Showing papers in "New Phytologist in 1984"

A hypothesis relating critical potassium concentrations for growth to the distribution and functions of this ion in the plant cell

Abstract: Summary The growth and metabolism of plants in response to different concentrations of tissue K is discussed in relation to current knowledge about the distribution and functions of this ion in plant cells. In the cytoplasm, K has an important role in providing the correct ionic environment for metabolic processes. The ionic requirements of protein synthesis seem to be particularly important in determining the composition of the cytoplasm. Potassium is not replaceable in its cytoplasmic functions and the plant probably needs to maintain the cytoplasmic concentration of K in the range of 100 to 200 mM. Potassium salts in the vacuole are involved in the generation of turgor but when unavailable they can be replaced by other solutes. Salts of other cations such as Na and Mg are often a readily available alternative to K but in their absence organic solutes must be accumulated. With these observations as a basis, a model is proposed in which, as the concentration of K in the tissue declines, the concentration in the cytoplasm is initially maintained constant, while that in the vacuole decreases. In order to maintain turgor, alternative solutes are accumulated in the vacuole as replacements for K. It is assumed that K in the vacuole can only drop to a certain minimum level and, once this is reached, any further decline of tissue K must be at the expense of that in the cytoplasm. This leads to a decrease in the rate of metabolic processes that depend on K and so to a decline in growth. The hypothesis explains the observed relationships between growth and concentrations of K in tissues, and their modification by Na and other cations.

The effect of phosphorus on the formation of hyphae in soil by thevesicular‐arbuscular mycorrhizal fungus, glomus fasciculatum

Abstract: Summary Hyphae formed in soil by the vesicular-arbuscular (VA) mycorrhizal fungus, Glomusfasciculatum (Thaxter sensu Gerd.) Gerd. and Trappe were extracted using the membrane filter techniqe and their length was estimated by the grid intersection method. The effect of phosphorus on the formation of this external mycelium was assessed after sampling procedures had been investigated. Phosphorus supply was varied from amounts severely deficient to those adequate for the growth of subterranean clover. After 6 weeks, the alleviation of severe phosphorus deficiency increased both the length of infected root and the length of external hyphae per centimetre of infected root. Further additions of phosphorus decreased both of these measurements. However, the level of added phosphorus at which the most external hyphae was formed per centimetre of infected root was higher than the level of phosphorus which gave the greatest length of infected root. The increase in phosphorus supply which gave the greatest increase in the length of external hyphae per centimetre of infected root also decreased the formation of vesicles within infected roots. At phosphate levels adequate for growth of mycorrhizal plants, there was little development of either external hyphae in soil or vesicles within the mycorrhizal roots.

A cost‐benefit analysis of photon absorption by photosynthetic unicells

Abstract: Summary An attempt is made to provide a cost-benefit analysis of light harvesting by microalgal cells. The costs relate to the number of mol photons which the cell must absorb to produce a quantity of light-harvesting apparatus containing 1 mol of chromophore; the fraction of the dry weight which is devoted to the light-harvesting machinery containing 1 mol of chromophore; and the number of mols of nitrogen which are used in producing the light-harvesting machinery containing one mol of chromophore. These costs relate to resources (photons; fraction of cell biomass; nitrogen) which may restrict the growth of microalgae in low-light environments (co-limitation by light and nitrogen in the case of nitrogen). The benefits relate to the photon absorption rate in a given light field (photon flux density and spectral distribution) per mol chromophore. Maximum photon absorption rate per mol chromophore requires that the specific absorption coefficient (e) of the pigment shall be high in the wavelength range to which the organism is exposed, and that self-shading (the ‘package’ effect) be minimized by having a large area of cell exposed to the incident photons per mol of chromophore. The costs of the light-harvesting machinery were estimated in terms of photons absorbed by the cell per mol of chromophore (plus associated protein and lipid) synthesized, using known biochemical pathways from carbon dioxide, nitrogen source (ammonium or nitrate) and photons to the light-harvesting apparatus. The fraction of the cell dry weight occupied by light-harvesting apparatus containing one mol of chromophore was deduced from the mass of protein and lipid associated with one mol of the various chromophores. The nitrogen cost was derived from the mols of nitrogen found in the light-harvesting machinery containing 1 mol of each of the various chromophores. These estimates show that, for all three criteria enumerated above, the cheapest tight-harvesting apparati are integral complexes containing chlorophylls a + b + carotenoids in chlorophytes and chlorophylls a+c2 (± C1) + carotenoids in Chromophytes, and the most expensive are the phycocyanins and allophycocyanins of Phycobiliphytes. The benefits of the various kinds of light-harvesting machinery were estimated in terms of the number of photons which were absorbed from a given light field per unit chromophore in solution and, more realistically, in vivo in cells of various sizes. The mean specific coefficient over the blue-green waveband characteristic of ‘aquatic shade’ showed that (in the absence of self-shading) the light-harvesting machinery characteristic of Chlorophytes and Chromophytes was generally superior to that of algae containing phycobilins and, especially, phycocyanin and allophycocyanin. When this disadvantage of the phycocyanins in terms of photon absorption rate per mol chromophore is compounded by considering the high energy (and fraction of biomass, and nitrogen) costs of synthesis of the phycocyanins, these pigments would appear to be contra-indicated as light-harvesting pigments for shade-adapted microalgae. Nevertheless, the phycocyanins occur in Cyanobacterial and Cryptophycean phytoplankters. A partial offset of the high costs of synthesis of the peripheral (phycobilin) light-harvesting complexes may derive from reduced H+ leakage through thylakoid membranes of organisms containing these complexes since the lipid bilayer area per mol chromophore is lower in Phycobiliphytes. A mismatch between prediction and reality analogous to that found for Phycobiliphyte exploitation of extreme shade environments is found when we examine the surface area of organism exposed to incident photons per mol chromophore. While many shade-adapted phytoplankters are small spheres up to a few tens of μm3 in volume or, if of larger volume, are cylinders of small radius or are flattened, there are also phytoplankters of shaded habitats with small projected areas per mol chromophore and hence with an inefficient use of light-harvesting machinery due to self-shading. It would appear that, while cost-benefit analyses of light-harvesting provide a partial answer to the problem of how microalgal cells can grow at very low photon flux densities, there are many exceptions to the generalisations which the cost-benefit analysis generates. These exceptions demand further study.

