Effect of phosphorus deficiency on acid phosphatase and phytase activities in common bean (Phaseolus vulgaris L.) under symbiotic nitrogen fixation

Changes in growth, symbiotic nitrogen fixation (SNF), acid phosphatase (ACP), and phytase activities to phosphorus availability (15 and 60 μmol KH₂PO₄ plant⁻¹ week⁻¹) were compared in two recombinant lines (115 and 147) of common bean. Plant growth, nodulation and SNF were genotype and P level-dependent. 147 was more affected by P shortage (15 μmol P) than 115. Four ACP types were revealed in the nodules of both lines, ACP1 exhibiting a higher specific activity under P shortage as compared to the 60 μmol P treatment, especially in 115. A single phytase was revealed for the nodules of both lines and was significantly enhanced by P deficiency. Three ACP types were found in roots and leaves, showing increasing activity under P deficiency, especially in 115. Regardless of P supply, leaf ACP specific activity was higher than that of nodules and roots in the both lines. Interestingly, phosphorus use efficiency for N₂ fixation significantly correlated to nodule ACP activity under P shortage in the both lines. The relatively better performance of 115 as compared to 147 under P deficiency could be partly ascribed to the ability of 115 to maintain higher ACP activity. This enzyme might be involved in the remobilization of the plant Pi and its utilization for SNF.     

Symbiotic effectiveness of Hup+ Rhizobium,VAM fungi and phosphorus levels in relation to nitrogen fixation and plant growth ofCajanus cajan

Effect of hydrogen uptake positive (Hup⁺) strain ofRhizobium sp. (pigeon pea) and VAM fungus (Glomus fasciculatum) was studied on the symbiotic parameters of pigeon pea (Cajanus cajan) cv. AL-15 at various levels of phosphorus. The Hup⁺ Rhizobium strain showed more nodulation, plant biomass and plant nitrogen content than its Hup⁻ counterpart. VAM infection in pigeon pea roots helped in translocating phosphorus from the soil and improved nitrogen fixation. Similarly, addition of phosphorus was found to play a positive role in enhancing all these parameters. Dual inoculation of Hup⁺ Rhizobium strain and VAM significantly increased nodulation, nitrogenase activity, plant nitrogen and phosphorus content and plant biomass compared to single inoculation of either organism and dual inoculation with Hup⁻ and VAM fungus.     

Interaction Effects of Ambient UV Radiation and Nutrient Limitation on the Toxic Cyanobacterium <Emphasis Type="Italic">Nodularia spumigena</Emphasis>

Nodularia spumigena is one of the dominating species during the extensive cyanobacterial blooms in the Baltic Sea. The blooms coincide with strong light, stable stratification, low ratios of dissolved inorganic nitrogen, and dissolved inorganic phosphorus. The ability of nitrogen fixation, a high tolerance to phosphorus starvation, and different photo-protective strategies (production of mycosporine-like amino acids, MAAs) may give N. spumigena a competitive advantage over other phytoplankton during the blooms. To elucidate the interactive effects of ambient UV radiation and nutrient limitation on the performance of N. spumigena, an outdoor experiment was designed. Two radiation treatments photosynthetic active radiation (PAR) and PAR +UV-A + UV-B (PAB) and three nutrient treatments were established: nutrient replete (NP), nitrogen limited (−N), and phosphorus limited (−P). Variables measured were specific growth rate, heterocyst frequency, cell volume, cell concentrations of MAAs, photosynthetic pigments, particulate carbon (POC), particulate nitrogen (PON), and particulate phosphorus (POP). Ratios of particulate organic matter were calculated: POC/PON, POC/POP, and PON/POP. There was no interactive effect between radiation and nutrient limitation on the specific growth rate of N. spumigena, but there was an overall effect of phosphorus limitation on the variables measured. Interaction effects were observed for some variables; cell size (larger cells in −P PAB compared to other treatments) and the carotenoid canthaxanthin (highest concentration in −N PAR). In addition, significantly less POC and PON (mol cell⁻¹) were found in −P PAR compared to −P PAB, and the opposite radiation effect was observed in −N. Our study shows that despite interactive effects on some of the variables studied, N. spumigena tolerate high ambient UVR also under nutrient limiting conditions and maintain positive growth rate even under severe phosphorus limitation.     

