La Dendrologia di Ulisse Aldrovandi

Abstract

Legume-Rhizobium symbiotic nitrogen (N₂) fixation plays a critical role in sustainable nitrogen management in agriculture. The nitrogen fixed by the root nodules not only affects the nitrogen cycle of nature, but is also of great economic importance. A number of physiological and biochemical processes in the nodules are affected by salt stress. The objective of this study was to evaluate the role of arbuscular mycorrhiza (AM) in moderating toxic effects of salt stress on nodular metabolism in Cajanus cajan (L.) Millspaugh (pigeonpea) cv. Manak. Exposure of plants to salinity stress (4, 6 and 8 dSm^?1) caused ionic imbalance, which resulted in increased Na⁺ and reduced K⁺ and Ca²⁺ contents in the nodules. Salinity induced increased synthesis and accumulation of proline and glycine betaine. Salt stress significantly increased the antioxidant enzyme activities in the nodules of all plants. Nodular growth suffered remarkably and a marked decline in nodule biomass was observed under salt stress. Leghemoglobin content and acetylene reduction activity (ARA) also declined under saline conditions. AM could significantly improve nodule dry mass, leghemoglobin content and nitrogenase activity, and phosphorus content under salt stress. Activities of antioxidant enzymes increased markedly in nodules of mycorrhizal-stressed plants. This study suggested a correlation between improved functional efficiency of nodules and higher osmolyte accumulation and enhanced antioxidant enzyme activities of AM plants under stressed conditions relative to the nodules of uninoculated plants.     

Nodulation,biomass and nitrogen content in Sesbania species

Sesbania rostrata developed nitrogen fixing nodules on the stem after spraying the plants with the bacterial culture TCSR 1. The number of stem nodules at 55 days after sowing was about 1200. Plants with stem nodules had a significantly reduced number of root nodules. The biomass of S. rostrata was comparable to the locally well adapted non-stem nodulating species S. aculeata. The %N and total nitrogen content were highest in S. rostrata compared to the other three species studied.     

Sitona weevils (Coleoptera: Curculionidae) as agents for rapid transfer of nitrogen from white clover (Trifolium repens L.) to perennial ryegrass (Lolium perenne L.)

The effects of root feeding by larvae of Sitona hispidulus (F.) (a common weevil pest of white clover) on the rate of transfer of nitrogen between plants of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) were investigated using a nutrient slant board technique. Clover plants, labelled with ¹⁵N were grown adjacent to ryegrass plants and were either infested with Sitona larvae or not infested. Ryegrass plants associated with the infested clover plants had a significantly higher dry matter yield and nitrogen content (75% and 74% respectively) than the uninvested plants, after 33 days exposure to insect herbivory. It was concluded that root feeding insects could play an important role in the cycling of nitrogen in grass/clover swards.     

Common Bean (Phaseolus vulgaris L.) Mutants Defective in Root Nodule Formation: I. PHYSIOLOGICAL CHARACTERIZATION

Two ineffectively nodulating, allelic mutants of the commonbean (Phaseolus vulgaris L.) lines RIZ30 and RIZ36 were studied.In both mutants the nodulation phenotype was characterized bythe formation of tumour-like swellings when inoculated withbean Rhizobium strains. Late formation of pink nodules was observedin mutant plants grown in pots with soil or in hydroponic conditionswith perlite. In the absence of mineral nitrogen, mutant growthand nodulation were poor; a low peak of acetylene reductionactivity being detected 45 d after inoculation. Mutant growthin the presence of mineral nitrogen was similar to that of thewild type. The mutants were also characterized by poor pod fertility(seed per pod). The nodulation and pod fertility phenotypesare specifically controlled at the root and shoot levels, respectively,as assessed with grafting experiments. Mutant shoots graftedon wild type, nitrogen-fixing roots produced a number of seedssimilar to the wild type despite the expression of the poorpod fertility phenotype. Nitrogen and seed yields of wild typeshoots, grafted on mutant, ineffectively nodulating roots wereseverely reduced when compared with control wild type plantsalthough nitrogen accumulation during vegetative phase was similar. Key words: Phaseolus valgaris, nitrogen fixation, nodulation mutants, grafting, nitrogen assimilation     

