The levels of ribitol, arabitol and mannitol in individual lobes of the lichen Parmelia conspersa (Ehrh. ex Ach.) ACH.

The levels of the soluble carbohydrates ribitol, arabitol and mannitol were measured in individual lobes of the lichen Parmelia conspersa (Ehrh. ex Ach.) Ach. Lobes were collected from a north and a south facing slate rock surface in South Gwynedd, Wales, U.K. on 4 days during 1990-1991. On each day sampled, the most significant variation in soluble carbohydrate levels was between the individual lobes of a thallus. In addition, carbohydrate levels were significantly greater on the south facing rock surface on 2 of the 4 days sampled. Factorial analyses of variance suggested that the levels of individual carbohydrates varied significantly between days but not between north and south facing rock surfaces. Mannitol levels varied less between days than arabitol levels. Levels of ribitol, arabitol and mannitol were positively correlated in individual lobes. A stepwise multiple regression suggested that on the north facing rock surface, arabitol and mannitol levels could be explained by variations in the level of ribitol. By contrast, on the south facing rock surface, the levels of fungal carbohydrates were less dependent on the level of ribitol and there was evidence of a relationship between arabitol and mannitol. Variations in carbohydrate production, allocation and metabolism could help to explain lobe growth variation in foliose lichens and the radial growth of lobes over a longer period of time. Greater carbohydrate production rather than differences in allocation and metabolism may explain the increased growth and frequency of P. conspersa on south facing rock surfaces in South Gwynedd. © 1994.

Concentration of mannitol and other soluble carbohydrates in the crustose lichen Rhizocarpon geographicum

In symbiotic lichens which have Trebouxia as the algal partner, photosynthesis by the algae results in the production of the soluble carbohydrate ribitol which is then transported to the fungus where it is converted to arabitol and mannitol. Within the fungus, arabitol may act as a short-term carbohydrate reserve while mannitol may have a more protective function and be important in stress resistance. The concentrations of ribitol, arabitol, and mannitol were measured, using gas chromatography, in the central areolae and marginal hypothallus of the crustose lichen Rhizocarpon geographicum (L.) DC. growing on slate rocks in north Wales, UK. The concentrations of all three soluble carbohydrates were greater in the central areolae than in the marginal prothallus. In addition, the ratio of mannitol in the prothallus to that in the areolae was least in July. The concentration of an individual carbohydrate in the prothallus was correlated primarily with the concentrations of the other carbohydrates in the prothallus and not to their concentrations in the areolae. Low concentration of ribitol, arabitol, and mannitol in the marginal prothallus compared with the central areolae suggests either a lower demand for carbohydrate by the prothallus or limited transport from areolae to prothallus and may explain the low growth rates of this species. In addition, soluble carbohydrates appear to be partitioned differently through the year with an increase in mannitol compared with arabitol in more stressful periods.

A pragmatic approach for engineering porous mannitol and mechanistic evaluation of particle performance

The importance of mannitol has increased recently as an emerging diluent for orodispersible dosage forms. The study aims to prepare spray dried mannitol retaining high porosity and mechanical strength for the development of orally disintegrating tablets (ODTs). Aqueous feed of d-mannitol (10% w/v) comprising ammonium bicarbonate, NH4HCO3 (5% w/v) as pore former was spray dried at inlet temperature of 110-170°C. Compacts were prepared at 151MPa and characterized for porosity, hardness and disintegration time. Particle morphology and drying mechanisms were studied using thermal (HSM, DSC and TGA) and polymorphic (XRD) methods. Tablet porosity increased from 0.20±0.002 for pure mannitol to 0.53±0.03 using fabricated porous mannitol. Disintegration time dropped by 50-77% from 135±5.29s for pure mannitol to 75.33±2.52-31.67±1.53s for mannitol 110-170°C. Hardness increased by 150% at 110°C (258.67±28.89N) and 30% at 150°C (152.70±10.58N) compared to pure mannitol tablets (104.17±1.70N). Increasing inlet temperature resulted in reducing tablet hardness due to generation of 'micro-sponge'-like particles exhibiting significant elastic recovery. Impact of mannitol polymorphism on plasticity/elasticity cannot be ruled out as a mixture of α and β polymorphs formed upon spray drying.