Rhizosphere bacterial population responses to root colonization by a vesicular-arbuscular mycorrh1zal fungus *†

Abstract: Summary A mixture of bacteria and a vesicular-arbuscular mycorrhizal fungus isolated from field-collected sods of blue grama (Bouteloua gracilis (H.B.K.) Griffiths) were tested for their interaction in the rhizosphere of pot-grown blue grama plants. Populations of the inoculated bacterial species and actinomycete populations, as influenced by the presence or absence of Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappe, were enumerated by dilution plate counts from rhizosphere and non-rhizosphere soil samples. Total bacterial counts and the population of one bacterial species in the non-rhizosphere soil of pots containing plants were significantly greater than in soil of pots without plants. The population of two bacterial species and actinomycetes were not significantly different in the non-rhizosphere soil of both mycorrhizal and non-mycorrhizal plant treatments when compared to the soil of pots without plants. In the rhizosphere of mycorrhizal plants, the total bacterial population and colony counts of one of the four bacterial isolates, when expressed as colony-forming units (CFU) per gram of root dry weight, were significantly reduced compared with controls. The numbers of CFU per gram of rhizosphere soil of one bacterial species were significantly increased by the presence of the mycorrhizal fungus. Although no significant negative correlation was observed between populations of bacterial species in the rhizosphere soils, significant positive correlations between specific bacterial populations depended on whether or not the roots were mycorrhizal.

Influence of vesicular–arbuscular mycorrhiza on the hydraulic conductivity of roots of two citrus rootstocks

Abstract: Summary Carrizo citrange (CC) and sour orange (SO) seedlings were grown in a low phosphorus (P) sandy soil and either inoculated with Glomus intraradices Schenck & Smith or fertilized with soluble P. Mycorrhizal seedlings had nutritionally sufficient levels of leaf P, non-mycorrhizal plants of similar size were P-deficient. The root–shoot ratio of both rootstocks was reduced by mycorrhizal colonization, but root hydraulic conductivity per unit root length of mycorrhizal CC and SO was more than twice that of non-mycorrhizal seedlings under well-watered conditions. Mycorrhizal plants also had significantly higher transpiration rates when standardized on a root length basis, and greater transpiration appeared to be related to the increased conductivity of roots. Flow of water to roots via hyphae alone could not account for the greater water uptake by mycorrhizal roots. Apparently, mycorrhizal enchancement of P nutrition was primarily responsible for the greater conductivity of roots since no differences were found between root hydraulic conductivity of mycorrhizal and non-mycorrhizal CC of equal P status under well-watered conditions.

