Content of water soluble carbohydrates under oxygen deprivation in plants with different flooding tolerance

We investigated the carbohydrate concentration of plant species naturally growing in habitats with a high risk of oxygen shortage (Senecio aquaticus Hill,Myosotis palustris (L.) L. em.Rchb) and congeneric species from drier sites (Senecio jacobaea L.,Myosotis arvensis (L.)Hill) Plants from the four species were cultivated in either nitrogen-flushed or aerated nutrient solution. Following oxygen shortage in the root environment a two—fourfold increase of carbohydrate content was found in the roots as well as in the shoots of the species examined. Although, the tendency in the response of both genera to the hypoxic conditions in the nutrient solution was the same, there were differences in concentration and composition of the water soluble carbohydrates. The flooding tolerantSenecio aquaticus accumulated the highest amounts of carbohydrates, in particular fructans (nearly 60% of the soluble carbohydrates, compared with 30% under aerated conditions). The increasing amount of sugars found under hypoxia-inducing conditions proved that substrate availability was not the limiting factor for survival under wetland conditions.     

Variation in the Structure and Response to Flooding of Root Aerenchyma in some Wetland Plants

The structure and response to flooding of root cortical aerenchyma(air space tissue) in a variety of wetland (flood-tolerant)species was investigated and compared with some flood-intolerantspecies. In some species aerenchyma consisted of enlarged schizogenousintercellular spaces and in others aerenchyma formation involvedlysigeny. Two types of lysigenous aerenchyma were distinguished.In the first the diaphragms between lacunae were arranged radiallyand consisted of both collapsed and intact cells. In the secondtype, which was confined to the Cyperaceae, the radial diaphragmscontained intact cells, and stretched between them were tangentially-arrangeddiaphragms of collapsed cells. Flooding in sand culture generally increased root porosity (airspace content) although there were exceptions. The flood-intolerantspecies Senecio jacobaea produced aerenchyma but did not survivelong-term flooding. Among the flood-tolerant species, Filipendulaulmaria did not produce extensive aerenchyma even when flooded.Eriophorum angustifolium and E. vaginatum produced extensiveaerenchyma under drained conditions which was not increasedby flooding. In Nardus stricta root porosity was increased bylow nutrient levels as well as by flooding. Aerenchyma, root cortex, wetland plants, waterlogging, flooding-tolerance, Ammophila arenaria, Brachypodium sylvalicum, Caltha palustris, Carex curia, Eriophorum vaginatum, Filipendula ulmaria, Glyceria maxima, Hieracium pilosella, Juncus effusus, Myosotis scorpioides, Nardus stricta, Narthecium ossifragum, Phalaris arundinacea, Senecio jacobaea, Trichophorum cespitosum     

Maternal effects and heterosis influence the fitness of plant hybrids

Here we tested two possible nonexclusive explanations for the maintenance of a hybrid swarm between Senecio jacobaea and Senecio aquaticus; first, that genotype-by-environment interactions involving water and nutrient clines are involved in hybrid fitness, and second, heterosis in early hybrid generations may provide an initial hybrid advantage that contributes to hybrid persistence. In three climate chamber studies, fitness and root growth were measured for parental species and natural and artificial F1 hybrids, in order to determine whether hybrids occur in habitats where they are more fit than parental species. Natural hybrids, which are generally back-crossed to S. jacobaea, always equaled S. jacobaea in growth characteristics. Maternal effects played a role in the fitness of F1 hybrids, with offspring from S. jacobaea mothers exhibiting higher fitness than those from S. aquaticus mothers, and compared with parental species and natural hybrids. Natural hybrids are not distributed in zones where they are most fit with respect to nutrient and water regimes. Superior fitness of early generation hybrids may contribute to hybrid swarm stability.     