A holistic multi evidence approach to study the fragmentation behaviour of crystalline mannitol

Mannitol is an essential excipient employed in orally disintegrating tablets due to its high palatability. However its fundamental disadvantage is its fragmentation during direct compression, producing mechanically weak tablets. The primary aim of this study was to assess the fracture behaviour of crystalline mannitol in relation to the energy input during direct compression, utilising ball milling as the method of energy input, whilst assessing tablet characteristics of post-milled powders. Results indicated that crystalline mannitol fractured at the hydrophilic (011) plane, as observed through SEM, alongside a reduction in dispersive surface energy. Disintegration times of post-milled tablets were reduced due to the exposure of the hydrophilic plane, whilst more robust tablets were produced. This was shown through higher tablet hardness and increased plastic deformation profiles of the post-milled powders, as observed with a lower yield pressure through an out-of-die Heckel analysis. Evaluation of crystal state using x-ray diffraction/differential scanning calorimetry showed that mannitol predominantly retained the β-polymorph; however x-ray diffraction provided a novel method to calculate energy input into the powders during ball milling. It can be concluded that particle size reduction is a pragmatic strategy to overcome the current limitation of mannitol fragmentation and provide improvements in tablet properties.

Lobe growth variation and the maintenance of symmetry in foliose lichen thalli

The aim of this study was to determine how thallus symmetry could be maintained in foliose lichens when variation in the growth of individual lobes may be high. Hence, the radial growth of a sample of lobes was studied monthly, over 22 months, in 7 thalli of Parmelia conspersa (Ehrh. Ex Ach.) Ach. And 5 thalli of P. glabratula ssp fuliginosa (fr. ex Duby) Laund. The degree of variation in the total radial growth of different lobes within a thallus over 22 months varied between thalli. Individual lobes showed a fluctuating pattern of radial growth from month to month with alternating periods of fast and slow growth. Monthly variations in radial growth of different lobes were synchronized in some but not in all thalli. Few significant correlations were found between the radial growth of individual lobes and total monthly rainfall or shortwave radiation. The levels of ribitol, arabitol and mannitol were measured in individual lobes. All three polyols varied significantly between lobes within a thallus suggesting that variations in algal phostosynthesis and in the partitioning of fungal polyols may contribute to lobe growth variation. The effect on thallus symmetry of lobes which grew radially either consistently faster or slower than average was studied. Slow growing lobes were overgrown, and gaps in the perimeter were eliminated by the growth of neighbouring lobes, in approximately 7 to 9 months. However, a rapidly growing lobe, with its neighbours removed on either side, continued to grow radially at the same rate as rapidly growing control lobes. The results suggested that lobe growth variation results from a combination of factors which may include the origin of the lobes, lobe morphology and the patterns of algal cell division and hyphal elongation in different lobes. No convincing evidence was found to suggest that exchange of carbohydrate occurred between lobes which would tend to equalize their radial growth. Hence, the fluctuating pattern of lobe growth observed may be sufficient to maintain a degree of symmetry in most thalli. In addition, slow growing lobes would tend to be overgrown by faster growing neighbours thus preventing the formation of indentations in the thallus perimeter.

Development and growth of the lichen Rhizocarpon geographicum

The development of new areolae on the marginal hypothallus of the lichen Rhizocarpon geographicum (L.) DC was studied after complete or partial removal of the central areolae. New areolae developed slowly on the isolated hypothalli over two years. Development was similar when the areolae were completely removed and when the central areolae were separated from the marginal hypothallus by ‘moats’ 2 to 5 mm in width. However, in intact thalli, the marginal areolae developed rapidly during Jan. – June 1986 but showed periods of retreat from the margin during Oct. - Dec. 1985 and July – Sept. 1986. These results suggested that primary areolae may develop from free-living algal cells trapped by the hypothallus while secondary areolae may develop from zoospores produced by the thallus. Complete removal of the areolae resulted in no measurable radial growth of the marginal hypothallus over 18 months. Removal of the central areolae to within 1 and 2 mm of the hypothallus significantly reduced growth. These results suggest that the areolae may supply the hypothallus with carbon for growth. When the marginal hypothallus was experimentally removed a new hypothallus developed within one year. Regeneration occurred initially by retreat of the marginal areolae and later by new hyphal growth. The concentration of ribitol, arabitol and mannitol was measured in the areolae and marginal hypothallus on four occasions in 1985/6 in a population growing on a steep south facing rock surface. The three carbohydrates were present in significantly higher concentration in the areolae than in the hypothallus. Hence, the slow growth of this species may result from inhibited transport of carbohydrate from areolae to hypothallus.