Diurnal changes in water relations and abscisic acid in field‐grown vitis vinifera cultivars

Abstract: Summary Diurnal changes in stomatal conductance and accompanying changes in the abscisic acid (ABA) content of expressed xylem sap were studied in unstressed grapevines (Vitis vinifera L. cvs Riesling and Silvaner) growing in the field in Adelaide, South Australia. Stomatal conductance increased rapidly during the early morning reaching a maximum in both cultivars at about 1000 h. Conductance during the latter part of the day was considerably reduced despite relatively mild environmental conditions. Quantitative analysis of ABA in untreated sap was carried out by high performance liquid chromatography (HPLC). The concentration of ABA in xylem sap varied from 1.5 × 10−4 mol m−3in the morning to 5 × 10−4 mol m−3in the late afternoon. Uptake of ABA from the xylem adequately accounted for diurnal changes in leaf ABA. Gas exchange analysis of detached field-grown leaves showed that ABA, supplied via the transpiration stream, caused substantial reductions in stomatal conductance and assimilation when supplied at a concentration equivalent to that in xylem sap [5 × 10−4 mol m−3 (+) ABA]. The reduction in assimilation was entirely stomatal in origin. It is suggested that ABA uptake from the xylem causes partial stomatal closure during the afternoon and plays a key role in the control of stomatal conductance in field grown vines by optimizing the relationship between CO2 uptake and water loss.

Studies on metal uptake by plants from serpentine and non-serpentine populations of thlaspi goesingense hálácsy (crycuferae).

Abstract: Plants of a serpentine (S) and a calcareous (C) population-type of Thlaspi goesingense Halacsy were raised from seed in various serpentine and artificial limestone soils with and without Zn amendment. S- and C-plants grown on metal-enriched substrates accumulated similar, extremely high concentrations of Ni, Zn, Co and Mn in the above-ground dry matter; they also showed a marked reduction of Ca uptake, which was combined with increased Mg uptake, in all the serpentine treatments. The fate of the Ni stored in seeds was followed when S-plants were grown in normal soil. Nickel was conserved and merely diluted within the plant dry matter as growth proceeded. Seedlings transplanted from serpentine soil to non-nickeliferous soil showed similar dilution effects, suggesting little or no overall Ni loss from the plant. The similar metal-uptake characteristics and performance of plants from the two populations studied suggests a constitutional metal tolerance within the species and the existence of a non-specific metal detoxification system.

The effects of elevated co2 on plants

Abstract: Summary Four populations of Phlox drummondii and one population each of Datura stramonium and Abutilon theophrasti were grown in six growth chambers at 300, 600 and 900 μ l l−1CO2, all other environmental variables remaining constant. Changes in timing and numbers of flowers produced were species- and population-dependent. In general, P. drummondii and D. stramonium flowered earlier under high CO2 while A. theophrasti was not affected. Significant population × CO2 interactions were found for several flower production characters in P. drummondii, indicating differential response to elevated CO2 levels even within a species. In D. stramonium, increased biomass in high CO2 caused significantly larger fruits to be formed, but there was no significant increase in seed number. In A. theophrasti, individual seed weight increased with increasing CO2, but total seed weight per plant remained constant. These results are discussed in relation to their possible implications to plant community structure, and the effects on higher trophic levels (e.g. pollinators and plant predators). Qualitative as well as quantitative changes in plants in response to high CO2 must be studied with care to ensure correct predictions of the effects of the global rise in CO2.

Differences between sexes in floral morphology, nectar production and insect visits in a dioecious species, silene dioica

Abstract: Summary Silene dioica (L.) Clairv. was studied in several wild populations near Swansea and in an experimental population to determine the extent of differences between the sexes in floral morphology, nectar production, and insect visits to flowers, and the effects of these differences on seed set. Male flowers are larger than female flowers and are borne more densely over a longer period. Male flowers have longer and more frequently white corona scales which may mimic the projecting white styles of female flowers. Male and female flowers were observed to secrete similar quantities of nectar sugar on the first morning of opening, but female flowers secreted more than males subsequently, and the total volumes of nectar secreted by female flowers were considerably greater. Male plants varied more than females in the total quantity of nectar sugar secreted per flower. Nectar concentration was nearly always greater in male flowers than in female flowers. Bumblebees, particularly Bombus hortorum (L.), B. pascuorum (Scop.) and B. terrestris (L.), were the most important visitors to the. flowers of S. dioica in the study sites, with some honeybees [Apis mellifera (L.)], butterflies (mainly Pieridae) and hoverflies (Syrphidae). Some visitors, including those that were most numerous at some sites, made most or all of their visits illegitimately. In the field in 1980 many visitors were observed to discriminate in favour of male flowers, which were much more numerous than female flowers. In the experimental population in 1981, many visitors discriminated in favour of female flowers, which were more numerous than in the field populations but were still substantially outnumbered by male flowers. Some individual bumblebees showed a change in preferences during the day, disciminating in favour of female flowers only during the afternoon, when the nectar resources available from female flowers were proportionately greater. The total numbers of visits that were observed to female and male flowers in the experimental population corresponded closely with the total quantities of nectar sugar available from each sex. The importance of the different insect visitors and of their discrimination between sexes in pollination is discussed. Seed set did not appear to be reduced by discrimination or insufficiency of pollinator visits in closely spaced plants, but was reduced in plants isolated by distances of 15 m or greater.