Spatial relationships between uninfected and infected cells in root nodules of soybean

In soybean (Glycine max (L.) Merr.) the uninfected cells of the root nodule are responsible for the final steps in ureide production from recently fixed nitrogen. Stereological methods and an original quantitative method were used to investigate the organization of these cells and their spatial relationships to infected cells in the central region of nodules of soybean inoculated with Rhizobium japonicum strain USDA 3I1B110 and grown with and without nitrogen (as nitrate) in the nutrient medium. The volume occupied by the uninfected tissue was 21% of the total volume of the central infected region for nodules of plants grown without nitrate, and 31% for nodules of plants grown with nitrate. Despite their low relative volume, the uninfected cells outnumbered the much larger infected cells in nodules of plants grown both without and with nitrate. The surface density of the interface between the ininfected and infected tissue in the infected region was similar for nodules in both cases also, the total range being from 24 to 26 mm²/mm³. In nodules of plants grown without nitrate, all sampled infected cells were found to be in contact with at least one uninfected cell. The study demonstrates that although the uninfected tissue in soybean nodules occupies a relatively small volume, it is organized so as to produce a large surface area for interaction with the infected tissue.     

The Effects of Salinity on Nitrogen Fixation and Trehalose Metabolism in Mycorrhizal Cajanus cajan (L.) Millsp. Plants

Arbuscular mycorrhizal (AM) fungi exist widely in natural ecosystems as well as in salt-affected soils and are considered suitable candidates for bio-amelioration of saline soils. Plants respond to salinity by accumulating sugars and other low-molecular-weight compatible solutes. One such compound is trehalose, which has been found to play an important role as a stress protectant. The aim of the present investigation was to study interactions between an AM fungus and salinity stress on growth, nitrogen fixation, and trehalose metabolism in Cajanus cajan (L.) Millsp. (pigeonpea). Two genotypes [Sel 85N (salt-tolerant) and ICP 13997 (salt-sensitive)] were subjected to saline treatments with and without mycorrhizal inoculations. Salinity reduced plant biomass (shoot and root) in both genotypes and resulted in a decline in shoot-to-root ratio (SRR); however, a smaller decline was observed in Sel 85N than in ICP 13997. AM colonization was reduced with increasing salinity levels but mycorrhizal responsiveness (MR) increased. Genotypic variability in nitrogen fixation and trehalose metabolism in response to salinity and mycorrhization was observed. An increment in nodule number was accompanied by a reduction in dry mass. Subsequently, nodular activity (leghemoglobin, acetylene-reduction activity [ARA], nitrogen content) was reduced under soil salinity, which was more profound in ICP 13997 than in Sel 85N. The symbiotic association with Glomus mosseae led to significant improvement in plant dry mass and nitrogen-fixing potential of nodules under salt stress. Salinity led to an increase in trehalose-6-P synthetase (TPS) and trehalose-6-P phosphatase (TPP) activities resulting in increased trehalose content in nodules, which was accompanied by inhibition of trehalose catabolism (trehalase activity). AM plants had lower trehalase activity under saline and nonsaline conditions. Thus, a symbiotic relationship between plant roots and G. mosseae might have resulted in salinity tolerance in a genotype-dependent manner.     

Genetic specificity of nitrogen nutrition in leguminous plants

Summary Mineral nitrogen did not increase grain yield and seed protein levels ofVicia faba L. andLupinus luteus L. in field trials and pot experiments. Fixed N₂ was substituted by mineral nitrogen in these cases because of inhibition of N₂ fixation by mineral nitrogen. Contrary to these results mineral nitrogen increased grain yields and seed protein amounts ofLupinus albus L.,Pisum sativum L., andGlycine max. (L.) Merr. The nitrogen effect was caused at an early stage by saving energy due to inhibition of N₂ fixation (measurement of gas exchange by means of IRGA). In case of the N application after flowering grain, yields and seed protein levels increased because the mineral N was an additional nitrogen source for plants. At this stage the plants had ceased fixing atmospheric nitrogen. The high sink activity of growing fruits induced a lack of assimilates in nodules (determined by means of¹⁴CO₂ application). The N effect was therefore the consequence of the lower assimilate pool for supplying root nodules in these plants in comparison withVicia faba L. andLupinus luteus L. Hence it follows that response to mineral nitrogen can be a criterion for discovering more effective Rhizobium-host combinations.     

