Encephalartos natalensis, Their Nutrient-Cycling Microbes and Enzymes: A Story of Successful Trade-Offs

Encephalartos spp. establish symbioses with nitrogen (N)-fixing bacteria that contribute to soil nutrition and improve plant growth. Despite the Encephalartos mutualistic symbioses with N-fixing bacteria, the identity of other bacteria and their contribution to soil fertility and ecosystem functioning is not well understood. Due to Encephalartos spp. being threatened in the wild, this limited information presents a challenge in developing comprehensive conservation and management strategies for these cycad species. Therefore, this study identified the nutrient-cycling bacteria in Encephalartos natalensis coralloid roots, rhizosphere, and non-rhizosphere soils. Additionally, the soil characteristics and soil enzyme activities of the rhizosphere and non-rhizosphere soils were assayed. The coralloid roots, rhizosphere, and non-rhizosphere soils of E. natalensis were collected from a population of >500 E. natalensis in a disturbed savanna woodland at Edendale in KwaZulu-Natal (South Africa) for nutrient analysis, bacterial identification, and enzyme activity assays. Nutrient-cycling bacteria such as Lysinibacillus xylanilyticus; Paraburkholderia sabiae, and Novosphingobium barchaimii were identified in the coralloid roots, rhizosphere, and non-rhizosphere soils of E. natalensis. Phosphorus (P) cycling (alkaline and acid phosphatase) and N cycling (β-(D)-Glucosaminidase and nitrate reductase) enzyme activities showed a positive correlation with soil extractable P and total N concentrations in the rhizosphere and non-rhizosphere soils of E. natalensis. The positive correlation between soil enzymes and soil nutrients demonstrates that the identified nutrient-cycling bacteria in E. natalensis coralloid roots, rhizosphere, and non-rhizosphere soils and associated enzymes assayed may contribute to soil nutrient bioavailability of E. natalensis plants growing in acidic and nutrient-poor savanna woodland ecosystems.

Differential patterns of nitrogen nutrition and growth cost of the indigenous Vachellia sieberiana and the introduced Chromolaena odorata in the savannah environment

Vachellia sieberiana fixes atmospheric nitrogen (N) and distributes it back into ecosystems. We hypothesize that biological nitrogen fixation in this plant species is limited by competition from the invasive shrub, Chromolaena odorata. Competition would therefore result in the legume plant switching its limited nitrogen (N) sources in phosphorus-poor soils in savannah ecosystems when resources have to be shared. This study investigated the different patterns of N use and growth costs by a native and an introduced leguminous shrubby species. We propose that the two species sharing the same environment might result in competition. The competitive effect would induce in the indigenous legume to better utilize atmospheric-derived N modifying plant growth kinetics and plant mineral concentrations. Seedlings of V. sieberiana were cultivated in natural soil inoculum with low levels of phosphorus (mg L-1 ± SE) of 3.67 ± 0.88. The experiments were divided into two treatments where (i) seedlings of V. sieberiana were subjected to competition by cultivating them together with seedlings of C. odorata, and (ii) seedlings of V. sieberiana were cultivated independently. Although V. sieberiana was subjected to competition, the N2-fixing bacteria that occupied the nodules was Mesorhizobium species, similar to plants not subjected to competition. Total plant biomass was similar between treatments although V. sieberiana plants subjected to competition accumulated more below-ground biomass and showed higher carbon construction costs than plants growing individually. Total plant phosphorus and nitrogen decreased in seedlings of V. sieberiana under competition, whereas no differences were observed in percent N derived from the atmosphere (%NDFA) between treatments. The specific nitrogen utilization rate (SNUR) was higher in V. sieberiana plants subjected to competition while specific nitrogen absorption rate (SNAR) showed the opposite response. Vachellia sieberiana is highly adapted to nutrient-poor savannah ecosystems and can withstand competition from invasive shrubs by utilizing both atmospheric and soil nitrogen sources.

Roots and Nodules Response Differently to P Starvation in the Mediterranean-Type Legume Virgilia divaricata

Virgilia divaricata is a tree legume that grows in the Cape Floristic Region (CFA) in poor nutrient soils. A comparison between high and low phosphate growth conditions between roots and nodules was conducted and evaluated for the plants ability to cope under low phosphate stress conditions in V. divaricata. We proved that the plant copes with low phosphate stress through an increased allocation of resources, reliance on BNF and enhanced enzyme activity, especially PEPC. Nodules had a lower percentage decline in P compared to roots to uphold its metabolic functions. These strategies partly explain how V. divaricata can sustain growth despite LP conditions. Although the number of nodules declined with LP, their biomass remained unchanged in spite of a plant decline in dry weight. This is achieved via the high efficiency of BNF under P stress. During LP, nodules had a lower % decline at 34% compared to the roots at 88%. We attribute this behavior to P conservation strategies in LP nodules that imply an increase in a metabolic bypass that operates at the PEP branch point in glycolysis. The enhanced activities of nodule PEPC, MDH, and ME, whilst PK declines, suggests that under LP conditions an adenylate bypass was in operation either to synthesize more organic acids or to mediate pyruvate via a non-adenylate requiring metabolic route. Both possibilities represent a P-stress adaptation route and this is the first report of its kind for legume trees that are indigenous to low P, acid soils. Although BNF declined by a small percentage during LP, this P conservation was evident in the unchanged BNF efficiency per weight, and the increase in BNF efficiency per mol of P. It appears that legumes that are indigenous to acid soils, may be able to continue their reliance on BNF via increased allocation to nodules and also due to increase their efficiency for BNF on a P basis, owing to P-saving mechanisms such as the organic acid routes.