Relationship between tissue sugar content, phloem import and lateral root initiation in wheat

Split‐root experiments were conducted to test the hypothesis that adjustments in lateral root initiation, as might occur in response to localized soil conditions, are determined by the sugar content of the root and do not depend on changes in the import of phloem‐translocated phytohormones. Wheat ( Triticum aesticum L. cv. Alexandria) seedlings were grown in hydroponics with their seminal roots divided between two compartments within the culture vessel. Two seminal roots of treated plants were supplied with standard nutrient solution supplemented with 50 m M glucose, whilst the remaining three roots received nutrient solution without glucose. Control plants had their roots divided in the same ratio, but both 'halves' received nutrient solution without glucose. Feeding glucose to one 'half' of the root system increased the frequency (number per unit length) of lateral root primordia in the fed axes. The increase was first observed 15 h after the start of treatment and was located within the apical 30 mm of root. At this time there was no significant treatment effect on the frequency of primordia in non‐fed axes. The enhanced initiation of lateral roots in glucose‐fed root tips was associated with an increase in their concentration of glucose and sucrose plus low molecular mass fructans. In contrast, there was a reduction in partitioning of ¹⁴C‐photosynthate to these root tips compared to the non‐fed roots of treated plants and controls. The results indicate that lateral root initiation can be stimulated by sugars in the absence of an increase in phloem translocation. It is proposed that proliferation of lateral roots in response to localized soil conditions, such as nutrient patches, may be signalled by an increase in sugar content of the tissue, rather than an altered flux of phytohormones or other material co‐transported with sucrose in the phloem.     

Natural hybridization between Senecio jacobaea and Senecio aquaticus: molecular and chemical evidence

Hybridization is known to be involved in a number of evolutionary processes, including species formation, and the generation of novel defence characteristics in plants. The genus Senecio of the Asteraceae family is highly speciose and has historically demonstrated significant levels of interspecific hybridization. The evolution of novel chemical defence characteristics may have contributed to the success of Senecio hybrids. Chemical defence against pathogens and herbivores has been studied extensively in the model species Senecio jacobaea, which is thought to hybridize in nature with Senecio aquaticus. Here, we use amplified fragment length polymorphisms (AFLPs) and pyrrolizidine alkaloid (PA) composition to confirm that natural hybridization occurs between S. jacobaea and the closely related species S. aquaticus. AFLPs are also used to estimate the ancestry of hybrids. We also demonstrate that even highly back-crossed hybrids can possess a unique mixture of defence chemicals specific to each of the parental species. This hybrid system may therefore prove to be useful in further studies of the role of hybridization in the evolution of plant defence and resistance.     

The effect of post-hypoxia on roots inSenecio andMyosotis species related to the glutathione system

This paper shows the effect of re-aeration following hypoxic pretreatment on the glutathione system in plants with different flooding tolerance. Re-aeration of hypoxically pretreated roots led to an increase of TBA-rm content indicating an accelerated lipid peroxidation (post-anoxic injury). Re-admission of oxygen resulted in a clear increase in the content of total glutathione in both flooding-intolerant speciesMyosotis arvensis andSenecio jacobaea. Simultaneously, the high ratio between reduced (GSH) and oxidized (GSSG) glutathione decreased in these species upon the onset of re-aeration, while the tolerantMyosotis palustris andSenecio aquaticus showed only little changes in contents of GSH and GSSG. An imbalance in GSH/GSSG ratio reflects oxidative stress. The glutathione reductase (GR) reacted very differently in the investigated genera. The metabolic response to varying oxygen pressure is much stronger in the flooding-intolerant species compared to species naturally growing in wetlands. The present results suggest that glutathione system is an important component in overcoming oxidative stress.     

Rabbit endozoochroy and seedbank build-up in an acidic grassland

The sources of seed for seedbank build-up in an acidic grassland were identified from analysis of differences in seedbank build-up over one year between plots where the input of rabbit pellets to seed-free soil had either been left or removed. In parallel, the flux of seed arriving in rabbit pellets was monitored. Pellet seed content and total seed input were highest in late summer/early autumn and again in the spring. The seed content of the pellets was dominated by a small number of species: Sagina apetala, Senecio jacobaea, Urtica dioica and Veronica arvensis. Smaller seeded species were more likely to be present as germinable seed in the pellets. Seedbank build-up as a result of wind, splash or adhesive dispersal totalled 547 seedsm². The additional effect of allowing seed input in pellets was 267 seedsm², though this increase was not significant. The total increase in seedbank over one year was equivalent to 15–20% of the seedbank present in undisturbed soil. Eight species showed a significant build-up of seed in the seedbank over one year as a result of all means of dispersal, but only Myosotis discolor showed a significantly higher soil germinable seed content in the plots where pellets had been allowed to remain in situ. The build-up of seed in the seedbank is contrasted with the build-up of vegetation on disturbed areas within the same system.     