Distribution ofFrankia in soils from forest and afforestation sites in northern Sweden

Summary The presence in soil ofFrankia, capable of forming nitrogen-fixing root nodules onAlnus incana (L.) Moench, was investigated. Intact soil cores from forested as well as disturbed sites were sampled and both alder-rich and alder-free sites were included in the study. Surface-sterilized alder seeds were sown in the soil cores which were kept in sterile culture tubes in a growth chamber. Root nodules with nitrogenase activity developed in soil cores from all sites studied. Thus, infective and effectiveFrankia was present in all of the soils sampled, even from sites free from actinorhizal plants and irrespective of pH and nitrogen content of the soils.     

Characteristic responses of three tropical legumes to the inoculation of two species of VAM fungi in Andosol soils with different fertilities

The growth and mineral nutrition responses were evaluated of three tropical legumes, cowpea (Vigna unguiculata L. cv Kuromame), pigeonpea [Cajanus cajan L. (Millsp.) cv ICPL 86009] and groundnut (Arachis hypogaea cv Nakateyutaka) inoculated with two different species of VAM fungi, Glomus sp. (Glomus etunicatum-like species) and Gigaspora margarita, and grown in Andosols with different fertilities [Bray II-P: topsoil (72 ppm), subsoil (<0.1 ppm)]. Percent fungal root colonization was high in cowpea and groundnut but relatively low in pigeonpea in both soil types. Despite the low rate of root infection, significant growth responses were produced, especially in the inoculated pigeonpea plant. In all legumes, shoot dry matter production was favoured by the inoculations. Increases in shoot biomass due to mycorrhizae were greater in the subsoil than in the topsoil. Mycorrhization raised shoot concentrations of P and Ca (in cowpea and groundnut) and P and K (in pigeonpea) in the topsoil. Whereas the P concentration in shoots in the subsoil was not positively affected by VAM fungi, particularly in cowpea and pigeonpea, the concentration of K in such plants was significantly increased by VAM treatment. The results also showed that mycorrhizal enhancement of shoot micronutrient concentrations was very rare in all plants, with negative effects observed in certain cases. Cu concentration, in particular, was not affected by VAM formation in any of the plants, and Mn and Fe in pigeonpea and groundnut, respectively, remained the same whether plants were mycorrhizal or not. In both soils the three legumes responded to Glomus sp. better than to Gigaspora margarita, and the effects of the VAM fungi on each of the crops relative to the controls were greater in the subsoil than in the topsoil. However, shoot growth of groundnut was not affected as much as cowpea and pigeonpea by the type of soil used. In spite of the relatively low infection of its root, pigeonpea was generally the most responsive of the three legume species in terms of mycorrhizal/nonmycorrhizal ratios.     

Shoot apex removal does not alter autoregulation of Modulation in soybean

The suppression of new nodule development in soybean (Glycine max (L.) Merr.) has been previously demonstrated to involve the shoot through reciprocal grafts between the wild-type cultivar Bragg and its supernodulating mutant nts382. Using the same grafting technique, but modified through the excision of the shoot apex region and emerging lateral shoots, we show here that autoregulation of nodule number still existed despite apex removal. This radical treatment lowered total nodule number per plant as well as root, shoot and nodule dry weight. Bragg shoots grafted onto nts382 roots gave wild-type nodulation (26 nodules, 15mg total nodule mass) as compared to nts382 shoots grafted onto Bragg roots (340 nodules, 277 mg total nodule mass). Specific nodule mass differed between supernodulating (about 0·5-1·0mg per nodule) and wild-type nodulating (2·3 mg per nodule) plants. In contrast to other growth characteristics, apex removal did not affect specific nodule size, except in plants with wild-type shoots and nts382 (supernodulation) roots. Apex removal only slightly affected the percentage of nodule weight per total root weight in nts382, but had a severe effect in wild type. Growth reductions varied between the normal and supernodulating plants. The fact that autoregulation of nodulation still functions in plants devoid of functional shoot apices suggests that the autoregulation signal may not be derived from the apex regions and that the leaf may be a likely source.     