Factors associated with lobe division in the lichen Parmelia conspersa (Ehrh. ex. Ach.)Ach.

The factors associated with lobe division were studied in thalli of the lichen Parmelia conspersa (Ehrh. ex Ach.)Ach. Lobe division was studied in sequences of adjacent lobes using spatial pattern analysis. In five large thalli, lobe division within the thallus margin was randomly distributed. Correlations between the degree of lobe division, the radial growth of the lobe and lobe morphology were studied in six thalli. Lobe division was positively correlated with either lobe width or area in four thalli. Correlations were observed with radial growth or morphology of the adjacent lobes in two thalli. Dividing and non-dividing lobes were removed from large thalli and glued to pieces of slate with their tips either at the same level or in front of neighbouring lobes. Dividing lobes divided more rapidly when their tips were glued in front of their neighbours. The levels of ribitol, arabitol and mannitol were measured within a 2 mm region of the tip in dividing and non-dividing lobes on four occasions in 1994. Carbohydrate levels were significantly increased in dividing compared with non-dividing lobes. In addition, the mean size of the algal cells was greater in non-dividing compared with dividing lobes especially at the lobe base. However, the percentage of zoosporangia and aplanosporangia did not vary significantly in dividing and non-dividing lobes. These results suggest that: 1) the pattern of lobe division within the thallus margin may be random, 2) lobe division may be determined by lobe size and the location of the lobe tip relative to the neighbouring lobes and 3) there may be an increase in the productivity of lobes associated with lobe division.

Does radial growth of the lichen Parmelia conspersa depend exclusively on growth processes at the lobe tip?

The objective of this study was to test the hypothesis that the radial growth of lobes of the lichen Parmelia conspersa depends largely on growth processes which occur at the lobe tip. First, individual lobes were removed from thalli and portions of the lobe removed to within various distances from the tip. Radial growth of the lobe was unaffected until less than 2 mm of the lobe tip remained. Second, the surfaces of individual lobes were painted with acrylic paint leaving different portions of the lobe exposed. Painting lobes to within 0.5 mm and 1 mm of the tip substantially reduced radial growth. Third, the levels of ribitol, arabitol and mannitol were measured in different regions behind the lobe tip on four occasions during 1994. The concentration of the three carbohydrates was greatest at the lobe tip and the levels declined linearly with distance from the tip. Fourth, painting one vertical half of the lobe tip did not affect radial growth but artificially bisecting the lobe tip with a scalpel reduced radial growth. Although transport of carbohydrate from other regions of the lobe cannot be ruled out, the results support the hypothesis that radial growth in P. conspersa depends largely on processes within a region approximately 2 mm behind the lobe tip.

Factors associated with degeneration of the thallus centre in foliose lichens

Degeneration of the older parts of foliose lichen thalli often lead to the formation of a space or 'window' in the centre of the colonies. The percentage of thalli of different size which exhibited 'windows' was studied in twenty saxicolous lichen populations in south Gwynedd, Wales. The proportion of thalli with 'windows' increased with thallus size. The size class at which 50% and 100% of thalli exhibited 'windows' varied between populations. Differences between populations were not correlated with distance from the sea, aspect, slope or porosity of the substrate or the total number of lichen species present. However, a higher percentage of smaller thalli had 'windows' on rock surfaces with a greater lichen cover. There were no significant differences in the levels of Ca, Mg, Cu or Zn in large (>4 cm) and small (<2 cm) Parmelia conspersa (Ehrh. ex Ach.) Ach. thalli or in the centres and marginal lobes of these thalli. The concentration of ribitol, arabitol and mannitol was significantly reduced in the centre of large thalli compared with the margin of large thalli and the centre of small thalli. However, carbohydrate levels were similar in the centre of large thalli and the margin of small thalli. The data suggest that loss of the thallus centre is a degenerative process related to thallus size. In the field, the formation of 'windows' may be related to the intensity of competition on a substrate. Central degeneration was not associated with a deficiency or an accumulation of Ca, Mg, Cu and Zn in the thallus centre. However, degeneration may be associated with a reduction in carbohydrates in the centre compared with the marginal lobes.