Growth and biomass of mycorrhizal mycelia in coniferous forests along short natural nutrient gradients

Hybridization may lead to unique phytochemical expression in plant individuals. Hybrids may express novel combinations or extreme concentrations of secondary metabolites or, in some cases, produce metabolites novel to both parental species. Here we test whether there is evidence for extreme metabolite expression or novelty in F1 hybrids between Senecio aquaticus and Senecio jacobaea. Hybridization is thought to occur frequently within Senecio, and hybridization might facilitate secondary metabolite diversification within this genus. Parental species express different quantities of several classes of compounds known to be involved in antiherbivore defence, including pyrrolizidine alkaloids, chlorogenic acid, flavonoids and benzoquinoids. Hybrids demonstrate differential expression of some metabolites, producing lower concentrations of amino acids, and perhaps flavonoids, than either parental species. Despite evidence for quantitative hybrid novelty in this system, NMR profiling did not detect any novel compounds among the plant groups studied. Metabolomic profiling is a useful technique for identifying qualitative changes in major metabolites according to plant species and/or genotype, but is less useful for identifying small differences between plant groups, or differences in compounds expressed in low concentrations.     

Reproductive fitness of hybrids between Senecio jacobaea and S. aquaticus (Asteraceae)

Natural hybridization is increasingly recognized as an important process for the ecology and evolution of natural plant populations and species. There is a great need to initiate more studies based on natural populations in order to elucidate the possible role of hybrids in nature. The reproductive success of early generation hybrids can make or break hybrid lineages and may determine the genetic structure of hybrid swarms or the potential for gene flow through future generations, but studies of hybrid reproductive success are lacking. Here we measured components of male and female reproductive success in Senecio jacobaea and S. aquaticus (Asteraceae) species and F(1) hybrids between these species under laboratory conditions, and we measured reproductive output from crosses producing F(1), F(2), and backcross (BC) generation hybrids. F(1) hybrids were readily produced, and on average, the success of crosses producing subsequent generations (F(2), BC) decreased (though remained substantial), but the success of crosses was highly dependent on the genotypes involved. Also, F(1) hybrids were bigger, produced more flowers, and therefore produced more pollen than parental plants. Finally, crosses between parents were asymmetrical, such that S. aquaticus produced more and larger F(1) seeds than did S. jacobaea.     

Phylogeny reconstruction in the gap between too little and too much divergence: the closest relatives of Senecio jacobaea (Asteraceae) according to DNA sequences and AFLPs

The species composition and molecular phylogeny of Senecio sect. Jacobaea (Asteraceae; Senecioneae) were studied to identify the closest relatives of Senecio jacobaea. Maximum parsimony and Bayesian inference analyses of DNA sequence data of the plastid (the trnT-L igs, the trnL intron, two parts of the trnK intron, and the psbA-trnH igs) and nuclear genome (ITS1, 5.8S, and ITS2) showed these markers to be suitable to assess the species composition of sect. Jacobaea, identifying 24 species as members of this section. Of these, nine species were not previously assigned to the section. The selected DNA sequence regions, however, showed too little sequence divergence to be optimal for phylogenetic inference within sect. Jacobaea. In contrast, AFLPs proved to be too variable to be used to study relationships between the basal lineages in sect. Jacobaea. Nonetheless, these markers are very useful to study the phylogeny of S. jacobaea and its closest relatives. The combined use of DNA sequence data and AFLPs allowed us to take a major step towards resolving phylogenetic relationships in sect. Jacobaea, identifying Senecio alpinus, Senecio pancicii and Senecio subalpinus (using DNA sequence data) or Senecio chrysanthemoides (using AFLPs) as the closest relatives of S. jacobaea.     

