Anthony J. Davy

Native oak chloroplasts reveal an ancient divide across Europe

Glacial refugia and postglacial migration are major factors responsible for the present patterns of genetic variation we see in natural populations. Traditionally postglacial history has been inferred from fossil data, but new molecular techniques permit historical information to be gleaned from present populations. The chloroplast tRNALeu1 intron contains regions which have been highly conserved over a billion years of chloroplast evolution. Surprisingly, in one of these regions which has remained invariant for all photosynthetic organisms so far studied, we have found intraspecific site polymorphism. This polymorphism occurs in two European oaks, Quercus robur and Q. petraea, indicating hybridisation and introgression between them. Two distinct chloroplast types occur and are distributed geographically as eastern and western forms suggesting that these oaks are each derived from at least two separate glacial refugia.

POPULATION BIOLOGY OF THE SALT MARSH ANNUAL SALICORNIA EUROPAEA agg.

SUMMARY tion to the adverse effects of hypersalinity in the upper marsh. (3) The demography of the two populations was examined. Permanent quadrats were mapped or photographed to follow the fate of seedlings; the reproductive and growth performances of individuals in relation to plant density were measured. (4) Overall, demographic trends in the two populations were similar. The probability of a seed giving rise to an adult plant was very low; most seeds died without germinating. Mortality of plants appeared to be density-independent, but there was a significant negative density-dependent relationship between number of seeds per plant and the density of Salicornia plants. (5) By midsummer the seed bank in the sediments was exhausted, so each generation of Salicornia at both sites appears to be distinct. (6) A model is given which describes the population dynamics of Salicornia in terms of density-dependent regulation of seed number and density-independent mortality. The reasons for the prevailing densities are discussed. In this study the behaviour of two populations of Salicornia europaea agg., from the upper marsh and from the open sandy mudflats of the lower marsh, respectively, has been examined in relation to environmental heterogeneity at Stiffkey salt marsh on the north Norfolk coast, England. In cultivation, plants from the two populations exhibit different

Salt stimulation of growth and photosynthesis in an extreme halophyte, Arthrocnemum macrostachyum

Halophytes that are capable of tolerating a wide range of salinity may grow best at intermediate salinities, but the physiological mechanisms underlying positive growth responses to salinity are not clear. This work investigated the growth of Arthrocnemum macrostachyum (Moric) C. Koch (a halophytic C3 shrub) over a wide range of salinities, and the extent to which its responses can be explained by photosynthetic physiology. Growth, gas exchange and chlorophyll fluorescence characteristics of plants were examined in a glasshouse experiment; tissue concentrations of photosynthetic pigments, ash, sodium, potassium, calcium and nitrogen were also determined. Plants showed marked stimulation of growth by salt, with a broad optimum of 171-510 mm NaCl for relative growth rate (RGR). Stimulation of RGR appeared to depend mainly on an increase in specific shoot area, whereas reduced RGR at high salinity (1030 mm) could be attributed to a combination of lower unit shoot (leaf) rate and lower shoot mass fraction. The non-saline treatment plants had the greatest fraction of non-photosynthetic, atrophied surface area. However, net photosynthesis (A) was also stimulated by NaCl, with an optimum of c. 510 mm NaCl. The responses of A to salinity could be accounted for largely by limitation by stomatal conductance (Gs) and intercellular CO(2) concentration (Ci). Even the most hypersaline treatment apparently had no effect on photosystem II (PSII) function, and this resistance could be an important strategy for this halophyte in saline soils. In contrast, Fv/Fm indicated that absence of salt represents an environmental stress for A. macrostachyum and this could be a contributory factor to salt stimulation of A. Notwithstanding the importance of the ability to develop and maintain assimilatory surface area under saline conditions, stimulatory effects on A also appear to be part of a suite of halophytic adaptations in this plant.

