Growth and respiration in two mangrove species at a range of salinities

Growth and dark respiration rates were measured in leaves and roots of seedlings of Avicennia marina (Forsk.) Vierh, (grey mangrove), and Aegiceras corniculatum (L.) Blanco (river mangrove). Plants were grown in a soil mixture at ambient temperatures and watered with 0.25 and 100% sea-water. Oxygen uptake was measured in excised root and leaf samples. In both species growth was maximal in 25% sea-water, and root respiration was lowest in 100% sea-water. Differences were found between the two species in the responses of leaf respiration to salinity. In A. corniculatum leaf respiration was raised in both 25 and 100% sea-water, while in A. marina only leaves in 100% sea-water showed higher rates of respiration. These results are consistent with the view that A. marina is the more salt-tolerant of the two species. In A. corniculatum the respiration rates of the hypocotyl were also measured, and were much higher in 100% sea-water than in the other two treatments. The results suggest that at high salinities there is a high metabolic cost in the shoots of both species, and that at such salinities rates of root respiration may be limited by the supply of substrate from the shoots.     

Effects of environmentally hazardous chemicals on the emergence and early growth of selected Australian plants

The effect of soil-incorporated copper, tri-allate, and anthracene on the emergence and early growth of three Australian native species (Banksia ericifolia, Casuarina distyla andEucalyptus eximia) and three crop species (Avena sativa, Cucumis sativus andGlycine max), was assessed using OECD Test Guideline 208. The crop species are sensitive species used in overseas phytotoxicity testing, and their responses were compared with those of the native species. Seeds were grown in pots in a glasshouse in a sandy loam soil at the chemical concentrations of 0, 10, 100, 1000 and 2000 mg kg^–1. LC50 and EC50 values were determined for each species. The most sensitive species was the monocotyledonA. sativa, while among the five dicotyledonsC. distyla was most sensitive. All three chemicals delayed emergence and affected seedling growth. The results indicate that the conditions of the OECD Test Guideline can be met under Australian conditions, but that the Guideline requires modification for use with Australian native species.     

Maximising adaptive potential in translocated populations of clonal saltmarsh plants: a case study on Wilsonia backhousei, Convolvulaceae

We investigated the implications of clonality for translocation of Wilsonia backhousei, a threatened, outbreeding, saltmarsh plant with tidally-dispersed fruit. Eight microsatellite loci were used to characterise samples from three estuaries in New South Wales, Australia, and to determine the size and distribution of genetically distinct individuals (genets). Within-population diversity was compared to the presence or absence of seed production using the t test. Ordinal logistic regression was used to investigate the relative influence on seed yield of soil characteristics (soil moisture, salinity, pH) and the number of clonal lineages within a 5 and 10 m radius. Principal coordinate analysis, analysis of molecular variance and Bayesian analysis were used to investigate the extent of gene flow within and among the three estuaries. We found individual genets could cover extensive areas (up to 225 m²) and apparently large populations could consist of only a few individuals. Populations that failed to produce seed had significantly less genetic diversity than populations that produced seed (P = 0.001). Seed yield showed a significant positive response to both increasing soil moisture content (P = 0.003) and increasing genetic diversity in a 5 m radius (P = 0.003). Gene flow was found to occur chiefly within estuaries though occasional longer-distance gene transfer was evident. To maximise adaptive potential in translocated populations of W. backhousei, we recommend sourcing propagules from multiple populations and planting representatives of the different populations in close proximity to facilitate sexual reproduction. These findings are likely to be applicable to other outbreeding clonal saltmarsh plants with tidally-dispersed fruit or seed.     

Purification and substrate specificity of an endo-dextranase of Streptococcus mutans K1-R

An extracellular endo-dextranase has been isolated from Streptococcus mutans K1-R. Incubation of cell-free culture fluid with sucrose permitted the removal of a large proportion of the extracellular d-glucosyltransferases by irreversible adsorption onto the insoluble glucans that these enzymes synthesize from sucrose. The remaining d-glucosyltransferases were separated from dextranase by precipitation with ammonium sulphate, chromatography on hydroxylapatite and DEAE-cellulose, followed by filtration on Ultrogel. The major products of action of the purified dextranase on (1→6)-α-d-glucans were isomaltotriose (IM₃), isomaltotetraose (IM₄), and isomaltopentaose (IM₅). Further hydrolysis of IM₄ and IM₅ occurred after prolonged incubation with excess of enzyme, to give d-glucose, IM₂, and IM₃. The relative rate of hydrolysis of isomaltose saccharides fell sharply with decreasing chainlength from IM₁₂ to IM₅. The hydrolysis of dextrans containing 96% or more of (1→6)-α-d-glucosidic linkages, expressed as apparent conversion into IM₃, was virtually complete, and substrates such as Streptococcus sanguis glucan, containing sequences of (1→6)-α-d-glucosidic linkages, were also effectively hydrolyzed. Dextranase activity towards the soluble glucan of Streptococcus mutans was limited, and there was no action on the insoluble glucan synthesized by S. mutans sucrose 3-d-glucosyltransferase.     