Dinophyte flagella: a cost‐benefit analysis

Abstract: The mobility of dinophytes allows them to migrate dielly, in a vertically stratified environment, between deeper regions where concentrations of nutrients (principally nitrate) are higher but light low, and less deep regions with lower nutrient concentrations but more light. When there is a substantial (several metres) depth range in which dinophytes can grow but, at any given depth, growth is restricted by both nitrate and light availability, vertical migration from a deeper station at night to a less deep station during the day could increase the availability of light and nitrate to a dinophyte cell over a complete light-dark cycle. Cost-benefit analysis shows that the benefits of migration (increased acquisition of photons and nitrate relative to non-migratory cells) could very substantially exceed the costs (the energy and nitrogen required to produce and operate the flagella apparatus and any additional nitrate or photon-harvesting machinery which may be required as a result of migration). It is therefore concluded that, in some habitats, vertical migration could increase the growth rates of dinophytes. The occurrence of such increases under natural conditions, and their selective significance, await further investigation.

Studies on nitrate reductase in british angiosperms

Abstract: Summary Leaf tissue from over 100 species, representing 40 families (mostly native British plants from natural populations), was assayed for in vivo nitrate reductase (NR) activity. From studies on the optimal assay conditions for the leaf tissue of 18 species, an assay procedure suitable for most species was devised. Higher levels of NR activity were measured in the leaves of ruderal species (mean NR activity = 4.39 ± 0.84 μmolh−1 g−1 fresh weight) than in woodland-edge species (mean NR activity = 1.36 ± 0.29 μmolh−1 g−1 fresh weight). A survey of NR activity in the leaves of 41 woody species showed that most contained significant levels of NR activity (> 1.0μmol h−1 g−1 fresh weight) suggesting that nitrate assimilation was appreciable in these species.

The effect of the fungus Atkinsonella hypoxylon (Clavicipitaceae) on the reproductive system and demography of the grass Danthonia spicata

Abstract: The Balansiae (Clavicipitaceae, Ascomycetes) consists of five genera of fungi that parasitize grasses, sedges and rushes. Most or all species produce alkaloids that are toxic to mammalian and insect herbivores. Previous reports suggest that infected grasses are more vigorous than uninfected grasses and that the proportion of infected individuals is greater in older populations. In this study individual plants or genotypes of the grass Danthonia spicata (L.) Beauv. Infected by the fungus Atkinsonella hypoxylon (Peck) Diehl. were compared with uninfected genotypes from the same population. Danthonia spicata typically bears dimorphic chasmogamous and cleistogamous flowers but in the study population infected plants were partially sterilized, producing only cleistogamous flowers; uninfected plants produced chasmogamous and cleistogamous flowers. Two experimental comparisons were conducted by propagating infected and uninfected genotypes into clones, or ramets, which were then planted back into their native site. Infected ramets had higher survival and growth rates, but lower flower production, than uninfected ramets both in random field transects and in field competition studies between D. spicata and the grass Anthoxanthum odoratum L. The results indicate infected individuals have higher survival and growth rates but reduced fecundity compared to uninfected individuals.

The development and ultrastructure of ectomycorrhizas

Abstract: Summary The ultrastructure of representative types of ectomycorrhizal root of pine was analysed after collection at different times of year from March to June, using scanning and transmission electron microscopy. In the Pre-Hartig net zone (PHZ) near the root apex, a well-developed sheath is present in which are incorporated sloughed root cap cells. There is evidence of dissolution of the host cell wall adjacent to fungal elements in some cells. Sheath structure in the Hartig net zone (HZ) is similar to that in the PHZ, but the residual cells here are collapsed root hairs. Glycogen and polyphosphate granules are abundant in the sheath in March but their number declines in later months. The Hartig net is made up of a pseudoparenchyma of infolded fungal elements and the cortical cells adjacent to it are generally living. No basic difference between niycorrhizal and non-mycorrhizal cortical cells was observed, thus indicating that, in contrast to pathogenic attack, no adverse reaction to the fungal presence occurs. Reasons for seasonal changes of storage compounds are discussed.

Post-medieval and recent changes in british vegetation: the culmination of human influence.