Predicting the hydraulic forces on submerged macrophytes from current velocity, biomass and morphology

Abstract Aquatic macrophytes are important in stabilising moderately eutrophic, shallow freshwater lakes in the clear-water state. The failure of macrophyte recovery in lakes with very soft, highly organic sediments that have been restored to clear water by biomanipulation (e.g. in the Norfolk Broads, UK) has suggested that the physical stability of the sediment may limit plant establishment. Hydraulic forces from water currents may be sufficient to break or remove plants. Our aim was to develop a simple model that could predict these forces from plant biomass, current velocity and plant form. We used an experimental flume to measure the hydraulic forces acting on shoots of 18 species of aquatic macrophyte of varying size and morphology. The hydraulic drag on the shoots was regressed on a theoretically derived predictor (shoot biomass × current velocity1.5). Such linear regressions proved to be highly significant for most species. The slopes of these lines represent species-specific, hydraulic roughness factors that are analogous to classical drag coefficients. Shoot architecture parameters describing leaf and shoot shape had significant effects on the hydraulic roughness factor. Leaf width and shoot stiffness individually did not have a significant influence, but in combination with shoot shape they were significant. This hydraulic model was validated for a subset of species using measurements from an independent set of shoots. When measured and predicted hydraulic forces were compared, the fit was generally very good, except for two species with morphological variations. This simple model, together with the plant-specific factors, provides a basis for predicting the hydraulic forces acting on the root systems of macrophytes under field conditions. This information should allow prediction of the physical stability of individual plants, as an aid to shallow-lake management.

Seasonal Patterns of Nitrogen Availability in Contrasting Soils in the Chiltern Hills

The availability of inorganic nitrogen in three strongly contrasting soils, which supported almost pure stands of Deschampsia caespitosa in the Chiltern Hills, was investigated during a study of the edaphic tolerance of the species. Seasonal changes in the rates of accumulation of nitrate- and ammonium-N, measured using an incubation method, were followed during 1971-72. The potential for nitrogen mineralization was assessed using incubations at 25? C while more realistic estimates of mineralization rates prevailing in the field were made with incubations at temperatures appropriate to the dates of collection. In the chalk soil (pH 7.6-8.3) and calcimorphic brown earth (pH 7.0-8.0) only nitrate-N accrued and at modest rates. The acid mull soil (pH 3.7-4.2) produced much more inorganic nitrogen; for much of the year this was substantially in the ammonium form but a pronounced spring peak in nitrification was observed during two successive years with some, although much less, nitrification evident for most of the study period. Nitrification could be detected in the calcareous soils even at temperatures as low as 2? C. The seasonal pattern of nitrogen release is discussed in relation to controlling environmental factors and microbial activity in the soil as well as the edaphic tolerance of D. caespitosa.

Using chloroplast DNA to trace postglacial migration routes of oaks into Britain

Postglacial migration is a major factor responsible for the patterns of genetic variation we see in natural populations. Fossil pollen data indicate that early postglacial colonists such as oak, were able to take both western and eastern migration routes into Britain. Analysis at a finer level is now permitted by the use of modern molecular techniques. A 13‐bp duplication in the chloroplast tRNALeul intron occurs in natural populations of East Anglian oaks, but is not found in other parts of Britain or from mainland Europe. The distribution of this marker suggests that the mutation occurred either in southern England, or during migration from the mainland, and became fixed in a source population from which East Anglia was colonized. Planting of non‐native trees for roadside boundaries and in the grounds of old houses and estates, explains the absence of the marker from some East Anglian oaks.

Silicon alleviates deleterious effects of high salinity on the halophytic grass Spartina densiflora.

The non-essential element silicon is known to improve plant fitness by alleviating the effects of biotic and abiotic stresses, particularly in crops. However, its possible role in the exceptional tolerance of halophytes to salinity has not been investigated. This study reports the effect of Si supply on the salinity tolerance of the halophytic grass Spartina densiflora; plants were treated with NaCl (0-680 mM), with or without silicon addition of 500 μM, in a glasshouse experiment. Plant responses were examined using growth analysis, combined with measurements of gas exchange, chlorophyll fluorescence and photosynthetic pigment concentrations. In addition, tissue concentrations of aluminium, calcium, copper, iron, potassium, magnesium, sodium, phosphorus and silicon were determined. Although high salinity decreased growth, this effect was alleviated by treatment with Si. Improved growth was associated with higher net photosynthetic rate (A), and greater water-use efficiency (WUE). Enhanced A at high salinity could be explained by beneficial effects of Si on the photochemical apparatus, and on chlorophyll concentrations. Ameliorative effects of Si were correlated with reduced sodium uptake, which was unrelated to a reduction in the transpiration rate, since Si-supplemented plants had higher stomatal conductances (G(s)). These plants also had higher tissue concentrations of essential nutrients, suggesting that Si had a positive effect on the mineral nutrient balance in salt-stressed plants. Si appears to play a significant role in salinity tolerance even in a halophyte, which has other, specific salt-tolerance mechanisms, through diverse protective effects on the photosynthetic apparatus, water-use efficiency and mineral nutrient balance.