Seasonal patterns of growth and nitrogen fixation in field-grown pea

The seasonal patterns of growth and symbiotic N₂ fixation under field conditions were studied by growth analysis and use of¹⁵N-labelled fertilizer in a determinate pea cultivar (Pisum sativum L.) grown for harvest at the dry seed stage. The patterns of fertilizer N-uptake were almost identical in pea and barley (the non-fixing reference crop), but more fertilizer-N was recovered in barley than in pea. The estimated rate of N₂ fixation in pea gradually increased during the pre-flowering and flowering growth stages and reached a maximum of 10 kg N fixed per ha per day nine to ten weeks after seedling emergence. This was the time of early pod-development (flat pod growth stage) and also the time for maximum crop growth rate and maximum green leaf area index. A steep drop in N₂ fixation rate occurred during the following week. This drop was simultaneous with lodging of the crop, pod-filling (round pod growth stage) and the initiation of mobilization of nitrogen from vegetative organs. The application of fertilizer-N inhibited the rate of N₂ fixation only during that period of growth, when the main part of fertilizer-N was taken up and shortly after. Total accumulation of fixed nitrogen was estimated to be 244, 238 and 213 kg N ha⁻¹ in pea supplied with nil, 25 or 50 kg NO ₃ ⁻ −N ha⁻¹, respectively. About one-fourth of total N₂ fixation was carried out during preflowering, one fourth during the two weeks of flowering and the remainder during post-flowering. About 55% of the amount of N present in pods at maturity was estimated to be derived from mobilization of N from vegetative organs. “Starter” N (25 or 50 kg NO ₃ ⁻ −N ha⁻¹) did not significantly influence either dry matter and nitrogen accumulation or the development of leaf area. Neither root length and root biomass determined 8 weeks after seedling emergence nor the yield of seed dry matter and nitrogen at maturity were influenced by fertilizer application.     

Co-inoculation with Glomus intraradices and Rhizobium tropici CIAT899 increases P use efficiency for N2 fixation in the common bean (Phaseolus vulgaris L.) under P deficiency in hydroaeroponic culture

Common bean (Phaseolus vulgaris L.) genotypes CocoT and Flamingo were inoculated with Rhizobium tropici CIAT899 and Glomus intraradices (Schenck & Smith) and grown under sufficient versus deficient phosphorus supply for comparing the effects of double inoculation on growth, nodulation, mycorrhization of the roots, phosphorus use efficiency and total nitrogen. Although the double inoculation induced a significant increase in all parameters whatever the phosphorus supply in comparison to control, significant differences were found among genotypes and treatments. Nevertheless, the highest phosphorus use efficiency and plant total nitrogen were found under P deficiency in combination with arbuscular mycorrhizal fungi. It is concluded that inoculation with rhizobia and arbuscular mycorrhizal fungi could improve symbiotic nitrogen fixation even under phosphorus deficiency.     

Effects of temperature, and availability of nitrogen and phosphorus on the abundance of Anabaena and Microcystis in Lake Biwa, Japan: an experimental approach

Under optimal nutrient conditions, both Microcystis sp. and Anabaena sp. isolated from Lake Biwa grew optimally at 28–32°C but differed in maximal growth rates, phosphate uptake kinetics, maximal phosphorus quotas, and growth responses to nitrogen and phosphorus limitation. The maximal growth rates of Microcystis and Anabaena were 1.6 and 1.25 divisions day⁻¹, respectively. With phosphate and nitrate in the growth-limiting range, the growth of Microcystis was optimal at an N : P ratio of 100 : 1 (by weight) and declined at lower (nitrogen limitation) and higher (phosphorus limitation) ratios. In contrast, Anabaena growth rates did not change at N : P ratios from 1000 : 1 to 10 : 1. Starting with cells containing the maximal phosphorus quota, Microcystis growth in minus-phosphorus medium ceased in 7–9 days, compared with 12–13 days for Anabaena. The phosphate turnover time in cultures starved to their minimum cell quotas was 7.9 min for Microcystis and 0.6 min for Anabaena. Microcystis had a higher K _s (0.12 μg P l⁻¹ 10⁻⁶ cells) and lower V _max (9.63 μg P l⁻¹ h⁻¹ 10⁻⁶ cells), than Anabaena (K _s 0.02 μg P l⁻¹ h⁻¹ 10⁻⁶ cells; V _max 46.25 63 μg P l⁻¹ h⁻¹ 10⁻⁶ cells), suggesting that Microcystis would not be able to grow well in phosphorus-limited waters. We conclude that in spite of the higher growth rate under ideal conditions, Microcystis does not usually bloom in the North Basin because of low availability of phosphorus and nitrogen. Although Anabaena has an efficient phosphorus-uptake system, its main strategy for growth in low-phosphorus environments may depend on storage of phosphorus during periods of abundant phosphorus supply, which are rare in the North Basin. Received: July 31, 2000 / Accepted: October 18, 2000     