Abstract: SUMMARY Although measurable climatic fluctuation in temperature and wetness has occurred between A.D. 1700 and 1984, lack of earlier records and the complicating effects of human influence militate against detection of vegetational response in Britain. Vegetational change during this period is thus regarded as almost entirely the result of anthropogenic factors. Agriculture, now affecting c. 80% of Britain's surface, is overwhelmingly the most important such factor. Original vegetation had been mostly destroyed or heavily modified by a.d. 1700, notably through removal of the former forest cover and draining of lowland wetlands. The last 300 years have seen continuing development of farming and other land uses, to the point where very little natural vegetation now remains, and the main extent of semi-natural types is located in the uplands. Many distinctive semi-natural vegetation types actually created by man over several centuries of low input-low output management have decreased greatly since A.D. 1700, and rate of loss has even accelerated since 1940-1950. Only 5% of permanent lowland grasslands now remain agriculturally unimproved neutral grassland and only an estimated 20% of the extent of pre-1940 calcareous grassland is now assignable to that category floristically. Lowland acidic heath has declined by 78% in area since A.D. 1830, and by 40% of the 1950 level. Lowland rich fens are reduced to scattered remnants, the largest expanse (the East Anglian Fenland) having decreased from 3380 km2 in ad 1630 to 10 km2 in 1984. Forest cover was reduced to 5·4% by 1924, but has increased to 9·4%, largely through planting of alien conifers in upland districts. Since 1930 an overall 46 % of ancient semi-natural broadleaved woodland has been lost, largely by conversion to conifer plantation but also through grubbing out. An estimated 300000 ha of ancient semi-natural woodland remains, representing 14% of existing woodland and 1·3% of Britain's land surface. The total extent of hedges in England and Wales was reduced by an estimated 140000 miles (28%) between 1946-1947 and 1974. Urban–industrial growth, including roads, railways, water-use, mineral extraction and energy generation, has directly obliterated large areas of vegetation and caused chemical pollution. Waste ground associated with these activities often develops semi-natural plant communities through spontaneous succession, and may become floristically rich. Pollution affects especially fresh water through both toxic residues and eutrophication from nutrient enrichment (including run-off of agricultural fertilizers). Atmospheric pollution causes acid deposition especially damaging locally to the lichen flora, and induces biological changes in lakes and rivers in districts of acidic rocks. Recreational pressures have caused increasing disturbance to vegetation locally, and less directly through working of Sphagnum bogs for horticultural moss litter and surface mining of limestone pavements for rockery stone. Collecting of plants as a hobby has also brought some species, notably rare ferns, orchids and‘alpines’to dangerously low population levels, or even local extinction. The British flora has been profoundly modified by human influence. Only 19 vascular species are known to have become extinct, but many have declined greatly during the last 300 years and, for species in some habitats, especially semi-natural grasslands, rate of decrease has accelerated since 1940. Of 317 nationally rare species 117 (37%) have declined by at least 33 % since 1930. Human agency has also introduced to Britain at least 239 plant species which now grow wild, and extended the distribution of at least 363 others. Some are still increasing but others, notably weeds of arable land, are decreasing again. Conservation measures include setting up nature reserves to contain adequate representation of all semi-natural and natural vegetation types, and of populations of all native plant species. The aim is to give protected area status to c. 10% of Britain's surface through appropriate control and management designed to maintain or enhance the nature conservation interest. At present c. 1% is managed as nature reserve of one or other category and another 6% has been designated as Sites of Special Scientific Interest. Wild plants are now legally protected against unauthorized picking and uprooting, and 61 rare species are protected by special measures. Wider environmental conservation measures to reduce damaging human impact on vegetation and flora include inputs to and by local authorities and their planning system, persuasion of landowners and resource managers, and pollution control; supported by an educational and publicity programme, and by relevant research and information.

Sucrose accumulation and the initiation of fructan biosynthesis in Lolium temulentum L.

Abstract: Summary Plants of Lolium temulentum L. grown at 20 °C were transferred to 5 °C at the beginning of the light period. Sucrose accumulation was much more marked at low temperatures, with increases in rate detectable about 1 h following transfer. Exposure to low temperature over 7 d resulted in appearance of fructan, initially as oligosaccharides, and eventually as polymers of higher molecular weight. Sucrose accumulation was most marked in the blades of mature leaves, whereas the accumulation of fructan was maximal in the meristernatic regions of developing leaves. Chilling also led to a rise in activity of sucrose: sucrose fructosyltransferase, the first enzyme specifically involved in the biosynthesis of fructans. This activity was also higher in leaf meristems than in other regions of the plant.

