Citric acid excretion and precipitation of calcium citrate in the rhizosphere of white lupin (Lupinus albus L.)

Abstract. White lupin ( Lupinus albus L.) was grown for 13 weeks in a phosphorus (P) deficient calcareous soil (20% CaCO₃, pH(H₂O)7.5) which had been sterilized prior to planting and fertilized with nitrate as source of nitrogen. In response to P deficiency, proteoid roots developed which accounted for about 50% of the root dry weight. In the rhizosphere soil of the proteoid root zones, the pH dropped to 4.8 and abundant white precipitates became visible. X-ray spectroscopy and chemical analysis showed that these precipitates consisted of calcium citrate. The amount of citrate released as root exudate by 13-week-old plants was about 1 g plant⁻¹, representing about 23% of the total plant dry weight at harvest. In the rhizosphere soil of the proteoid root zones the concentrations of available P decreased and of available Fe, Mn and Zn increased. The strong acidification of the rhizosphere and the cation/anion uptake ratio of the plants strongly suggests that proteoid roots of white lupin excrete citric acid, rather than citrate, into the rhizosphere leading to intensive chemical extraction of a limited soil volume. In a calcareous soil, citric acid excretion leads to dissolution of CaCO₃ and precipitation of calcium citrate in the zone of proteoid roots.     

Home range use and the exploitation of gum in the marmosetCallithrix jacchus jacchus

Three wild groups of common marmoset, Callithrix jacchus jacchus,in north-east Brazil, of approximately similar size, had home ranges between 2.5 and 6.5 ha. But their core areas were similar in size between 1.0 and 1.5 ha, with a monthly area of heavy use between 1.1 and 1.6 ha. The groups were selective in the use of their home ranges, even though they were small: they used some areas heavily and others lightly. The core areas had higher densities of trees that produced gum exudates than did other parts of the home ranges. Our data suggest that a group of marmosets in this habitat may require a minimum of about 50 gum trees in its home range at a minimum density of about 50 trees/ha. In addition, the animals require suitable trees in which to sleep. We suggest that patches of forest with these desirable properties remain relatively fixed in size and location over the years and that individual animals are constantly in flux between them.     

Rhizosphere microorganism effects on soluble amino acids,sugars and organic acids in the root zone ofAgropyron cristatum,A. smithii andBouteloua gracilis

Three axenic and rhizosphere microorganism-inoculated shortgrass steppe plant species were evaluated for possible differences in residual organic carbon and nitrogen present as sugars, organic acids and amino acids. IntroducedAgropyron cristatum was compared toA. smithii andBouteloua gracilis, which are dominant species in the native shortgrass steppe. These plants, grown for 90 days in root growth chambers, showed differences in residual organic carbon and nitrogen per gram of root, and rhizosphere microbe presence resulted in additional changes in these compounds. The root biomass ofB. gracilis was significantly increased with microbes present. TheAgropyron species had significantly lower amino acid levels with microbes present, while under the same conditions, theB. gracilis showed significant decreases in residual sugars. Based on the amino acids, sugars and organic acids, the C/N ratio of the sterileA. cristatum was higher than forB. gracilis. Rhizosphere microbe presence did not result in changes in these C/N ratios. These results suggest thatA. cristatum, with microbes present, will have lower levels of amino acids present, whileB. gracilis, with a lower C/N ratio, will have sugars used to a greater extent by the rhizosphere microbes. This resulted in the higher levels of residual soluble organic C and N in the rhizosphere ofB. gracilis, in comparison with the introducedA. cristatum. These differences may be critical in influencing the course of nutrient accumulation and plant competition in short-grass steppe communities, and in understanding basic aspects of plant-rhizosphere microorganism interactions.     

Strain-specific salt tolerance and chemotaxis ofAzospirillum brasilense and their associative N-fixation with finger millet in saline calcareous soil

Three salt-tolerantAzospirillum brasilense strains were isolated from the roots of finger millet grown in saline calcareous soil and characterized. The effect of various salts on growth and N₂ase activity of these strains was tested and strain STR1 was found more tolerant at higher concentrations of Cl^-, SO₄ ² and HCO₃ ^-. Bicarbonate was found to be the most toxic. The content and concentrations of root exudates of finger millet genotypes were different and chemotaxis to sugars, amino acids, organic acids and root exudates was strain specific. Under salt stress, significant interactions between strains and genotypes of finger millet resulted in different responses of N₂ase activity, endo- and exorhizospheric population, dry weight of root, shoot and grain yield.     

An Artificial “gum-tree” for marmosets (Callithrix j. jacchus)

Marmosets (Callithrix, Cebuella) in the wild gouge wells in trees and eat the exudates that accumulate there. An artificial gum-tree was made of wooden dowel and filled with Acacia Senegal exudate (gum arabic) dissolved in water. Three families of marmosets avidly gouged and consumed gum from this device, showing all of the behavioral patterns described in nature. The gum-tree cost little and was easy to make.     