Weed invasion in East Africa: insights from herbarium records

The invasion process was documented from data on the labels of specimens stored in the East African Herbarium at the National Museums of Kenya. We analysed data from seven abundant alien plant species: Ageratum conyzoides L. (Asteraceae), Capsella bursa‐pastoris (L.) Med . (Brassicaceae ) , Datura stramonium L. (Solanaceae), Galinsoga parviflora Cav. (Asteraceae), Tagetes minuta L. (Asteraceae), Spergula arvensis L. (Caryophyllaceae) and Stellaria media (L.) Vill. (Caryophyllaceae) and compared these data with the spread of two native weeds: Cynoglossum coeruleum A.DC. (Boraginaceae) and Senecio discifolius Oliv. (Asteraceae). Although all investigated species had been already recorded before the Second World War, most specimens were collected between 1960 and 1980. This regional spread is correlated with a change in the agricultural systems of Kenya. The early records of alien weeds were restricted to higher altitudes. With the increasing human population and the associated increase of agricultural activities (e.g. irrigation in arid areas) the weed species from South America were able to spread to lower altitudes.     

Involvement of abscisic acid and ethylene in the responses of citrus seedlings to salt shock

The responses of salt‐sensitive citrus rootstocks to 200 m M NaCl were periodically determined on seedlings of citrange Carrizo ( Citrus sinensis [L.] Osbeck × Poncirus trifoliata [L.] Raf) during 30 days. The stressed seedlings adjusted osmotically, reduced stomatal conductance, increased proline content and ethylene production, and showed massive leaf abscission (92%). The salt shock also increased abscisic acid (ABA) and aminocyclopropane‐1‐carboxylic acid (ACC) in roots, xylem fluid and leaves, and in addition promoted Cl⁻ accumulation. The pattern of change of ABA, ACC and proline followed a two‐phase response: an initial transient increase (10‐12 days) overlapping with a gradual and continuous accumulation. This biphasic response appears to be compatible with the proposal that the transitory hormonal rises are induced by the osmotic component of salinity, whereas the Cl⁻ increase determines the subsequent accumulations. During the second phase, Cl⁻ levels correlated with abscission in leaves. Production of leaf ethylene was also concomitant with the increase in the abscission rate. Salt‐induced abscission was either reduced with CoCl₂ (52%) or inhibited with silver thiosulphate (14%). The results suggest that in salt‐stressed citrus, leaf abscission is induced by the chloride build‐up through a mechanism that stimulates leaf ACC synthesis and further conversion to ethylene.     

Host-plant choice and larval growth in the cinnabar moth: do pyrrolizidine alkaloids play a role?

Witte et al. (1992) described two distinct chemotypes of Senecio jacobaea L. Asteraceae, a chemotype with jacobine as one of the major pyrrolizidine alkaloids (PAs) and a chemotype with erucifoline as one of the major PAs. We hypothesized that the presence of erucifoline might be the factor responsible for the lack of success of the cinnabar moth on Senecio erucifolius L. Asteraceae and the S. jacobaea erucifoline chemotype. We performed a survey of the distribution of the two chemotypes in the Netherlands and compared this with the distribution map of Tyria jacobaeae L. Lepidoptera, Arctiidae. The distribution of the two chemotypes in the Netherlands is poorly correlated with the distribution of the cinnabar moth. The jacobine chemotype occurs along the coast and the erucifoline chemotype predominantly inward.An oviposition experiment showed that the cinnabar moth did not discriminate between the two chemotypes of S. jacobaea and S. erucifolius. Larval performance did not differ between the two chemotypes and species. Although the distribution of S. jacobaea jacobine chemotype is loosely associated with the abundance of the cinnabar moth the oviposition and growth experiments indicate that other factors than the presence of erucifoline play a role in this association.The absence of recordings of S. erucifolius as a foodplant for the cinnabar moth might be explained by the phenology of the foodplant. Ovipositing females of the univoltine cinnabar moth prefer flowering plants for oviposition. S. erucifolius starts flowering about 1–2 month later than S. jacobaea just after the peak density of moths.     