Effects of soil trenching and coring on the formation of ectomycorrhizas on birch seedlings grown around mature trees

Abstract: Initially non-mycorrhizal seedlings of birch (Betula pendula Roth.) were grown for a season around two 11-year-old birch trees on an experimental site south of Edinburgh. Some seedlings (non-isolated) were planted in undisturbed positions, others (trenched) in volumes of soil isolated from the mature tree by trenches, and yet others (cored) in cores of soil (8 × 10 cm diameter) that had been removed and replaced immediately. Mycorrhizas of Lactarius pubescens (Fr. ex Krombh.) Fr., Lactarius glyciosmus (Fr. ex Fr.) Fr., and Leccinum scabrum (Bull. ex Fr.) S. F. Gray and Leccinum roseofracta Watl. developed abundantly on the non-isolated seedlings but usually very poorly on the trenched and cored seedlings. That some development of Leccinum mycorrhizas occurred in trenched areas is attributed to incomplete severance of roots. In contrast, mycorrhizas of other fungi, for example Hebeloma spp. and Inocybe spp., developed most abundantly on trenched and cored seedlings rather than on non-isolated seedlings. The results demonstrate an important difference between types of ectomycorrhizal fungus. Lactarius and Leccinum spp. seem to depend on a continuing supply of photosynthate from a mature tree in order to colonize seedling roots, whereas several other mycorrhizal fungi show no such dependence. The results also indicate an important role for mycelial strands in establishment of mycorrhizas by some fungi (Lactarius pubescens and Leccinum spp.) and thus parallel previous reports on the importance of mycelial strands in colonization of substrata by saprophytic and pathogenic fungi.

New nodulating legume trees from south‐east brazil

Abstract: Summary An extensive search for active (acetylene reducing) nitrogen-fixing root nodules was made amongst Brazilian forest legumes. Seven new nodulated species, including two new nodulating genera, were found in the Caesalpinioideae, 18 new species in the Mimosoideae and 30 new-species, including four new genera, in the Papilionoideae. All have potential importance in reafforestation. Fifty-eight new records of non-nodulating species are included. The morphology of all nodules was studied and is discussed in relation to taxonomic position. Preliminary data on nodule structure are given.

Anatomical features of the seed coat of acacia kempeana (mueller) which relate to increased germination rate induced by heat treatment

Abstract: Entry of water and subsequent germination of hard seeds of Acacia kempeana occurs after the strophiole lifts and cracks. The ‘opening’ of a strophiole is induced by heat and is effected through the splitting of thin-walled cells lying beneath short palisade cells within the strophiole. Estimated pressure increases of greater than 25 % within the vascular bundle, which passes directly under the thin-walled cells, might influence strophiolar lifting. The vascular bundle between the hilum and the adjacent strophiole is virtually devoid of xylern vessels, thereby preventing water entry through the hilum. However, following the strophiole, the vascular bundle obtains a maximum number of about 35 xylern vessels and is itself enclosed in this region by a noticeable group of over 150 thin-walled cells. Many of these thin-walled cells appear crushed along the route of the vascular bundle through the mesophyll layer to the end opposite the hilum. At the non-hilum end, the vascular bundle almost touches the internal side of the seed coat and thus entry of water would be facilitated through this zone to the internal endosperm and embryo.

Amines in barley laves infected by brown rust and their possible relevance to formation of ‘green islands’

Abstract: Summary Free amines, present in healthy leaves of barley and in leaves infected by brown rust (Puccinia hordei Otth.), have been partially characterized. The concentrations of five amines present in both kinds of tissue rise following infection and at least three extra amines are detectable in rusted tissue. The concentration of the polyamine, spermidine, increases rapidly by 6 d after inoculation and, at sporulation, it is six to seven times that in comparable healthy tissues. The possible involvement of polyamines in the ‘green island’ syndrome is discussed.

Comparative development of infection by three vesicular-arbuscular mycorrhizal fungi

Abstract: Summary The effect of inoculum density on the development of infection by Gigaspora decipiens Hall and Abbott, Glomus fasciculatum (Thaxter sensu Gerdemann) Gerdemann and Trappe and Glomus tenue Greenall (Hall) was investigated. From individual propagules, infection increased logarithmically for each fungus but, at higher inoculum levels, the infectivities differed. Of the three fungi, Gi. decipiens was the least, and Gl. fasciculatum the most infective. The fungi showed different strategies for infection spread. Glomus fasciculatum produced larger infection segments, and spread within the root more extensively than the other fungi. Gigaspora decipiens and GL tenue were more dependent on external spread by the formation of secondary entry points. The intensity of infections was affected by inoculum density, but not by the age of the infection.

