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Soil nitrogen (N) acts as an inhibitor to the biological process of N 2 fixation in legumes. Conversely, the phosphorus (P) supply fosters N 2 fixation by promoting nodule formation, facilitating energy transfer, activating enzymes, and enhancing nitrogenase activity. This could help counteract the inhibitory effects that high soil N levels can have on N 2 fixation in legumes. However, there is limited knowledge regarding whether P supply can mitigate the inhibition of N 2 fixation. The objective of this work was to evaluate the effects of two levels of P and five N levels on N 2 fixation, shoot, root, and nodule biomass produced, and metabolic changes of leaves and nodules of Pisum sativum and Vicia faba. N 2 fixation was determined by measuring the natural abundance of 15N (%Ndfa) in shoots and roots, and metabolites were evaluated using GC-MS. The results showed that increasing N levels led to a reduction of N 2 fixation at the two P levels in both species. However, P supply reduced the inhibitory effect of N addition on N 2 fixation due to higher plant biomass production, which resulted in an increased mass of nodules. Under P deficiency, N addition affected N 2 fixation due to energy production in the nodules, which led to the accumulation of sugars and organic acids, especially in P. sativum. The response of N 2 fixation to a high N supply indicates that P. sativum and V. faba downregulated N 2 fixation but maintained viable nodules to resume the process if the N supply in the soil decreases. • Phosphorus increases biomass of Pisum sativum and Vicia faba fostering N 2 fixation. • P supply facilitates legumes in overcoming N 2 fixation inhibition induced by soil N by nodule resource allocation. • At low P supply, N 2 fixation inhibition is the result of sugar and organic acids accumulation in Pisum sativum. [ABSTRACT FROM AUTHOR]

Background and aims: Legume N2 fixation is highly sensitive to drought. Elevated [CO2] (e[CO2]) decreases stomatal conductance (gs) and improves water use efficiency (WUE), which may result in soil water conservation and allow N2 fixation to continue longer under drought. Using a Free-Air CO2 Enrichment (FACE) approach, this study aimed to elucidate whether e[CO2] improves N2 fixation of Pisum sativum L. under drought.Methods: In a FACE system, plants were grown in ambient [CO2] (~400 ppm) or e[CO2] (~550 ppm) and subjected to either terminal drought or well-watered treatments. Measurements were taken of photosynthesis, soil water dynamics, water soluble carbohydrates (WSC), amino acids (AA) and N2 fixation.Results: Lower gs under e[CO2] increased water use efficiency at leaf and plant level, and this translated to slower soil water depletion during drought. Elevated [CO2] increased WSC and decreased total AA concentrations in nodules, and increased nodule activity under drought. N2 fixation was stimulated (+51%) by e[CO2] in proportion to biomass changes. Under e[CO2] a greater proportion of plant total N was derived from fixed N2 and a smaller proportion from soil N uptake compared to a[CO2].Conclusion: This study suggests that e[CO2] increased WUE and this resulted in slower soil water depletion, allowing pea plants to maintain greater nodule activity under drought and resulting in appreciable increases in N2 fixation. Our results suggest that growth under e[CO2] can mitigate drought effects on N2 fixation and reduce dependency on soil N resources especially in water-limited agro-ecosystems. [ABSTRACT FROM AUTHOR]