Factors determining the growth curve of the foliose lichen Parmelia conspersa

Growth curves of the foliose lichen Parmelia conspersa (Ehrh. Ex Ach.)Ach. Were obtained by plotting radial growth (RGR, mm yr-1) of the fastest measured lobe, the slowest measured lobe, a randomly selected lobe, and by averaging a sample of lobes from each thallus against thallus diameter. Growth curves derived from the fastest-growing lobe and by averaging lobes were asymptotic and could be fitted by the growth model of Aplin and Hill. Mean lobe width increased with thallus size, reaching a maximum at approx. 4.5 cm thallus diameter. In four out of six thalli, radial growth of lobes over four months was positively correlated with initial lobe width or area. The RGR of isolated lobes was unaffected until the base of the lobe was removed to within 1-2 mm of the tip. The concentration (micrograms mg-1 biomass) of ribitol, arabitol and mannitol was greater in the marginal lobes of large than in small thalli. The results suggested that the growth curve of P. conspersa is determined by processes that occur within individual marginal lobes and can be explained by the Aplin and Hill model. Changes in lobe width and in the productive capacity of individual lobes with thallus size are likely to be more important factors than the degree of translocation within the lobe in determining the growth curve.

The role of formulation excipients in the development of lyophilised fast-disintegrating tablets

Despite recent Success, many fast-disintegrating tablets (FDTs) still face problems of low mechanical strength, poor mouth-feel and higher disintegration times. This Study aimed to optimise FDTS using a progressive three-stage approach. A series of hardness, fracturability and disintegration time tests were performed on the formulations at each stage. During Stage 1, tablets were prepared in concentrations between 2% and 5% w/w, and were formulated at each concentration as single and combination bloom strength gelatin (BSG) using 75 and 225 BSGs. Analysis revealed that both hardness and disintegration time increased with an increase in gelatin concentration. A combination (5% gelatin) FDT comprising a 50:50 ratio of 75:225 BSGs (hardness: 13.7 +/- 0.9 N and disintegration time: 24.1 +/- 0.6 s) was judged the most ideal, and was carried forward to Stage II: the addition of the saccharides sorbitol, mannitol and sucrose in concentrations between 10% and 80% w/w. The best properties were exhibited by mannitol-containing formulations (50%-hardness: 30.9 +/- 2.8 N and disintegration time: 13.3 +/- 2.1 s), which were carried forward to the next stage: the addition of viscosity-modifying polymers to improve mouth-feel and aid pre-gastric retention. Addition of carbopol 974P-NF resulted in the enhancement of viscosity with a compromise of the hardness of the tablet, whereas Pluronic F127 (6%) showed an increase in disintegration time and viscosity with retention of mechanical propel-ties. (C) 2008 Elsevier B.V. All rights reserved.

Do the lichens Xanthoparmelia conspersa (Ach.) Hale and Rhizocarpon Ram. Em. Th. Fr. subgenus Rhizocarpon utilise exogenous carbohydrates for radial growth?

To field test the hypothesis that lichen thalli can use environmental sources of carbon, solutions of ribitol, arabitol and mannitol were added to intact thalli of Xanthoparmelia conspersa (Ach.) Hale and a yellow species of Rhizocarpon (Rhizocarpon Ram. Em. Th. Fr. subgenus Rhizocarpon). In addition, ribitol and an arabitol/mannitol mixture were added to the marginal hypothalli of Rhizocarpon thalli after removal of the areolae. Carbohydrates were added at the beginning of 2- or 3-month growth periods for up to 15 months at concentrations approximately three times the levels estimated to be in the thalli. Addition of carbohydrates to intact thalli of both species had no effect on total radial growth but addition of mannitol significantly increased growth of X. conspersa thalli in the September/October growth period in one experiment. However, this effect was not repeated in a subsequent experiment in which different concentrations of mannitol were added to intact thalli. Addition of ribitol to hypothalli of Rhizocarpon resulted in significantly increased growth in the first few months of the experiment, growth then declining to levels below that of untreated thalli. The data suggest that although hypothalli of Rhizocarpon may have the ability to utilise exogenous carbohydrates for growth, there was little evidence that intact thalli of either species utilise environmental sources of carbon in the field.