Yields and accumulations of N and P in farmer-managed intercrops of maize–pigeonpea in semi-arid Africa

Maize (Zea mays L.) is a major staple food in Sub-Saharan Africa but low soil fertility, limited resources and droughts keep yields low. Cultivation of maize intercropped with pigeonpea (Cajanus cajan L. Millsp.) is common in some areas of eastern and southern Africa. The objectives of this study were (1) to investigate dry matter, nitrogen (N) and phosphorus (P) accumulation in different plant components of maize–pigeonpea intercropping systems and (2) to report the effects of the intercrops on soil fertility. Maize–pigeonpea intercrops were compared to sole maize grown using farmersȁ9 practices. Intercropping maize and pigeonpea increased (P < 0.05) total system yield compared to sole maize in terms of biomass, N and P accumulation. Pigeonpea planted in maize did not reduce (P < 0.05) the accumulation of dry matter, N nor P in the maize grain. The harvest indices of maize, calculated on basis dry matter, N or P did not differ either (P < 0.05). Total soil C and N contents and inorganic N content, nitrate and ammonium, were not affected by two seasons of maize–pigeonpea intercropping compared to sole maize (P > 0.11). Nitrate and ammonium levels in soil were still not affected by the treatments after the soils were incubated in anaerobic conditions for 8 days at 37°C (P > 0.11). However, pigeonpea added up to 60 kg of N ha⁻¹ to the system and accumulated up to 6 kg of P ha⁻¹ and only 25% of this N and P were exported in the grain. In conclusion, beside the added grain yield of pigeonpea in the intercropped systems, pigeonpea increased the recirculation of dry matter, N and P, which may have a long-term effect on soil fertility. Furthermore, the stems from pigeonpea contributed to household fuel wood consumption. The intercropped system thus had multiple benefits that gave significant increase in combined yield per unit area without additional labour requirements. The main requirement in order to up-scale the maize–pigeonpea intercropping approach is sufficient supply of high-quality pigeonpea seeds.     

Activity of antioxidant enzymes in response to cadmium in Crotalaria juncea

The aromatic amine, -phenethylamine, was identified in various field-grown leguminous plants by analyses with HPLC, GC, GC-MS and ¹H-NMR. High concentration of -phenethylamine was generally detected only in mature root nodules, but not in other plant organs such as root, stem, leaf, pod and grain. Occurrence was specific to the root nodules formed by Bradyrhizobium infection. Ten of eleven legume crops including soybean [Glycine max (L.) Merr.], pigeon pea [Cajanus cajan (L.) Millsp.], adzuki bean (Vigna angularis), mung bean [V. radiata (L.) Wilczek] and cowpea (V. unguiculata) contained this aromatic amine, but groundnut (Arachis hypogaea L.) also nodulated by Bradyrhizobium sp. did not. Root nodules collected from garden pea (Pisum sativum L.), broad bean (Vicia fava L.), kidney bean (Phaseolus vulgaris L.) and various other herbaceous legumes nodulated by Rhizobium sp., Mesorhizobium sp., Sinorhizobium sp. or Azorhizobium caulinodans, and root-nodulated, woody non-legumes, nodulated by Frankia spp., contained little -phenethylamine.The amount of -phenethylamine in Bradyrhizobium-infected nodules varied with the legume species and their cultivars, and most significantly, with nodule age. In field-grown soybean plants, nodule -phenethylamine attained maximum concentration at the flowering stage and far exceeded that of the major polyamines of soybean nodules, putrescine and spermidine.     