Action of alpha-1,6-glucan glucohydrolase on oligosaccharides derived from dextran

The action of α-1,6-glucan glucohydrolase on α-(1→6)-D-glucosidic linkages in oligosaccharides that also contain an α-(1→2)-, α-(1→3)-, or α-(1→4)-D-glucosidic linkage has been investigated. The enzyme could hydrolyse α-(1→6)-D-glucosidic linkages from the non-reducing end, including those adjacent to an anomalous linkage. α-(1→6)-D-Glucosidic linkages at branch points were not hydrolysed, and the enzyme could neither hydrolyse nor by-pass the anomalous linkages. These properties of α-1,6-glucan glucohydrolase explain the limited hydrolysis of dextrans by the exo-enzyme. Hydrolysis of the main chain of α-(1→6)-D-glucans will always stop one D-glucose residue away from a branch point. The extent of hydrolysis by α-1,6-glucan glucohydrolase of some oligosaccharide products of the action on dextran of Penicillium funiculosum and P. lilacinum dextranase, respectively, has been compared. Differences in the specificity of the two endo-dextranases were revealed. The Penicillium enzymes may hydrolyse dextran B-512 to produce branched oligosaccharides that retain the same 1-unit and 2-unit side-chains that occur in dextran.     

Salinity, growth and root respiration in the grey mangrove, Avicennia marina

Relationships between growth parameters and root respiration under various conditions of salinity were investigated in seedlings of the grey mangrove Avicennia marina (Forsk.) Vierh. Growth, root/shoot ratios, leaf succulence and osmotic potential of leaves were measured for seedlings grown for 6–8 weeks in 100, 50, 25 and 0% seawater. Oxygen uptake of root segments, from distal to proximal ends of roots, was measured for all treatments. Total growth was maximal in 25% seawater, highest leaf succulence was obtained in 50% seawater, and highest leaf osmotic potential in 100% seawater. Oxygen uptake in distal root segments, as measured both by Clark oxygen electrode and Warburg manometry, showed a stimulation in the presence of salt that closely paralleled growth stimulation. The rates of respiration were highest in 25% seawater. The oxygen uptake was not stimulated by salt per se, since concentrations higher than 25% were associated with a decline in rate of oxygen uptake from the maximum. Values for the respiratory quotient approximated to one in all treatments. Avicennia marina has been reported to exclude from its roots about 90% of the salt in the surrounding medium. It might have been expected that increased concentrations of salt in the growth medium would be associated with a standard salt respiration response in the roots; however, this was not obtained.     

Metabolism of the reserve polysaccharide of Streptococcus mitior (mitis): is there a second alpha-1,4-glucan phosphorylase?

The alpha-1,4-glucan phosphorylase (alpha-1,4-glucan: orthophosphate glucosyltransferase; EC 2.4.1.1) associated with the particulate cell fraction of Streptococcus mitior strain S3 was compared with the soluble maltodextrin phosphorylase that had been previously isolated from the same organism (Walker et al., 1969). The particulate enzyme was more sensitive to the glycogen content of the cell than the soluble euzyme; its activity was highest when the cells were grown under conditions favoring high glycogen storage. Substrate specificities of the two high activity towards endogenous glycogen, whereas low-molecular-weight maltodextrins were the preferred substrates for the soluble phosphorylase. The purification of the particulate phosphorylase included incubation of the particulate fraction in 160 mM sodium phosphate-10 mM sodium citrate-0.1% (wt/vol) Triton X-100 buffer (pH 6.7) and ion-exchange chromatography on diethylamino-ethyl- Sephadex A-50. The purified enzyme was fully soluble. The value for the purification factor was variable and depended on (i) the substrate used and (ii) whether the synthetic or the degradative reaction was being measured. The solubilization resulted in considerable changes in the properties of the phosphorylase: the pH optimum for activity was raised from 6.0 to 7.0-7.5 and the substrate specificity was altered. Consequently, the purified enzyme bore greater similarity to the soluble maltodextrin phosphorylase. The reported results are best explained in terms of a single phosphorylase, the specificity which is determind by its binding state in the cell. The enzyme acts as a glycogen phosphorylase in the particulate state and as a maltodextrin phosphorylase when soluble. The equilibrium between the two forms is related to the glycogen content of the cells.