The relative contribution of symbiotic N2 fixation and other nitrogen sources to grassland ecosystems along an altitudinal gradient in the Alps

The significance of symbiotic N₂ fixation in legumes (Trifolium alpinum L., T. nivale Sieber, T. pratense L., T. badium Schreber, T. thalii Vill., T. repens L., Lotus alpinus [DC.] Schleicher, L. corniculatus L., Vicia sativa L.) and other N sources for the N budget of grassland ecosystems was studied along an altitudinal gradient in the Swiss Alps. The total annual symbiotic N₂ fixation was compared with other sources of N for plant growth of the total plant community (mineralisation and wet deposition). The contribution of symbiotically fixed N to total above-ground N yield of the swards decreased from at least 16% to 9% with increasing altitude where legumes were present. This decrease was due to a decrease in the yield proportion of legumes from 15% at 900 and 1380 m a.s.l. to 5% at 2100 and 2300 m a.s.l. (no legumes were found above 2750 m a.s.l.) and not to a decline in the activity of symbiotic N₂ fixation. With increasing altitude legumes are more patchily distributed. The high symbiotic N₂ fixation of individual plants up to their altitudinal limit is not primarily the result of low mineral N availability since an addition of NH₄ ⁺ or NO₃ ⁻ fertiliser at 2300 m a.s.l. led either to no decrease or only to a minor decrease in symbiotic N₂ fixation. At 1380 m a.s.l., N mineralisation (13.45 g N m⁻² yr⁻¹) appeared to be the main source of N for growth of the sward; N from symbiosis (at least 1.0 g to 2.6 g N m⁻² yr⁻¹) and wet deposition (0.4 g to 0.6 g m⁻² yr⁻¹) was not a significant N source for plant growth at this altitude. At 2100 m a.s.l., the combined amounts of N from symbiotic N₂ fixation (at least 0.1 g N m⁻² yr⁻¹) and wet deposition (0.3 g N m⁻² yr⁻¹) appeared to be similarly important for plant growth as soil N mineralisation (0.47 g N m⁻² yr⁻¹). At high altitudes, wet N deposition and symbiotic N₂ fixation together represent a significant source of N for the grassland ecosystem while at low altitudes these N inputs appear to be much less important. This revised version was published online in June 2006 with corrections to the Cover Date.     

Quantitative trait locus analysis of symbiotic nitrogen fixation activity in the model legume <Emphasis Type="Italic">Lotus japonicus</Emphasis>

Many legumes form nitrogen-fixing root nodules. An elevation of nitrogen fixation in such legumes would have significant implications for plant growth and biomass production in agriculture. To identify the genetic basis for the regulation of nitrogen fixation, quantitative trait locus (QTL) analysis was conducted with recombinant inbred lines derived from the cross Miyakojima MG-20 × Gifu B-129 in the model legume Lotus japonicus. This population was inoculated with Mesorhizobium loti MAFF303099 and grown for 14 days in pods containing vermiculite. Phenotypic data were collected for acetylene reduction activity (ARA) per plant (ARA/P), ARA per nodule weight (ARA/NW), ARA per nodule number (ARA/NN), NN per plant, NW per plant, stem length (SL), SL without inoculation (SLbac−), shoot dry weight without inoculation (SWbac−), root length without inoculation (RLbac−), and root dry weight (RWbac−), and finally 34 QTLs were identified. ARA/P, ARA/NN, NW, and SL showed strong correlations and QTL co-localization, suggesting that several plant characteristics important for symbiotic nitrogen fixation are controlled by the same locus. QTLs for ARA/P, ARA/NN, NW, and SL, co-localized around marker TM0832 on chromosome 4, were also co-localized with previously reported QTLs for seed mass. This is the first report of QTL analysis for symbiotic nitrogen fixation activity traits.     