On the ecology of short‐lived forbs in chalk grasslands: life‐history characteristics

Abstract: Summary The life-histories of Carlina vulgaris L., Daucus carota L., Euphrasia officinalis L., Gentianella germanica (Willd). Borner, Linum catharticum L., and Scabiosa columbaria L. in a chalk grassland in the Netherlands are described on the basis of frequent observations during a period of three and a half years. E. officinalis and a small part of the L. catharticum population were found to be annual. G. germanica and the major part of the L. catharticum population were biennial, whereas the other species took several years to reach maturity. All species except the major part of the Scabiosa columbaria population were monocarpic. Vegetation cover influenced the occurrence of short-lived forbs, but a large variation is found in the data. In D. carota, E. officinalis and Scabiosa columbaria seed production had a marked effect on seedling density in the following year. Shoot growth in Scabiosa columbaria seedlings varied at various vegetation densities. Furthermore, a marked correlation was found between rosette size and the probability of becoming mature in the subsequent year. Within one grassland, life-history types within the group of short-lived species are found to vary. On the one hand, there are the small-seeded annuals with high mortality, but rather constant density of mature plants (Euphrasia), and on the other hand there are the more conservative life histories (Carlina, Daucus and Scabiosa), with larger seeds, lower mortality rates, but higher age of first reproduction and considerable variation in the density' of individuals.

Competition for infection between vesicular‐arbuscular mycorrhizal fungi

Abstract: Summary The competitive abilities of three VAM fungi were investigated. Various pairs of fungi were interacted and the amounts of infection produced by mixed and single inocula were compared. The outcome of the interactions varied according to the fungi involved. Aggressiveness appeared to be inversely related to infectivity. In the interactions examined, both Glomus fasciculatum (Thaxter sensu Gerdemann) Gerdemann & Trappe and Glomus tenue (Greenall) Hall were less aggressive than Gigaspora decipiens Hall and Abbott. No interaction occurred between Gl. fasciculatum and GL tenue. Except in the interaction between Gl. fascicidatum and Gi. decipiens, the total amount of infection formed by mixed inoculation was similar to that formed by single inoculation of the more infective fungus of the interacting pair. The outcome of the interactions were shown to be affected, in some circumstances, by inoculum frequency, time of harvest, most incubation of the inoculum, and the spatial distributions of the interacting propagules.

Selenium toxicity in perennial ryegrass and white clover

Abstract: Summary Shoot concentrations of Se associated with a 10% reduction in dry matter yield were established for perennial ryegrass (Loliumperenne L.) and white clover (Trifolium repens L.) grown in sand culture. Selenite– treated plants had lower shoot concentrations of Se than those treated with selenate; the critical toxic values associated with a 10% reduction in growth were 48 and 320μg Se g− 1 dry matter shoots for ryegrass plants, and 160 and 330μeg1 dry matter shoots for white clover plants, respectively. Differences in the mode of absorption of selenite and selenate, and in the distribution and chemical form of Se found in the plant after absorption, probably account for this disparity in toxicity. While both selenate and selenite increased the Se concentration in the tissues of the plants to high levels, a greater proportion of the absorbed Se was transported to the shoots of the selenate-treated plants than of those treated with selenite. Increasing supplies of selenate were associated with a reduction in chlorophyll content. Contrary to the concept of a common uptake mechanism in the roots for selenate and sulphate, increasing supplies of selenate had a synergistic effect on S concentrations in the shoots rather than the expected antagonistic effect.

The influence of burial and dry-storage upon cyclic changes in dormancy, germination and response to light in seeds of various arable weeds

Abstract: In general, shallow burial (25 mm) reduced seedling emergence except in the case of B. sterilis, which also germinated at a depth of 150 mm but failed to emerge. Cyclic changes in dormancy of buried seeds were observed. Although sensitivity to light was enchanced by burial, germination and response to light quality varied between seasons. Dormancy of most species, including the winter annuals A. myosuroides, Veronica persica and Viola arvensis, was least in autumn and most in summer. Papaver rhoeas was least dormant in spring and S. media was least dormant in summer. Generally, red light promoted germination whereas far-red light or darkness was not stimulatory. However, A. myosuroides was more sensitive to far-red light in autumn than at other times of the year. Natural day-light promoted germination of A. myosuroides, S. media and Veronica arvensis. Germination of B. sterilis was delayed in natural daylight. Transfer of A. myosuroides, Papaver rhoeas, Veronica arvensis and Viola arvensis from darkness to daylight increased germination above that obtained under natural daylight. Germination of freshly collected seeds of Alopecurus myosuroides, Arrhenatherum elatius, Poa annua, Poa trivialis, Plantago major and Viola arvensis was promoted by short irradiations of red light. Dry-storage did not affect germination of Alopecurus myosuroides, Poa annua, Plantago major or Viola arvensis, but did affect dormancy and light requirement of Poa trivialis. Burial of seeds of Alopecurus myosuroides, Poa trivialis and Plantago major increased their sensitivity to far-red light. It is concluded that cyclic changes in seed dormancy and associated light sensitivity are determined seasonally and result from changing environmental factors and that they are of adaptive importance for survival.