Sugar and fructan accumulation during metabolic adjustment between respiration and fermentation under low oxygen conditions in wheat roots

In terms of gene expression and carbohydrate metabolism, the response of wheat seedlings to hypoxia is dramatically different from the anoxic response. Total carbohydrate content of roots increased 4-fold during 6 days of hypoxia, with a 17-fold increase in fructans. In contrast, anoxically treated roots depleted all soluble carbohydrates and died within 72 h. Gas exchange measurements (CO₂ release vs. O₂ uptake) demonstrate that hypoxia establishes a new balance between fermentation and aerobic respiration in the roots without altering the flux of carbon through glycolysis. Furthermore, the respiratory component of this new balance is 55% higher in roots that have been hypoxically pretreated compared to non-hypoxically pretreated roots. The establishment of this new homeostasis under hypoxia involves the induction of glycolytic (aldolase and enolase) and fermentative enzymes (pyruvate decarboxylase, alcohol dehydrogenase, and lactate dehydrogenase). Enzyme induction is generally complete within 24 h with mRNA induction occurring primarily during Period I (0–6 h of hypoxia), and maximal enzymes activities attained during Period II (6–24 h of hypoxia). Accumulation rates of Suc, hexoses, and fructans also change during Periods I and II. By the start of Period III (24–144 h of hypoxia), the metabolic adjustments are complete and fructans are the major carbohydrate accumulated. In anoxia, the pattern of enzyme induction was dramatically different: aldolase was not induced and declined throughout the treatment. Alcohol dehydrogenase, pyruvate decarboxylase, and lactate dehydrogenase were induced as in hypoxia, but rapidly declined within 72 h of anoxia. Only enolase exhibited a similar expression pattern in both anoxia and hypoxia.     

Anoxia tolerance in tobacco roots: effect of overexpression of pyruvate decarboxylase

Plant survival during flooding relies on ethanolic fermentation for energy production. The available literature indicates that the first enzyme of the ethanolic fermentation pathway, pyruvate decarboxylase (PDC), is expressed at very low levels and is likely to be rate-limiting during oxygen deprivation. The authors expressed high levels of bacterial PDC in tobacco to study the modulation of PDC activity in vivo, and assess its impact on the physiology of ethanolic fermentation and survival under oxygen stress. In contrast to leaves, wild-type normoxic roots contained considerable PDC activity, and overexpression of the bacterial PDC caused only a moderate increase in acetaldehyde and ethanol production under anoxia compared to wild-type roots. No significant lactate production could be measured at any time, making it unlikely that lactate-induced acidification (LDH/PDC pH-stat) triggers the onset of ethanol synthesis. Instead, the authors favour a model in which the flux through the pathway is regulated by substrate availability. The increased ethanolic flux in the transgenics compared to the wild-type did not enhance anoxia tolerance. On the contrary, rapid utilisation of carbohydrate reserves enhanced premature cell death in the transgenics while replenishment of carbohydrates improved survival under anoxia.     

Evaluation of the importance of lactate for the activation of ethanolic fermentation in lettuce roots in anoxia

According to the Davies–Roberts hypothesis, plants primarily respond to oxygen limitation by a burst of lactate production and the resulting pH drop in the cytoplasm activates ethanolic fermentation. To evaluate this system in lettuce ( Lactuca sativa L.), seedlings were subjected to anoxia and in vitro activities of alcohol dehydrogenase (ADH, EC 1.1.1.1), pyruvate decarboxylase (PDC, EC 4.1.1.1) and lactate dehydrogenase (LDH, EC 1.1.1.27) and concentrations of ethanol, acetaldehyde and lactate were determined in roots of the seedlings. The in vitro activities of ADH and PDC in the roots increase in anoxia, whereas no significant increase was measured in LDH activity. At 6 h, the ADH and PDC activities in the roots kept in anoxia were 2.8- and 2.9-fold greater than those in air, respectively. Ethanol and acetaldehyde in the roots accumulated rapidly in anoxia and increased 8- and 4-fold compared with those in air by 6 h, respectively. However, lactate concentration did not increase and an initial burst of lactate production was not found. Thus, ethanol and acetaldehyde production occurred without an increase in lactate synthesis. Treatments with antimycin A and salicylhydroxamic acid, which are respiratory inhibitors, to the lettuce seedlings in the presence of oxygen increased the concentrations of ethanol and acetaldehyde but not of lactate. These results suggest that ethanolic fermentation may be activated without preceding activation of lactate fermentation and may be not regulated by oxygen concentration directly.     