Experimental studies of hypothallus growth in the lichen Rhizocarpon geographicum

Removal of the areolae of the crustose lichen Rhizocarpon geographicum (L.)DC. resulted in either low or no measurable radial growth of the marginal hypothallus. Radial growth of the hypothallus was also significantly reduced compared with intact thalli when (1) areolae were removed to within 1 and 2 mm of the hypothallus and (2) a 5 mm wide ‘moat’ was created between the areolae and the hypothallus. Adding ribitol (0.01 M) to isolated hypothalli at 3-month intervals over 15 months results in total radial growth c. 60% that of intact thalli. Adding an arabitol/mannitol mixture (0.05 M arabitol, 0.03 M mannitol) increased radial growth compared with deionized water and ribitol treatments. Adding ribitol (0.7 M), arabitol (0.2 M) and mannitol (0.08 M) to the areolae of intact thalli had no significant effects on radial growth of the hypothallus. On a south-facing rock surface, isolated hypothalli grew at a similar rate to intact thalli for 2 months. Growth then declined and the hypothalli disappeared from the rock surface within 6 months. The effects of addition of carbohydrate suggest that the marginal hypothallus has the capacity to utilize exogeneous materials. However, in intact thalli in the field, the radial growth of the hypothallus is likely to be a result of transfer of materials from the areolae through hyphal connections.

Radial growth and carbohydrate levels in the lichen Parmelia conspersa on north and south facing rock surfaces

Parmelia conspersa (Ehrh. Ex Ach.)Ach. is a foliose lichen found more frequently on south facing compared with north facing rock surfaces in South Gwynedd, Wales, UK. The radial growth of thalli of P. conspersa from a north and a south facing rock surface was measured in situ at intervals of two months for 1 yr during 1990/1991. Mean annual radial growth rates were greater on the south compared with the north facing rock surface. In addition, the pattern of radial growth varied during the year with maximum growth recorded in the Feb/Mar. period especially on the south facing rock surface. The levels of ribitol, arabitol and mannitol were measured in individual lobes of P. conspersa collected from the same rock surfaces on 4 days (2 Jun; 7 July and 30 Nov. 1990 and 29 Mar. 1991) during 1990/1991. The total of the three carbohydrates varied between days; the levels of arbitol and ribitol being significantly lower in the 7 July sample on both north and south facing rock surfaces. In addition, the levels ribitol, arabitol and mannitol were higher on the south facing rock surface especially in the summer samples. The ratio of arabitol plus mannitol to ribitol and the mannitol/arabitol ratio varied more between days sampled than between north and south facing rock surfaces. The level of ribitol in individual thalli was positively correlated with arabitol on the north facing and with mannitol on the south facing slope. These results suggest that differences in the radial growth of P. conspersa thalli with aspect are more likely to reflect higher rates of photosynthesis on the south facing rock surface rather than large difference in the way carbohydrates were partitioned on the different surfaces. Lower radial growth rates may place P. conspersa at a competitive disadvantage on north facing rock surfaces.

Effect of osmotic and matrix potential on Saprolegnia diclina and S. ferax

Growth of biomass and sporulation of pathogenic and non-pathogenic Saprolegnia species was markedly decreased at reduced water potentials. Oogonium and zoosporangium formation were more sensitive to reduced osmotic and matrix potentials than growth in biomass. Although little difference was observed between the effects of matrix and osmotic potentials, the Saprolegnia species investigated responded differently to those solutes utilized in control of osmotic potential. Biomas, oogonium and zoosporangium formation were greater in the presence of reduced osmotic potentials mediated by mannitol than equivalent potentials mediated by potassium chloride. Endogenous potassium levels varied little with reduced matrix or osmotic potentials. Conversly, mannitol content of colonies exposed to reduced osmotic potentials mediated by mannitol initailly increased while endogenous amino acid levels were observed to rise in response to moderately reduced water potentials. Sensitivity of Saprolegnia species to reduced potantials and effects on substrate colonization are discussed in the light of these observations.