Differential growth costs and nitrogen fixation in Cytisus multiflorus (L'Hér.) Sweet and Cytisus scoparius (L.) Link are mediated by sources of inorganic N

Shrubby legumes in Mediterranean-type ecosystems face strong nutrient limitations that worsen in summer, when water is absent. Nitrogen-fixing legumes are likely to be able to switch between soil N and atmospheric N (N₂ ) sources to adjust the C costs of N acquisition in different seasons. We investigated the utilisation of different inorganic N sources by two indigenous shrubby legumes (Cytisus multiflorus and Cytisus scoparius). Plant performance in terms of photosynthesis and biomass production was also analysed. Plants were cultivated in sterile river sand supplied with Hoagland nutrient solution, grown in N-free nutrient solution and inoculated with effective rhizobial strains from nodules of adult plants of the same species. A second treatment consisted of plants given 500 μm NH₄ NO₃ added into the nutrient solution. In a third treatment, plants were watered with another source of N (500 μm NH₄ NO₃ ) as well as being inoculated with effective rhizobial strains. The application of NH₄ NO₃ to the legumes resulted in a larger increase in plant dry matter. Carbon construction costs were higher in plants supplied with mineral and symbiotic N sources and always higher in the endemic C. multiflorus. Differences in photosynthesis rates were only observed between species, regardless of the N source. Non-fertilised inoculated plants had more effective root nodules and a clear dependence on N₂ fixation. We propose that the ability of C. scoparius to change N source makes it a plastic species, which would account for its broader distribution in nature.

Evolution of vegetation and soil nutrients after uranium mining in Los Ratones mine (Cáceres, Spain)

The evolution of vegetation structure following mine rehabilitation is rather scarce in the literature. The concentration of long-lived radionuclides of the (238)U series might have harmful effects on living organisms. We studied soil properties and the natural vegetation occurring along a gradient in Los Ratones, an area rehabilitated after uranium mining located in Cáceres, Spain. Soil and vegetation were sampled seasonally and physical and chemical properties of soil were analysed, including natural isotopes of (238)U, (230)Th, (226)Ra and (210)Pb. Species richness, diversity, evenness and plant cover were estimated and correlated in relation to soil physical and chemical variables. The location of the sampling sites along a gradient had a strong explanatory effect on the herbaceous species, as well as the presence of shrubs and trees. Seasonal effects of the four natural isotopes were observed in species richness, species diversity and plant cover; these effects were directly related to the pH values in the soil, this being the soil property that most influences the plant distribution. Vegetation in the studied area resembles that of the surroundings, thus proving that the rehabilitation carried out in Los Ratones mine was successful in terms of understorey cover recovery.

Influence of soil texture on the distribution and availability of 238U, 230Th, and 226Ra in soils

The influence of soil texture on the distribution and availability of (238)U, (230)Th, and (226)Ra in soils was studied in soil samples collected at a rehabilitated uranium mine located in the Extremadura region in south-west Spain. The activity concentration (Bqkg(-1)) in the soils ranged from 60 to 750 for (238)U, from 60 to 260 for (230)Th, and from 70 to 330 for (226)Ra. The radionuclide distribution was determined in three soil fractions: coarse sand (0.5-2mm), medium-fine sand (0.067-0.5mm), and silt and clay (<0.067 mm). The relative mobility of the natural radionuclides in the different fractions was studied by comparison of the activity ratios between radionuclides belonging to the same radioactive series. The lability of these radionuclides in each fraction was also studied through selective extraction from the soils using a one-step sequential extraction scheme. Significant correlations were found for (238)U, (230)Th, and (226)Ra between the activity concentration per fraction and the total activity concentration in the bulk soil. Thus, from the determination of the activity concentration in the bulk soil, one could estimate the activity concentration in each fraction. Correlations were also found for (238)U and (226)Ra between the labile activity concentration in each fraction and the total activity concentration in bulk soil. Assuming that there is some particle-size fraction that predominates in the process of soil-to-plant transfer, the parameters obtained in this study should be used as correction factors for the transfer factors determined from the bulk soil in previous studies.