Soybean (Glycine max (L.) Merr.) is considered to be susceptible to flooding, a major agronomic problem in the world, and nitrogenase activity rapidly declines due to oxygen deficiency in root nodules. We investigated nodule acclimation to flooding at the morphological level using a soybean cultivar possessing the ability to form secondary aerenchyma. After 1 week of treatment, lenticels were formed on the surface of the root nodules, and secondary aerenchyma were observed through the lenticels under both irrigated and flooded conditions. As the plant grew, the nodule epidermis came off, and well-developed secondary aerenchyma covered the nodule surface. The secondary aerenchyma originated from the secondary meristem (phellogen) girdling the sclerenchyma, and the degree of development was greater in flooded nodules than in irrigated ones. Although root nodulation and total nitrogenase activity (TNA) decreased under flooded conditions, there were no differences in shoot N concentration, specific nitrogenase activity (SNA) and relative ureide-N in the xylem bleeding sap between plants in the irrigated and flooded conditions. Under flooded conditions, however, when the entry of oxygen into the secondary aerenchyma formed in the hypocotyl was inhibited by vaseline treatment (pasting on the surface of the hypocotyl), the shoot N concentration, TNA, SNA, the ureide-N concentration and the relative ureide-N in the sap declined remarkably. These results suggested that secondary aerenchyma formation in soybean plants is a morphological acclimation response to flooding stress, and that one of the functions is to supply atmospheric oxygen to root nodules, which consequently enables nodule activity to be maintained.

Abstract: Sewage sludge has been used as N fertilizer because it contains some of inorganic N, principally as nitrate and ammonium ions. However, sewage sludge addition to legumes could result in impaired nodule metabolism due to the presence of inorganic N from sludge. A greenhouse experiment was conducted to examine the effects of sewage sludge on growth, photosynthesis, nitrogen assimilation and nodule metabolism in alfalfa (Medicago sativa L. cv. Aragón). Plants were grown in pots with a mixture of perlite and vermiculite (2:1, v/v). The experiment included three treatments: (1) plants inoculated with rhizobia and amended with sewage sludge at rate of 10% (w/w) (RS); (2) plants inoculated with rhizobia without any amendment (R); and (3) non-inoculated plants fed with ammonium nitrate (N). N2-fixing plants had lower growth and sucrose phosphate synthase activity but higher photosynthesis than nitrate-fed plants because they compensated the carbon cost of the rhizobia. However, sewage sludge-treated plants evidenced a loss of carbon sink strength due to N2 fixation by means of decreased photosynthetic capacity, leaf chlorophylls and N concentration in comparison to untreated plants. Sewage sludge did no affect nodulation but decreased nodule enzyme activities involved in carbon and N metabolisms that may lead to accumulation of toxic N-compounds. [ABSTRACT FROM AUTHOR]

A study evaluated the response of two varieties of beans (Phaseolus vulgaris L.) to different soil moisture levels during a dry season. The soil moisture regimes maintained throughout the growth period were field capacity, 70-75 %, 50-55 % or 20-25 % available soil moisture. Plant growth, yield and nodulation were optimal when plants grew at high soil moisture levels. With increasing stress, all measured parameters of both varieties were reduced. However, polebeans, with its vine type of growth was affected to a greater degree than bushbeans. In contrast, nodulation and nodule activity of both variables was affected by moisture stress. A second experiment evaluated the effects of different soil moisture levels over the growth cycle of bushbeans, which produce greater yields under drier conditions. The highest yields were obtained at higher moisture levels throughout the growth cycle. Moisture stress upto flowering reduced yields to a greater extent than when the plants were subjected to reduced soil moisture after flowering appearance. Some casual mechanisms of the results of the experiments and possible implications for incorporating this popular vegetable legume in rainfed agricultural systems are presented. [ABSTRACT FROM AUTHOR]