Genotypical differences among graminaceous species in release of phytosiderophores and uptake of iron phytosiderophores

Graminaceous species can enhance iron (Fe) acquisition from sparingly soluble inorganic Fe(III) compounds by release of phytosiderophores (PS) which mobilize Fe(III) by chelation. In most graminaceous species Fe deficiency increases the rate of PS release from roots by a factor of 10–20, but in some species, for example sorghum, this increase is much less. The chemical nature of PS can differ between species and even cultivars.The various PS are similarly effective as the microbial siderophore Desferal (ferrioxamine B methane sulfonate) in mobilizing Fe(III) from a calcareous soil. Under the same conditions the synthetic chelator DTPA (diaethylenetriamine pentaacetic acid) is ineffective.The rate of Fe(III)PS uptake by roots of graminaceous species increases by a factor of about 5 under Fe deficiency. In contrast, uptake of Fe from both synthetic and microbial Fe(III) chelates is much lower and not affected by the Fe nutritional status of the plants. This indicates that in graminaceous species under Fe deficiency a specific uptake system for FePS is activated. In contrast, the specific uptake system for FePS is absent in dicots. In a given graminaceous species the uptake rates of the various FePS are similar, but vary between species by a factor of upto 3. In sorghum, despite the low rate of PS release, the rate of FePS uptake is particularly high.The results indicate that release of PS and subsequent uptake of FePS are under different genetic control. The high susceptibility of sorghum to Fe deficiency (lime-chlorosis) is most probably caused by low rates of PS release in the early seedling stage. Therefore in sorghum, and presumably other graminaceous species also, an increase in resistance to lime chlorosis could be best achieved by breeding for cultivars with high rates of PS release. In corresponding screening procedures attention should be paid to the effects of iron nutritional status and daytime on PS release as well as on rapid microbial degradation of PS.     

Herbivore-induced changes in plant carbon allocation: assessment of below-ground C fluxes using carbon-14

Effects of above-ground herbivory on short-term plant carbon allocation were studied using maize (Zea mays) and a generalist lubber grasshopper (Romalea guttata). We hypothesized that above-ground herbivory stimulates current net carbon assimilate allocation to below-ground components, such as roots, root exudation and root and soil respiration. Maize plants 24 days old were grazed (c. 25–50% leaf area removed) by caging grasshoppers around individual plants and 18 h later pulse-labelled with¹⁴CO₂. During the next 8 h,¹⁴C assimilates were traced to shoots, roots, root plus soil respiration, root exudates, rhizosphere soil, and bulk soil using carbon-14 techniques. Significant positive relationships were observed between herbivory and carbon allocated to roots, root exudates, and root and soil respiration, and a significant negative relationship between herbivory and carbon allocated to shoots. No relationship was observed between herbivory and¹⁴C recovered from soil. While herbivory increased root and soil respiration, the peak time for¹⁴CO₂ evolved as respiration was not altered, thereby suggesting that herbivory only increases the magnitude of respiration, not patterns of translocation through time. Although there was a trend for lower photosynthetic rates of grazed plants than photosynthetic rates of ungrazed plants, no significant differences were observed among grazed and ungrazed plants. We conclude that above-ground herbivory can increase plant carbon fluxes below ground (roots, root exudates, and rhizosphere respiration), thus increasing resources (e.g., root exudates) available to soil organisms, especially microbial populations.     

Role of proteinaceous amino acids released in root exudates in nutrient acquisition from the rhizosphere

The role of proteinaceous amino acids in rhizosphere nutrient mobilization was assessed both experimentally and theoretically. The degree of adsorption onto the soil's solid phase was dependent on both the amino acid species and on soil properties. On addition of amino acids to both soil and freshly precipitated Fe(OH)₃, no detectable mobilization of nutrients (K, Na, Ca, Mg, Cu, Mn, Zn, Fe, S, P, Si and Al) was observed, indicating a very low complexation ability of the acidic, neutral and basic amino acids. This was supported by results from a solution equilibria computer model which also predicted low levels of amino acid complexation with solutes present in the soil solution. On comparison with the Fe(OH)₃ and equilibria data obtained for the organic acid, citrate, it was concluded that amino acids released into the rhizosphere have a limited role in the direct acquisition of nutrients by plants. The effectiveness of root exudates such as amino acids, phytosiderophores and organic acids in nutrient mobilization from the rhizosphere is discussed with reference to rhizosphere diffusion distances, microbial degradation, rate of complexation and the root's capacity to recapture exudate-metal complexes from the soil.     

我国半乳甘露聚糖胶资源利用前景初探

半乳甘露聚糖植物胶加工是我国70年代中期兴起的新兴产业。本文从我国植物资源发展的角度。重点论证了瓜尔豆（Cyanopsis tetragonoloba（L.）Taubert.）、田菁（Sesbania cannabina(Retz.)Pers.)、胡芦巴（Trigonella foenum-graecum L.）的生产情况,认为胡芦巴适应性强、产量高、易于机械化大面积种植与轮作,并具有改良土壤以及经济效益好等优势。因此,发展半乳甘露聚糖胶资源,大量种植胡芦巴是最好的选择。