Ammonium effects on function and structure of nitrogen-fixing root nodules of Alnus incana (L.) Moench

Cloned plants of Alnus incana (L.) Moench were inoculated and grown without combined nitrogen for seven weeks. The effects of ammonium on the function and structure of the root nodules were studied by adding 20 mM NH₄Cl (20 mM KCl=control) for four days. Nitrogenase activity decreased to ca. 50% after one day and to less than 10% after two days in ammonium treated plants, but was unaffected in control plants. The results were similar at photon flux densities of 200 and 50 mol m^-2 s^-1. At the higher light level the effect was concentration dependent between 2 and 20 mM NH₄Cl. The recovery was slow, and more than 11 d were needed for plants treated with 20 mM ammonium to reach initial activity. The distribution of ¹⁴C to the root nodules after assimilation of ¹⁴CO₂ by the plants was not changed by the ammonium treatment. Microscopical studies of root nodules showed high frequencies of endophyte vesicles being visually damaged in nodules from ammonium-treated plants, but not in nodules from control plants. When nitrogenase activity was restored, visually damaged vesicles were again few, whereas young developing vesicles were numerous. The slow recovery, the ¹⁴C-translocation pattern, and the structural changes of the endophyte indicate a more complex mechanism of ammonium influence than simply a short-term reduction in supply of carbon compounds to the nodules.     

Phenotypic and genetic diversity in <Emphasis Type="Italic">Sinorhizobium meliloti</Emphasis> and <Emphasis Type="Italic">S. medicae</Emphasis> from drought and salt affected regions of Morocco

Background  

Sinorhizobium meliloti and S. medicae are symbiotic nitrogen fixing bacteria in root nodules of forage legume alfalfa (Medicago sativa L.). In Morocco, alfalfa is usually grown in marginal soils of arid and semi-arid regions frequently affected by drought, extremes of temperature and soil pH, soil salinity and heavy metals, which affect biological nitrogen fixing ability of rhizobia and productivity of the host. This study examines phenotypic diversity for tolerance to the above stresses and genotypic diversity at Repetitive Extragenic Pallindromic DNA regions of Sinorhizobium nodulating alfalfa, sampled from marginal soils of arid and semi-arid regions of Morocco.     

Shoot/root assimilate allocation and nodulation ofVigna unguiculata seedlings as influenced by shoot light environment

Spectral balance of light received by southern pea [Vigna unguiculata (L.) Walp.] seedling shoots affected photoassimilate allocation among leaves, stems and roots. A higher ratio of far-red (FR) relative to red (R) light resulted in longer stems, higher shoot/root biomass ratio, less massive roots and fewer nodules. The same response pattern to FR/R ratio was obtained in a controlled environment with artificial light sources, or in sunlight where the FR/R ratio was modified by reflection from different colored soil surfaces or by FR reflected from competing plants. The importance of early shoot/root photoassimilate allocation and nodulation may differ according to soil nitrogen availability and moisture content.     

Influence of the food plants on the degree of parasitism of larvae ofHeliothis armigera byCotesia kazak

Influence of the food plants ofHeliothis armigera (Hb.) on the degree of parasitism by exotic parasiteCotesia kazak Telenga (Hymenoptera: Braconidae) was studied in cages in the laboratory on 7 food plants such as cotton (Gossypium hirsutum L.), tomato (Lycopersicon esculentum Mill), okra [Abelmoschus esculentus (L.) Moench], Dolichos (dolichos lablab L.), pigeonpea [Cajanus cajan (L.) Millsp.], Cowpea (Vigna unquiculata (L.) and chickpea (Cicer arietium L.). To determine the preference of the parasite 2 test methods were employed. In single plant choice test cotton was most preferred. Next in order of preference were tomato and okra. Dolichos, pigeonpea, cowpea and chickpea were least preferred. In multiple choice test, however, cotton and okra were preferred followed by tomato. Parasites were seen visiting these plants very frequently and high parasitism was recorded on these plants. Chick pea, pigeon pea, cowpea and Dolichos were the least preferred food plants. There appears to be some difference in fecundity as affected by some food plants. Exposure on okra, cotton and tomato resulted in higher cocoon production as compared to pigeonpea, Dolichos, cowpea and chickpea. There was, however, no difference in sex-ratio and longevity of the progeny as affected by food plants. This exotic parasite should be released first in crops such as cotton, okra and tomato on whichH. armigera is a very serious pest in India and elsewhere. Contribution No. 140/86 of the Indian institute of Horticultural Research, Bangalore-560 089     