Soil Phosphorus Fractions and Symbiotic Nitrogen Fixation across a Substrate-Age Gradient in Hawaii

We evaluated soil phosphorus (P) fractions, other soil characteristics, and rates of symbiotic N₂ fixation across a substrate-age gradient in Hawaii that was dominated by the leguminous tree Acacia koa (koa). Patterns of soil P observed on this gradient were compared to those on a slightly wetter gradient dominated by the nonfixer Metrosideros polymorpha (ohia). Along both gradients, concentrations of primary-mineral P fell sharply between the young and intermediate-aged sites, while labile inorganic P declined most steeply between the intermediate-aged and old sites. The most marked difference between the two gradients was that total soil carbon (C), nitrogen (N), and P, as well as nonoccluded organic P, were more variable across the ohia gradient, increasing to the intermediate-aged sites, then declining sharply at the old site. On the koa gradient, specific nitrogenase activity, measured by the acetylene-reduction (AR) assay, decreased three- to eightfold between the young site and the intermediate-aged and old sites, respectively. Nodule biomass showed no clear pattern. N₂ fixation rates, estimated by combining AR activity and nodule biomass measurements, were up to 8 kg N · ha⁻¹ · y⁻¹ at the young site and no more than 2 kg N · ha⁻¹ · y⁻¹ at the older sites, suggesting that koa may be a modest source of N in these Hawaiian forests. Received 26 September 2000; accepted 15 February 2002     

The response ofAzolla pinnata R. Brown to the split application of phosphorus and the transfer of assimilated phosphorus to flooded rice plants

A field experiment was conducted at the Bangladesh Rice Research Institute, Joydebpur, Dhaka during the late wet season. Basal application of P at both 5 and 10 kg ha⁻¹ significantly increased total biomass production and nitrogen fixation byAzolla pinnata R. Brown (local strain). Addition of both 5 and 10 kg P ha⁻¹ in equal splits at inoculation and at six day intervals thereafter during growth periods of 12, 24 and 36 days increased biomass production and nitrogen fixation by Azolla over that attained with the basal application. Biomass and nitrogen fixation using a split application of 5 kg P ha⁻¹ exceeded that attained with basal application of 10 kg P ha⁻¹ and split application of 10 kg P ha⁻¹ resulted in 0.58, 11.2, and 18.3 t ha⁻¹ more biomass, and 0.47, 18.9, and 18.3 more kg fixed N ha⁻¹ at 12, 24 and 36 days, respectively, than the same amount applied as a basal application. Analyses indicated that the critical level of dry weight P in Azolla for sustained growth was in the range of 0.15–0.17%. Compared with the control, where no P was added, and additional 30 and 36 kg N ha⁻¹ were fixed after 24 and 36 days, respectively, when P was provided at 10 kg ha⁻¹ using a split application. A separate field study showed that flooded rice plants received P from incorporated Azolla with about 28% of the P present in the supplied Azolla being incorporated into the rice plants.     

Mycorrhizal colonization decreases respiration of common bean nodulated root in hydroaeroponic culture

The bean-rhizobia symbiosis allows atmospheric nitrogen fixation through nodule formation. Nevertheless, nodule establishment in Mediterranean areas is subjected to various biotic and abiotic constraints such as phosphorus soils deficiency. This study compares plant-growth response to moderate (75 μmol KH₂PO₄ plant^?1 week^?1) versus severe phosphorus deficiency (30 μmol KH₂PO₄ plant^?1 week^?1) after inoculation with Rhizobium tropici CIAT 899 and Glomus intraradices of four Phaseolus vulgaris lines contrasting in P use efficiency (PUE) for their symbiotic nitrogen fixation (SNF) in hydroaeroponic culture. After 5 weeks of growth under glasshouse conditions, the oxygen consumption related to nitrogen fixation was measured on intact nodulated roots. The obtained results revealed that mycorrhizal colonization decreased the nodulated-roots O₂ consumption of P. vulgaris under both P deficiencies although it increased the growth of all plant organs and the nodulation with a large genotypic variability. Moreover, mycorrhizal colonization was higher under severe P deficiency than under moderate one. In conclusion, the tripartite inoculation improved growth parameters under severe P-deficiency with a decrease in nodulated root O₂ consumption.     