Demography of Hieracium Pilosella in a Breck Grassland

(1) A 2-year demographic study was made of four adjacent populations of Hieracium pilosella on contrasting soil types in Breckland, in order to examine whether the population fluxes were different. Three of the four sites were rabbit-grazed, but at the fourth a rabbit-proof exclosure was constructed. (2) Recruitment of rosettes to all populations was the result of clonal growth, as seedling establishment failed to occur. However, sexual reproduction and clonal growth were closely linked; it was observed that an immature inflorescence needed to be produced before the production of stolons and the development of rosettes at the ends of the stolons could occur. Plants senesced after undergoing reproduction. (3) All the populations showed overall exponential decay rates, upon which was super-imposed an annual rhythm of natality and mortality. Initially all four populations showed fairly similar overall exponential decay rates, but midway through the study the rate of decline abruptly increased in the populations growing in acidic sand and chalk soil, with the result that one population became almost extinct, and in the other the age of the majority of individuals in the population consisted of young plants less than 0.5 years of age. (4) The absence of grazing resulted in the population in the exclosure consisting of a greater proportion of plants 2 years or older compared with the other sites. (5) The significance of these differences in the population fluxes at the sites is discussed in relation to changes in Breckland populations of H. pilosella reported earlier by Watt (1962), and to the exceptional drought of 1975-6.

Regenerative Potential of Elymus Farctus From Rhizome Fragments and Seed

(1) Both newly-germinated seedlings and shoots from single-node rhizome fragments of Elymus farctus were able to emerge after experimental burial with sand at a depth of 127 mm, but not from 178 mm. (2) Multi-node fragments had a greater probability of emergence than single-node ones at any depth, and also could emerge from greater depths. (3) Inhibition between buds on multi-node fragments led to reserves being concentrated into a single dominant shoot. Distal buds were more likely to become dominant than proximal buds. (4) Nodes of rhizomes taken from the dunes at monthly intervals showed seasonal variation in their ability to produce roots and shoots under controlled conditions. This regenerative ability was inversely related to the growth of the parent plants at the time of sampling, and could be increased by supplying exogenous nitrogen as nitrate. Seasonal patterns were still apparent, however, and concentration of total water-soluble carbohydrate was implicated as a further limiting factor. (5) Multi-node fragments are more flexible in their response to burial, because the likelihood of viable shoot production is increased and buds and resources are conserved against the possibility of further disturbance, whereas seed germination is an all-or-nothing gamble.

Carry-over of Differential Salt Tolerance in Plants Grown from Dimorphic Seeds of Suaeda splendens

BACKGROUND AND AIMS Halophytic species often show seed dimorphism, where seed morphs produced by a single individual may differ in germination characteristics. Particular morphs are adapted to different windows of opportunity for germination in the seasonally fluctuating and heterogeneous salt-marsh environment. The possibility that plants derived from the two morphs may also differ physiologically has not been investigated previously. METHODS Experiments were designed to investigate the germination characteristics of black and brown seed morphs of Suaeda splendens, an annual, C(4) shrub of non-tidal, saline steppes. The resulting seedlings were transferred to hydroponic culture to investigate their growth and photosynthetic (PSII photochemistry and gas exchange) responses to salinity. KEY RESULTS Black seeds germinated at low salinity but were particularly sensitive to increasing salt concentrations, and strongly inhibited by light. Brown seeds were unaffected by light, able to germinate at higher salinities and generally germinated more rapidly. Ungerminated black seeds maintained viability for longer than brown ones, particularly at high salinity. Seedlings derived from both seed morphs grew well at high salinity (400 mol m(-3) NaCl). However, seedlings derived from brown seeds performed poorly at low salinity, as reflected in relative growth rate, numbers of branches produced, F(v)/F(m) and net rate of CO(2) assimilation. CONCLUSIONS The seeds most likely to germinate at high salinity in the Mediterranean summer (brown ones) retain a requirement for higher salinity as seedlings that might be of adaptive value. On the other hand, black seeds, which are likely to delay germination until lower salinity prevails, produce seedlings that are less sensitive to salinity. It is not clear why performance at low salinity, later in the life cycle, might have been sacrificed by the brown seeds, to achieve higher fitness at the germination stage under high salinity. Analyses of adaptive syndromes associated with seed dimorphism may need to take account of differences over the entire life cycle, rather than just at the germination stage.