No relation between drought stress and ethylene production in Norway spruce

The relationship between water availability and ethylene production was studied in 24‐year‐old trees of Norway spruce [ Picea abies (L.) Karst.] growing on experimental plots with different water availability. Ethylene and oxygen were collected from the cambial/xylem region and heartwood in the stem using non‐destructive sampling methods. Xylem sap flow was measured in stems using a heat‐balance technique. Pre‐dawn water potential of shoots was used to assess the water status of the trees. Growth was calculated from increments in stem basal area. The highest ethylene concentrations were found in irrigated trees that also, as compared to the other treatments, showed the most rapid flow rate of sap, the highest pre‐dawn water potential, and the most rapid growth. By contrast, the lowest ethylene concentrations were measured in trees to which artificial drought was induced. Such trees also showed the lowest water transport, lowest water potential and relatively slow growth. Thus, no signs of drought‐induced ethylene production were found in this study, contrary to the general contention of a positive relation between drought stress and ethylene production.     

The effect of enhanced ultraviolet-B radiation on germination and seedling development of plant species occurring in a dune grassland ecosystem

The germination of seeds of seven plant species occurring in a dune grassland vegetation of the Netherlands, was studied at four levels of UV-B radiation simulating unto 45% stratospheric ozone reduction during April. With the exception of seeds of Senecio jacobaea, germination of the dune grassland species was not affected by enhanced UV-B irradiance. Although a clear UV-B fluence-response relationship was not observed, the germination rate of S. jacobaea seeds and maximal germination percentage were reduced at enhanced UV-B. Germination rate in the dark was higher than germination in the light for Oenothera biennis, Plantago lanceolata, Rumex obtusifolius and S. jacobaea. Total dry biomass accumulation of seedlings was not affected by increased UV-B radiation in any of the species tested. Clear-cut differences in UV-absorbance of methanolic extracts were observed between species. Enhanced UV-B irradiance stimulated UV-absorbance of seedling extracts of Holcus lanatus and Verbascum thapsus. A clear UV-B fluence-response relationship was observed for both species. The results indicate that germination of the studied plant species probably will not be adversely affected by the expected stratospheric ozone reduction in The Netherlands.     

Signal transduction in blue light‐induced oxygen uptake in Chlorella cells

In DCMU‐poisoned wild‐type and in non‐photosynthetic pigment mutant cells of Chlorella kessleri , grown heterotrophically with glucose as a carbon source and with nitrate as sole nitrogen source, the known blue light‐enhanced uptake of oxygen and breakdown of starch were reduced by staurosporine and K252a, both potent inhibitors of protein kinase C. This corresponded to sensitivity to these inhibitors of blue light‐induced uptake of nitrate of such organisms. Cells grown with ammonia as sole nitrogen source responded to short wavelength visible irradiation with an increase in oxygen uptake, and this, too, was inhibited by staurosporine and K252a. However, these cells did not show any blue light‐enhanced uptake of nitrate. From these results, enhanced consumption of oxygen under blue light cannot be a consequence of blue light‐induced protein phosphorylation involved in the light‐dependent uptake of nitrate. However, existence of a specific protein phosphorylation within the process of enhancement of oxygen uptake under blue light is not yet proven by the data. There might be a master reaction that induces both processes independently, or there may be influences of other light‐induced processes which lead to enhanced starch breakdown, thereby supplying the glucose for oxidative degradation.