Dissolved Organic Nitrogen Release and Bacterial Activity in the Upper Layers of the Atlantic Ocean

The variability of the percentage of extracellular dissolved organic nitrogen (DON) release (PER), along with the relationship between DON release and bacterioplankton activity, was examined during five oceanographic cruises, carried out in the upwelling region of the NW Iberian Peninsula, the SW Bay of Biscay, and a latitudinal transect in the Atlantic Ocean (50°N–35°S). Rates of nitrogen uptake, DON release, and bacterial production were measured at 66 stations and sampled between August 1998 and October 2000. The percentage of DON release relative to the gross uptake of ammonium (PER^NH4+) ranged from 3 to 46%, whereas that relative to total nitrogen (NH₄ ⁺ + NO₃ ⁻ + urea) gross uptake (PER^total) varied between 21 and 82%. The highest values for both PER^NH4+ and PER^total were found in oligotrophic oceanic waters (<0.25 mg chlorophyll a m⁻³). In oceanic environments, a positive relationship was found between nitrogen uptake and DON release rates, with a log–log linear regression slope significantly lower than 1, suggesting an inverse relationship between PER and gross nitrogen uptake rates. In contrast, in areas with higher productivity levels (>6 mg chlorophyll a m⁻³), such as the continental shelf off the NW Iberian Peninsula, PER held constant as nitrogen uptake increased. These results suggest the dominance of different processes controlling DON release in oceanic and neritic zones. DON release rates accounted for less than 15% of the variability observed in bacterial production rates, suggesting a weak response of bacterioplankton to phytoplankton on short time scales (hours). Furthermore, nitrogen budgets showed an excess of DON release in relation to bacterial requirements.     

Direct and interaction-mediated effects of environmental changes on peatland bryophytes

Ecosystem processes of northern peatlands are largely governed by the vitality and species composition in the bryophyte layer, and may be affected by global warming and eutrophication. In a factorial experiment in northeast China, we tested the effects of raised levels of nitrogen (0, 1 and 2 g m⁻² year⁻¹), phosphorus (0, 0.1 and 0.2 g m⁻² year⁻¹) and temperature (ambient and +3°C) on Polytrichum strictum, Sphagnum magellanicum and S. palustre, to see if the effects could be altered by inter-specific interactions. In all species, growth declined with nitrogen addition and increased with phosphorus addition, but only P. strictum responded to raised temperature with increased production of side-shoots (branching). In Sphagnum, growth and branching changed in the same direction, but in Polytrichum, the two responses were uncoupled: with nitrogen addition there was a decrease in growth (smaller than in Sphagnum) but an increase in branching; with phosphorus addition growth increased but branching was unaffected. There were no two-way interactions among the P, N and T treatments. With increasing temperature, our results indicate that S. palustre should decrease relative to P. strictum (Polytrichum increased its branching and had a negative neighbor effect on S. palustre). With a slight increase in phosphorus availability, the increase in length growth and production of side-shoots in P. strictum and S. magellanicum may give them a competitive superiority over S. palustre. The negative response in Sphagnum to nitrogen could favor the expansion of vascular plants, but P. strictum may endure thanks to its increased branching.     

Symbiotic N2 fixation in pea and field bean estimated by15N fertilizer dilution in field experiments with barley as a reference crop

Summary The total amount of nitrogen derived from symbiotic nitrogen fixation in two pea and one field bean cultivar, supplied with 50 kg N ha⁻¹ at sowing (‘starter’-N), was estimated to 165, 136, and 186 kg N ha⁻¹, respectively (three-year means). However, estimates varied considerably between the three years. At the full bloom/flat pod growth stage from 30 to 59 per cent of total N₂ fixation had taken place. The proportion of total N derived from N₂ fixation at maturity was higher in seeds than in vegetative plant parts and amounted to 59.5, 51.3 and 66.3 per cent of total above-ground plant N in the two pea cultivars and field bean, respectively (three-year means). The recovery of fertilizer N was 62.2, 70.2, 52.1, and 69.5 per cent in the two pea cultivars, field bean and barley, respectively. Growth analysis indicated that barley did not meet the claims for an ideal reference crop in the¹⁵N fertilizer dilution technique for estimating N₂ fixation in pea and field bean. ‘Starter’-N neither increased the seed yield nor the N content of the grain legumes.     