Cowpea [Vigna unguiculata (L.) Walp.] plants were grown in a split-root system using a calcined montmorillonite clay (Turface) as the growth medium. At transplanting, either the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus macrocarpum Tul. & Tul. and 100 mg hydroxyapatite (HAP) or 100, 200 or 400 mg HAP kg-1 without the VAM fungus were mixed into side 1 of the split-root system. Side 2 received only 100 mg HAP kg-1. After 30 d, a combined inoculum of four Rhizobium isolates (TAL169, TAL173, TAL658 and IRC256) was applied to the surface of all pots. At harvest (60 d), root systems were separated from shoots, and nodule activity (acetylene reduction), plant dry weight, nitrogen (N) and phosphate (P) content and the percentage of VAM colonization were determined. There was no significant effect of treatment on either the N or P content of leaves or roots. The VAM fungus, however, significantly (P < 0.01) increased shoot dry weight. Root dry weight and nodule activity were significantly increased only on the VAM side of the split-root system. Rhizobia from 30 nodules per half-root system were identified using fluorescent antibody techniques. Strain TAL173 was detected from 95 % of the nodules and neither the VAM fungus nor P application affected competitive interaction by rhizobia for nodulation sites. Our work suggests the existence of a localized, non-systemic, non-P-mediated influence of a VAM fungus on root dry weight and nodule activity in cowpea. [ABSTRACT FROM AUTHOR]

Soybean [Glycine max (L.) Merr. cv. Hobbit] plants nodulated by Bradyhizobium japonicum strain USDA 110 were grown in pot cultures in severely P‐ and N‐deficient soil and either colonized by the vesicular‐arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe or fertilized with a high (HP) or low (LP) level of KH2PO4 (0.6 or 0.3 mM, respectively), After 7 weeks of growth, nodule and chloroplast activities (C2H2 reduction and CO2 exchange rate) were determined. Photosynthetic P‐use efficiency of CO2 fixation was significantly higher in VAM than in HP plants, while that of nitrogenase activity was lower. The LP plants were intermediate in both respects. The ratio of nodule to chloroplast activity [mol C2H2 reduced (mol CO2 fixed)−1] was highest in HP and lowest in VAM plants. Root colonization by the VAM fungus significantly increased nodule number and dry weight and reduced nodule specific mass and activity in comparison to HP plants. In spite of lower nodule activity, VAM plants were significantly larger and had higher N concentrations than the HP plants. The results suggest nonnutritional. VAM‐elicited and host‐mediated effects on the symbiotic functions of the legume association. [ABSTRACT FROM AUTHOR]

The objective of this research was to study the root and nodule responses to drought of mycorrhizal or nitrogen-fixing alfalfa plants in relation to root cyto-kinin levels. Double symbiotic plants showed significantly higher root and nodule activities during drought that may be related to smaller decreases in cytokinin content of mycorrhizal plants. [ABSTRACT FROM PUBLISHER]

• Nitrate and ammonium systemically induced a rhythmic pattern of symbiotic nitrogen fixation (SNF). • The nitrate ion had a local effect on the nodule activity while ammonium did not. • During the first hours after ammonium supply, the nodule transcriptome remained virtually unaffected. • After nitrate supply downregulation was observed for genes encoding leghaemoglobin, NCR peptides and nicotianamine synthase. Symbiotic nitrogen fixation (SNF) has a high energetic cost for legume plants; legumes thus reduce SNF when soil N is available. The present study aimed to increase our understanding regarding the impacts of the two principal forms of available N in soils (ammonium and nitrate) on SNF. We continuously measured the SNF of Medicago truncatula under controlled conditions. This permitted nodule sampling for comparative transcriptome profiling at points connected to the nodules' reaction following ammonium or nitrate applications. The N component of both ions systemically induced a rhythmic pattern of SNF, while the activity in control plants remained constant. This rhythmic activity reduced the per-day SNF. The nitrate ion had additional local effects; the more pronounced were a strong downregulation of leghaemoglobin, nodule cysteine-rich (NCR) peptides and nodule-enhanced nicotianamine synthase (neNAS). The neNAS has proven to be of importance for nodule functioning. Although other physiological impacts of nitrate on nodules were observed (e.g. nitrosylation of leghaemoglobin), the main effect was a rapid ion-specific and organ-specific change in gene expression levels. Contrastingly, during the first hours after ammonium applications, the transcriptome remained virtually unaffected. Therefore, nitrate-induced genes could be key for increasing the nitrate tolerance of SNF. [ABSTRACT FROM AUTHOR]