Spontaneous nodules induce feedback suppression of nodulation in alfalfa

A small subpopulation of alfalfa (Medicago saliva L.) plants grown without fixed nitrogen can develop root nodules in the absence of Rhizobium. Cytological studies showed that these nodules were organized structures with no inter- or intracellular bacteria but with the histological characteristics of a normal indeterminate nodule. Few if any viable bacteria were recovered from the nodules after surface sterilization, and when the nodular content was used to inoculate alfalfa roots no nodulation was observed. These spontaneous nodules were formed mainly on the primary roots in the region susceptible to Rhizobium infection between 4 and 6 d after seed imbibition. Spontaneous nodules appeared as early as 10 d after germination and emerged at a rate comparable to normal nodules. The formation of spontaneous nodules on the primary root suppressed nodulation in lateral roots after inoculation with R. meliloti RCR2011. Excision of spontaneous nodules at inoculation eliminated the suppressive response. Our results indicate that the presence of Rhizobium is not required for nodule organogenesis and the elicitation of feedback regulation of nodule formation in alfalfa.Abbreviation RT root tip This work was supported by an endowment to the Racheff Chair of Excellence of the University of Tennessee, and the Soybean Promotion Board, Haskinsville, Tenn., USA. We are indebted to Noel Gerahty for performing the acetylene-reduction assays, and Dr. E.T. Graham for allowing the use of microscope facilities.     

Assimilation of Ammonium Nitrogen by Heterotrophic and Symbiotrophic White Lupine Plants

The activities of glutamate dehydrogenase, asparagine synthetase, and total glutamine synthetase in the organs of the white lupine (Lupinus albus L.) plants were measured during plant growth and development. In addition, the dynamics of free amino acids and amides in plant organs was followed. It was shown that the change in the nutrition type was important for controlling enzyme activities in the organs examined and, consequently, for directing the pathway of ammonium nitrogen assimilation. As long as the plants remained heterotrophic, glutamine-dependent asparagine synthetase of cotyledons and glutamine synthetase of leaves apparently played a major role in the assimilation of ammonium nitrogen. In symbiotrophic plants, root nodules became an exclusive site of asparagine synthesis, and the role of leaf glutamine synthetase increased. Unlike glutamine synthetase and asparagine synthetase, glutamate dehydrogenase activity was present in all organs examined and was less dependent on the nutrition type. This was also indicated by a weak correlation of glutamate dehydrogenase activity with the dynamics of free amino acid and amide content in these organs. It is supposed that glutamine synthetase plays a leading role in both the primary assimilation of ammonium, produced during symbiotic fixation of molecular nitrogen in root nodules, and in its secondary assimilation in cotyledons and leaves. On the other hand, secondary nitrogen assimilation in the axial organs occurs via an alternative glutamate dehydrogenase pathway.     

Effect of plant nutrient supply on nodule effectiveness and rhizobium strain competition for nodulation ofLotus pedunculatus

Summary The effect of nutrient supply on nodule formation and competition between Rhizobium strains for nodulation ofLotus pedunculatus was studied. Limiting plant growth by decreasing the supply of nutrients in an otherwise nitrogen-free medium, increased the size but decreased the number and the nitrogenase activity of nodules formed by a fast-growing strain of Lotus Rhizobium (NZP2037). In contrast decreasing nutrient supply caused only a small decline in the size, number and nitrogenase activity of nodules formed by a slow-growing strain (CC814s). Providing small quantities of NH₄NO₃ (50 to 250 g N) to plants grown with a normal supply of other nutrients stimulated nodule development by both Rhizobium strains and increased the nitrogenase activity of the NZP2037 nodules. Differences in the level of effectiveness (nitrogen-fixing ability) of nodules formed by different Rhizobium strains on plants grown with a normal supply of nutrients were less apparent when the plants were grown with decreased nutrient supply or when the plants were supplied with low levels of inorganic N.Inter-strain competition for nodulation ofL. pedunculatus between the highly effective slow-growing strain CC814s and 7 other fast- and slow-growing strains, showed CC814s to form 42 to 100% of the nodules in all associations. The greater nodulating competitiveness of strain CC814s prevailed despite changes in the nutrient supply to the host plant. A tendency was observed for partially effective Lotus Rhizobium strains to become more competitive in nodule formation when plant growth was supplemented with low levels of inorganic nitrogen.