The nodule conductance to O<Subscript>2</Subscript> diffusion increases with high phosphorus content in the <Emphasis Type="Italic">Phaseolus vulgaris-</Emphasis>rhizobia symbiosis

Although recent studies have addressed the effects of phosphorus (P) deficiency on nodule O₂ permeability, little attention has been given to the relationship between nodule P status and nodule permeability. To study these traits, four recombinant inbred lines, namely RILs 34, 83, 115, 147 and one local variety (Concesa) of common bean (Phaseolus vulgaris) were inoculated with RhM11 (a native rhizobial strain from Haouz area of Marrakesh), and grown in hydroaeroponic culture under P-sufficiency (250 μmol P plant⁻¹ week⁻¹) versus P-deficiency (75 μmol P plant⁻¹ week⁻¹) conditions. At the flowering stage, the biomass of plants and nodules and their P contents was determined after measuring O₂ uptake by nodulated roots (Conr) and nodule conductance to O₂ diffusion (g_n). The results showed that P-deficiency significantly decreased plant growth and nodulation, though there were differences between bean genotypes. P-deficiency also induced a decrease in nodule P content (31%) in both sensitive (83, 147) and tolerant lines (34, 115), a 42 and 27% reduction in shoots of sensitive and tolerant lines, respectively. These decreases were associated with significant variations in nodule surface and O₂ permeability among bean genotypes and P-nutrition. Under P-deficiency, g_n increased more for the sensitive (39%) than for the tolerant lines (27%). This increase was linked with a rise both in the P levels in nodules and shoots, as well as in the efficiency of symbiotic nitrogen fixation as determined by nodule-dependent biomass production for the sensitive lines. Furthermore, positive correlations were found between O₂ permeability, g_n and P content both in nodules and shoots (r ² = 0.94** and r ² = 0.96**). We conclude that nodule variations in Conr and g_n are related to nodule P content, and may contribute to the adaptation of energy metabolism in N₂-fixing bean nodules to P-deficiency.     

Effects of light intensity and nitrogen starvation on growth, total fatty acids and arachidonic acid in the green microalga Parietochloris incisa

The effects of light and nitrogen deficiency on biomass, fatty acid content and composition were studied in Parietochloris incisa, the unicellular freshwater chlorophyte accumulating very high amounts of arachidonic-acid-rich triacylglycerols. P. incisa cultures grown on complete nutrient medium and under high light (400 μmol photons m^{− 2} s⁻¹) showed the highest rate of growth in comparison to medium (200 μmol photons m⁻² s⁻¹) and low (35 μmol photons m⁻² s⁻¹) light intensity. Cultures grown under high light (on complete BG-11 medium) attained higher volumetric contents of total fatty acids and arachidonic acid due to greater increase in biomass. Nitrogen starvation brought about a strong increase in the arachidonic acid proportion of total fatty acids. Thus, adjustments to cultivation conditions could serve as an efficient tool for manipulation of yield and relative content of arachidonic acid in P. incisa. The significance of the changes in lipid metabolism for adaptation of P. incisa to high-light stress and nitrogen deficiency is also discussed.     

Phosphorus fertilization and tillage effect on dinitrogen fixation in soybeans

Summary Tillage has been shown to affect the uptake of phosphorus (P) and yield of soybeans, [Glycine max (L.) Merr.], but there is little information concerning the effects of P fertilization on nitrogen (N₂) fixation in soybeans under no-tillage. Two field experiments were conducted in 1980 and 1981 to determine the effects of soil P on N₂ fixation under no-tillage and to study the interaction of P fertilization and tillage of N₂ fixation, nutrient uptake, and yield of soybeans. In Exp. I, P was applied in 1977 at five rates up to 384 kg P ha⁻¹ and the effects of residual soil P were evaluated in 1980 and 1981 under no-tillage management. Nitrogen fixation rates, as measured by acetylene reduction assay, were significantly affected by soil P in Exp. I, but the assay proved to be a poor technique for estimating total plant N in these tests. Acetylene reduction rates and plant P increased rapidly as soil P increased from 2 to 20 mg kg⁻¹, with little additional increase above 20 mg P kg⁻¹. In Exp. II, rates (0, 32, 64, and 128 kg P ha⁻¹) and time (fall, spring and fall plus spring) of P application were compared under conventional tillage and no tillage. However, plant P increased with increasing levels of applied P. Applied P had no affect on acetylene reduction rates but rates were greater for no-tillage than conventional tillage at the V9 and R5 stages of growth in 1981. Plant uptake of P was more efficient under no-tillage than under conventional tillage in 1980 and 1981. Application of 64 kg P ha⁻¹ under no-tillage resulted in equivalent plant P levels as the 128 kg P ha⁻¹ applied under conventional tillage.     