Simulation of germination of pioneer species along an experimental drought gradient

The germination of ten plant species from the Iberian Peninsula was assessed along a water deficit gradient between -0. 1652 (moist) and -0.4988 MPa (dry) of osmotic potential, created by addition of increasing concentrations of polyethylene glycol (PEG 6000) to distilled water in which plants were grown hydroponically. The level and rate of germination of Daucus carota and Thapsia villosa significantly decreased with decreasing psi. Seeds of Dactylis glomerata and Dittrichia viscosa had positive germination responses to low osmotic potentials; germination of Epilobium hirsutum was not affected by osmotic potential. Germination of Medicago arabica, Cynosurus cristatus, Cistus ladanifer and Cistus albidus, was no favored by the addition of polyethylene glycol (PEG). Germination of Foeniculum vulgare and Thapsia villosa was inhibited by PEG.

Seed germination in response to chemicals: effect of nitrogen and pH in the media

Seed germination generally presents a peak in the next growing season after a fire. Among other factors associated with fire are the increase of soil nitrogen and changes in the pH of the soil. In this study, we addressed the question, whether or not the germination response of eight species is linked with the increase in pH and nitrogenous compounds in the germination media? We assessed the separate and combined effects of nitrogenous compounds and pH on the percentage and rate of germination of seeds of Medicago arabica (L.) Hudson, Epilobium hirsutum L., Foeniculum vulgare Miller, Daucus carota L., Thapsia villosa L., Cynosurus cristatus L., Dactylis glomerata L. and Rumex crispus L. All these species are well represented in the Mediterranean ecosystems of the central-west Spain. Water and CaCl2 were used as controls. Nitrogenous compounds increased percent germination (level) and rate in three of the species studied. High pH negatively affected the germination rate of seeds from most species, but had no effect on the per cent germination of any of the species. The higher concentration of the nutritious solutions affected negatively the germination level and rate. The different germination responses of seeds of the studied species could not be exclusively attributed to pH values in the media, whereas the amount and form of Nitrogen in the media has a greater effect on it. These differences in germination are species dependent.

Analysis of the legume-rhizobia symbiosis in shrubs from central western Spain

AIMS

This work analyses the diversity of rhizobia associated with some of the predominant shrubby legumes in central-western Spain. Symbiotic promiscuity and effectiveness were studied using cross-inoculation experiments with shrubby species.

MATERIAL AND RESULTS

Six new bradyrhizobia strains were isolated from nodules collected from wild plants of six leguminous species, Cytisus balansae, C. multiflorus, C. scoparius, C. striatus, Genista hystrix and Retama sphaerocarpa. These isolates were genetically characterized by 16S rDNA partial sequencing and random amplification of polymorphic DNA-PCR fingerprinting. The phylogenetic analysis revealed that these isolates could represent three new Bradyrhizobium species. Shrubby legumes and bradyrhizobia displayed a high symbiotic promiscuity both for infectivity and effectiveness. Symbioses were effective in more than 70% of the associations established by four of the six plant species.

CONCLUSIONS

Native woody legumes in western Spain are nodulated by Bradyrhizobium strains. The high degree of symbiotic promiscuity and effectiveness highlights the complex dynamics of these communities in wild ecosystems under a Mediterranean-type climate. Furthermore, the results from this study suggest a potential importance of inoculation for these legume species in soil-restoration projects.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first study, to our knowledge, that combines both molecular analysis and pot trials to study the rhizobia-legume symbiosis for wild legumes.

Effect of smoke, charred wood, and nitrogenous compounds on seed germination of ten species from woodland in central-western Spain

The effect of smoke, charred wood, and nitrogenous compounds on germination was tested on 10 species of the Cistaceae, Poaceae, Fabaceae, and Asteraceae, from fire-prone, shrubby woodlands in central-western Spain. Dry seeds were exposed to smoke, by watering with distilled water-charred wood suspensions, or NaNO2, KNO3, NH4Cl, and NH4NO3. Smoke enhanced germination in 9 of 10 of the species. In species of Poaceae, germination was stimulated by 20 min of smoke exposure. In Asteraceae and Fabaceae species, 10 min of smoke exposure was the most effective treatment for enhancing germination. Three species--Cistus ladanifer, Cistus crispus, and Cistus monspeliensis--had a positive response to 20 min of smoke exposure; germination of Cistus salviifolius L. was also enhanced after 10 min. The effect of charred wood was variable, with no consistent germination pattern within the families. Trifolium angustifolium and Retama sphaerocarpa showed no stimulation of germination under most of the charred wood concentrations. Similarly, germination of Senecio jacobea under the charred wood treatment did not surpass that of the control. NaNO2 promoted seed germination in Dactylis glomerata (10 mM), Cistus ladanifer (1, 10, and 25 mM), and Cistus crispus (1 and 10 mM). KNO3 enhanced germination in Dactylis glomerata (1 and 25 mM), Dittrichia viscosa (10 and 25 mM), C. ladanifer (1, 10, and 25 mM), Cistus crispus (1 and 25 mM), and C. salviifolius aud C. monspeliensis (25 mM). NH4Cl induced germination of Dactylis glomerata and Dittrichia viscosa (1 mM), and Cistus species germinated best in 25 mM of this salt. NH4NO3 induced germination only in Cistus species. Holcus lanatus had the highest level of germination regardless of treatment.