Anatomical variation in first leaves of nine tritucum genotypes, and its relationship to photosynthetic capacity

Abstract: Summary The variation in over 20 leaf characters including morphological, anatomical and cellular features have been examined in nine Triticum genotypes at three levels of ploidy. The characters included transectional leaf area, leaf width, distance between vascular bundles, volume occupied by each cell type, cell size and number, and chloroplast size and number. Analysis of these characters showed that the major components of the variation were related to the ploidy level and the genotype of the species and cultivars examined. The measured characters most strongly influenced by ploidy level were cell size, chloroplast number and leaf thickness. Genetic variation accounted for the variation in all other measured characters. The interaction of genetic variation with the major effects attributable to ploidy level creates a characteristic anatomical ‘fingerprint’ for each genotype. The observation that photosynthetic capacity is negatively correlated with the ratio of mesophyll cell size to nuclear genome size, and with no other character, is discussed.

Phosphate uptake by goodyera repens in relation to mycorrhizal infection

Abstract: Summary The rate of P uptake per unit length of root of adult plants of the orchid Goodyera repens Br., infected with the mycorrhizal endophyte Rhizoctonia goodyerae-repentis Constantin and Dufour, was up to 100 times greater than that of non-mycorrhizal plants. When the active extramatrical mycelium was reduced by treatment with thiabendazole fungicide, the relative growth rate of mycorrhizal plants was reduced in low P conditions but not in P rich conditions. Rates of P uptake into fungicide-treated plants were Jess than those of mycorrhizal plants at both P levels. Translocation of 32P from a point source to the plant via the extramatrical mycelium was demonstrated and could be inhibited by the application of thiabendazole. In a model system incorporating a perlite/Vermiculite substrate, translocation of 32P via the endophyte took place over distances up to 9 cm. The mycorrhiza of adult G. repens appears similar to other mycorrhizas in its ability to enhance growth and P uptake.

Development of infection by a vesicular–arbuscular mycorrhizal fungus in brassica napus l. and trifolium subterraneum l.

Abstract: Summary The sequence of stages involved in the development of infection by Glomus caledonium (Nicol. & Gerd.) Trappe & Gerd. in Brassica napus L. (spring rape) and Trifolium subterraneum L. (subterranean clover) was examined. When plants grown at similar relative levels of phosphate were compared, B. napus reduced the rate of all stages of the processes leading to colonization. Although maximum values of germination, hyphal extension, and appressorial formation and adhesion were similar in B. napus and T. subterraneum, in B. napus fewer appressoria produced penetration pegs and some penetration pegs failed to initiate colonies. In addition, in B. napus only a third of the colonies possessed arbuscules whereas in T. subterraneum all colonies had arbuscules. The rate of death of epidermal and cortical cells was similar between the species and did not explain the reduced colonization of B. napus. Chemical rather than physical factors associated with B. napus appeared to be involved in reducing the rate of development of each growth stage.

Newferry and the boreal-atlantic transition.

Abstract: The establishment and expansion of alder [Alnus glutinosa (L.) Gaertner] in Britain between approx. 8000 and 5000 years ago is discussed in relation to changes of climate and sea-level, and to human influences. Detailed pollen analyses of Mesolithic layers at Newferry, Co. Antrim are presented. Evidence from this and other sites suggest widespread disturbance of forest cover which, by locally reducing competition, (cf. McVean, 1956) appears to have been one of the factors in the establishment of alder. Estuarine habitats are suggested as having importance in the spread of alder but the onset of climatic wetness was probably not a key factor (as bas often hitherto been supposed) other than in the sense of being permissive.

Observations on free‐living trebouxia de puymalyand pseudotrebouxia archibald, and evidence that both symbionts from xanthoria parietina (l.) th. fr. can be found free‐living in nature

Abstract: Summary The presence of free-living Trebouxia and/or Pseudotrebouxia is documented. Several free-living phycobionts were brought into culture and compared to the cultured Xanthoria parietina phycobiont. With the aid of both morphological and immunological criteria, we provide evidence that both the mycobiont and phycobiont of X. parietina can be found free-living in nature. The potential for resynthesis between bionts under natural conditions is discussed.