Nitrogen and phosphorus removal through laboratory batch cultures of microalga Chlorella vulgaris and cyanobacterium Planktothrix isothrix grown as monoalgal and as co-cultures

Batch cultures of the green microalga Chlorella vulgaris and cyanobacterium Planktothrix isothrix and their corresponding co-cultures were grown in municipal wastewater in order to study their growth as well as the nitrogen (NH₄–N) and phosphorus (PO₄³⁻–P) removal. The cultures were grown under two irradiances of 20 and 60 μmol photons m⁻² s⁻¹ in shaken and unshaken conditions. The co-culture of unshaken Chlorella and Planktothrix showed the greatest growth under both irradiances. The monoalgal Planktotrix cultures showed better growth when unshaken than when shaken, whereas Chlorella cultures grew better when mixed, but only at the higher irradiance. The highest percentage of nitrogen removal (up to 80%) was attained by the unshaken co-cultures of Chlorella and Planktothrix. The amount of nitrogen recycled in the biomass reached up to 85% of that removed. Shaken monoalgal cultures of Chlorella showed phosphorus removal under both irradiances. They completely removed the initial phosphorus concentration (7.47 ± 0.17 mg L⁻¹) within 96 and 48 h under 20 and 60 μmol photons m⁻² s⁻¹, respectively.     

Influence of host cultivars and Bradyrhizobium strains on the growth and symbiotic performance of soybean under salt stress

This paper examines the importance of salt tolerance of host cultivars, Bradyrhizobium strains, and host-Bradyrhizobium combinations on the symbiotic nitrogen fixation potential of soybean under NaCl and KCl salt stress. Plants were grown in a soil medium, and the experiments were conducted under controlled environment growth room conditions. Bradyrhizobium growth was examined in yeast-mannitol broth andB. japonicum strains tolerant of NaCl and KCl (80 mM) stress were identified. Soybean cultivar Williams, which was sensitive to salt stress, performed poorly both in growth and symbiotic nitrogen fixation, irrespective of whether it was matched with a tolerant or sensitive Bradyrhizobium strain. Tolerant cultivar Manchu sustained nodulation and nitrogen fixation, irrespective of whether it was matched with a tolerant or sensitive Bradyrhizobium strain. Evidence presented here suggests a need, first to select soybean cultivars that are tolerant to salt stress, and then to match them with tolerant and effective Bradyrhizobium strains.     

Symbiotic effectiveness and response to mannitol-mediated osmotic stress of various chickpea–rhizobia associations

Thirty-six symbiotic associations involving six chickpea cultivars against six rhizobial strains were evaluated for symbiotic performance and responses to osmotic stress applied by mannitol (50 mM) in aerated hydroponic cultures. Analyses in different symbioses were focused on biomass production, nodulation, nitrogen fixation, and their modulation under osmotic stress conditions, as well as expression of nodular antioxidant enzymes. Mesorhizobium ciceri reference (835) and local (CMG6) strains, as well as the local (C₁₁) M. mediterraneum allowed the best symbiotic efficiency for all chickpea cultivars. The osmotic stress induces severe decrease ranging 30–50% in aerial biomass and 50–70% for nitrogen fixation. Nevertheless, plants inoculated with M. ciceri (835) and M. mediterraneum (C₁₁) preserve a relatively high growth (4 g plant⁻¹) with nitrogen-fixing activity (25 μmols h⁻¹ plant⁻¹). The bacterial partner was the most important factor of variance of the analysed parameters in osmotic stress or physiological conditions where it gets to 60–85%. The strains allowing the best competent symbioses were proposed for field assays. Under osmotic stress, nodular peroxidase (POX) and ascorbate peroxidase (APX) activities were significantly enhanced. The increase of POX and APX was inversely correlated with the inhibition of aerial biomass production (p = 0.05) and nitrogen-fixing capacity (p = 0.01), suggesting a protective role of these enzymes in nodules. Superoxide dismutase (SOD) was also activated in stressed nodules. However, the spectacular decrease in catalase (CAT) activity discounts its involvement in osmotic stress response.