morphological, biochemical and mol % (G+C) DNA contents. Two isolates were identified asKlebsiella pneumoniae (NIAB-1) andBeijerinckia sp. (Iso-2).15N enrichment studies confirmed the nitrogen fixing ability of the isolates. The effects of different levels of combined nitrogen (NO 3 − & NH 4 + ), pH (5.5–9.0) and salt (NaCl) on nitrogenase activity of the isolates were determined at various time intervals. All isolates exhibited nitrogenase activity even in the presence of 5 mmol/l NO 3 − or NH 4 + in a semi-solid medium after 24 h of growth. Maximum nitrogenase activity was observed at alkaline pH and all isolates were able to tolerate up to 3% NaCl in the medium.

Variation in ability of Rhizobium leguminosarum isolates to fix dinitrogen symbiotically in the presence of ammonium nitrate

To assess the variation in symbiotic effectiveness of N2-fixing Rhizobium leguminosarum isolates in the presence of NH4NO3, peas were inoculated with 38 isolates and grown for 4 weeks with weekly additions of 2 mM NH4NO3. Acetylene reduction was inhibited relative to a N-free control in all isolates but the inhibition varied from 60 to 100%. There were also significant differences (p = 0.05) between isolates with varying inhibition of C2H2 reduction in H2 uptake, H2 evolution, and leghemoglobin content. Ten isolates were selected for further study of the importance of NH4NO3 concentration (0, 1,2, and 5 mM) and plant growth stage (3,4, and 5 weeks) to characters associated with N2 fixation and plant growth. Plant age, NH4NO3 treatment, and isolate were all significant sources of variation (p ≤ 0.01) for each character measured but interactions between these factors were also significant. There were significant differences between isolates in the rate of decrease in C2H2 reduction with increasing NH4NO3 concentration (C2H2 reduction responsiveness to NH4NO3) and this responsiveness term was correlated with C2H2 reduction rates at 2 and 5 mM NH4NO3 (p = 0.01). Acetylene reduction rates were correlated with shoot dry weights at 0.1, and 2 mM NH4NO3 (p = 0.001). These results suggest the possibility of enhancing N2 fixation in peas through selection of rhizobia with maximum effectiveness at low levels of combined nitrogen.

Occurrence of aerobic nitrogen fixing bacteria in wetland and dryland plants

Abstract We recently suggested that N2-fixing bacteria other than Azospirillum are present in greater abundance in the roots of wetland rice (2). Using a low concentration of tryptic soy agar, we isolated aerobic heterotrophic bacteria that required low levels of combined nitrogen for nitrogenase activity from wetland rice roots (1). In a liquid medium under batch culture conditions, the bacteria were unable to grow without combined nitrogen under either aerobic or anaerobic conditions. Neither could they grow nor exhibit nitrogenase activity in a semi-solid N-free medium unless the medium was supplemented with a small amount of yeast extract, casamino acids or any amino acid. Among the nitrogenase positive aerobic heterotrophic isolates from the inner tissue of rice roots that accounted for 80% of the isolated heterotrophs, about 90% were found to be Pseudomonas-like, which require a low level of combined nitrogen. We further report here the occurrence of aerobic nitrogen fixing bacteria in association w...

Symbiotic characteristics of Trifolium ambiguum seedlings grown in tube culture as affecting subsequent symbiotic vigour

A hexaploid line of Caucasian clover (Trifolium ambiguum), inoculated with two strains of Rhizobium trifolii, was grown in tubes of nitrogen-free medium under bacteriologically-controlled conditions. Observations were made on various symbiotic characteristics of the seedlings. After 30 days, they were transplanted into pots of sand and vermiculite watered with nutrient solution lacking nitrogen and grown for another 100 days. Uninoculated plants supplied with various levels of combined nitrogen were used for comparison. R, trifolii strain CC283b was classified as highly effective and was greatly superior in nitrogen-fixing ability to strain CC231a which had been used routinely in Australia for many years for inoculation of experimental sowings of Caucasian clover. In this line of T, ambiguum, the inherent plant-to-plant variation was accentuated in plants relying on symbiotically-fixed nitrogen. This effect was strong in plants inoculated with the intermediately-effective strain CC231 a but mild in plants inoculated with the highly-effective strain CC283b, i.e., a larger component of the variability amongst CC231a plants was due to the symbiosis than was the case for CC283b plants. Seedlings inoculated with the intermediately-effective strain and selected on the basis of 'within-tube' characteristics such as time to initial nodulation, symbiotic effectiveness rating, and nodule size produced more dry matter when grown in pots than the unselected population and were as productive as plants inoculated with the highly-effective strain. In contrast, with the highly-effective strain, selected plants grown in pots produced very little more dry matter than the unselected population. It was concluded that the use of 'within-tube' symbiotic characteristics as a means of selection for improved production will only be successful when a substantial component of the plant-to-plant variability is due to the symbiosis.

Bimodal response by nodulated legumes to combined nitrogen

This paper reports on the occurrence of a bimodal growth response by nodulated lucerne to a range of levels of combined nitrogen and considers the role of temperature and genotypic effects in varying this response. The response is characterised by a peak at low levels of combined nitrogen followed by a trough and a further peak at high levels of nitrogen. Host growth rate, rate of development of nitrogen fixing capacity and combined nitrogen uptake are factors implicated in varying this bimodal response. Any beneficial response to combined nitrogen is restricted to the pre-linear phase of growth. The need for high legume growth rates to achieve fixation at high soil nitrogen levels is also shown.