Population and leaf-level variation of iridoid glycosides in the invasive weed Verbascum thapsus L. (common mullein): implications for herbivory by generalist insects

Plant–insect interactions, which are strongly mediated by chemical defenses, have the potential to shape invasion dynamics. Despite this, few studies have quantified natural variation in key defensive compounds of invasive plant populations, or how those defenses relate to levels of herbivory. Here, we evaluated variation in the iridoid glycosides aucubin and catalpol in rosette plants of naturally occurring, introduced populations of the North American invader, Verbascum thapsus L. (common mullein; Scrophulariaceae). We examined two scales that are likely to structure interactions with insect herbivores—among populations and within plant tissues (i.e., between young and old leaves). We additionally estimated the severity of damage incurred at these scales due to insect chewing herbivores (predominantly grasshoppers and caterpillars), and evaluated the relationship between iridoid glycoside content and leaf damage. We found significant variation in iridoid glycoside concentrations among populations and between young and old leaves, with levels of herbivory strongly tracking leaf-level investment in defense. Specifically, across populations, young leaves were highly defended by iridoids (averaging 6.5× the concentration present in old leaves, and containing higher proportions of the potentially more toxic iridoid, catalpol) and suffered only minimal damage from generalist herbivores. In contrast, old leaves were significantly less defended and accordingly more substantially utilized. These findings reveal that quantitative variation in iridoid glycosides is a key feature explaining patterns of herbivory in an introduced plant. In particular, these data support the hypothesis that defenses limit the ability of generalists to feed on mullein’s well-defended young leaves, resulting in minimal losses of high-quality tissue, and increasing performance of this introduced species.     

Increased nitrogen availability influences predator–prey interactions by altering host-plant quality

Little is known about how plant nutritional and defensive qualities interact to influence predator–prey interactions. To address this need, we provided the neo-tropical milkweed, Asclepias curassavica, with two levels of nitrogen availability and examined how altered host-plant quality influenced the responses of a specialist aphid, Aphis nerii, and a coccinellid predator, Harmonia axyridis. Aphis nerii uses A. curassavica for multiple resources, including nutrition and sequestration of cardenolides for defense against natural enemies. Increased nitrogen availability improved A. curassavica quality by decreasing carbon-to-nitrogen (C:N) ratios and cardenolide concentrations, resulting in A. nerii that also had lower C:N ratios and cardenolide concentrations. Aphis nerii population growth was higher on plants with high nitrogen availability, compared with aphids on plants with low nitrogen availability. In no-choice feeding trials, Harmonia axyridis consumed more high C:N ratio aphids, suggesting a potential compensatory response to reduced aphid nutritional quality. Additionally, H. axyridis were able to consume more low-quality aphids at the expense of increasing exposure to increased cardenolide concentrations, suggesting that interactions between H. axyridis and A. nerii may be strongly influenced by prey nutritional quality. This work highlights the need to consider how variation in plant quality influences herbivore nutritional and defensive quality when examining mechanisms that influence predator–prey interactions.     

Variation in resource availability changes the impact of invasive thistles on native bunchgrasses.

The threat posed by invasive nonnative plants to native plant populations is one of the largest challenges facing both conservation biology and restoration ecology. California has been highly impacted by invaders, although many relict stands of native plants are found on shallow, rocky soils with limited resources. The abiotic conditions of these sites may strongly influence the performance of an invasive plant and its effect on resident native species. In addition, the maturity of native plants in these sites may modulate an invader's impact; larger, well-established plants may be better able to resist invaders. In this study we examined how the impact of an invasive thistle (Centaurea solstitialis) on a native perennial bunchgrass (Nassella pulchra) changed in response to variation in soil depth, soil water availability, and bunchgrass maturity. We measured plant performance in terms of survival, growth, reproduction, and predawn water potential. We found that soil depth, water availability, and bunchgrass maturity acted in concert to influence the impact of the invasive thistle on the native bunchgrass. Both species performed better in deep soils, especially during dry years. The combination of shallow soil and low water availability reduced C. solstitialis performance and ameliorated its negative effect on N. pulchra growth and reproduction. Higher water availability resulted in a stronger negative effect of C. solstitialis on N. pulchra in both shallow and deep soils. However, as N. pulchra matured and increased in size, we saw a steady decline in C. solstitialis growth and reproductive output. Higher water availability increased the performance of C. solstitialis in shallow soils. C. solstitialis may thus have a stronger impact on N. pulchra and be more able to invade relict stands of N. pulchra in shallow soils during high-rainfall years. However, established stands of N. pulchra appear to be more resistant to invasion by C. solstitialis as N. pulchra plants grow older and larger.     

Variation in composition of predator-attracting allelochemicals emitted by herbivore-infested plants: Relative influence of plant and herbivore

Summary During foraging, natural enemies of herbivores may employ volatile allelochemicals that originate from an interaction of the herbivore and its host plant. The composition of allelochemical blends emitted by herbivore-infested plants is known to be affected by both the herbivore and the plant. Our chemical data add new evidence to the recent notion that the plants are more important than the herbivore in affecting the composition of the volatile blends. Blends emitted by apple leaves infested with spider mites of 2 different species,T. urticae andP. ulmi, differed less in composition (principally quantitative differences for some compounds) than blends emitted by leaves of two apple cultivars infested by the same spider-mite species,T. urticae (many quantitative and a few qualitative differences). Comparison between three plant species — apple, cucumber and Lima bean — reveals even larger differences between volatile blends emitted upon spider-mite damage (many quantitative differences and several qualitative differences).     

Geographic covariation of chemical quality of the host alga<Emphasis Type="Italic"> Fucus vesiculosus</Emphasis> with fitness of the herbivorous isopod<Emphasis Type="Italic"> Idotea baltica</Emphasis>

Environmental and/or genetic among-site variation in plant quality may influence growth and fecundity of specialized herbivores inhabiting a particular site. Such variation is important as it generates spatial variation in selection for traits related to plant–herbivore interaction. Littoral macroalgae are known to respond plastically to environmental variation by modifying their chemistry or morphology. We studied geographic variation in phlorotannin, nitrogen, protein, and sugar (fucose, mannitol, and melibiose) concentrations of the brown alga Fucus vesiculosus at 12 sites separated by 0.5 to 40 km in the naturally fragmented Archipelago Sea in the northern Baltic Sea. By this regional variation in algal chemistry we attempted to explain among-population variation in size and fecundity of the crustacean herbivore Idotea baltica. We observed high spatial variation in all the measured chemical characteristics of F. vesiculosus, as well as in female size and the number of eggs produced by the herbivores. Spatial variation in nitrogen or protein contents of the alga did not explain the variation of herbivore traits. However, egg size positively covaried with spatial variation in the concentration of mannitol, the major storage carbohydrate of the alga. Such a positive relationship may arise if I. baltica can utilize the nutritive value of a mannitol-rich diet thereby being better able to provision the developing eggs with energy-rich metabolites. Unexpectedly, the concentration of phlorotannins, secondary metabolites having a putative role in defense against herbivory, positively covaried with the size of the herbivore. Among-population variation in host plant chemistry and covariation of that with herbivore growth and reproduction imply that herbivores respond to the local quality of their host plants, and that geographical structuring of populations has to be taken into account in studies of plant–herbivore interactions.Communicated by M. Kühl, Helsingør     

Glucosinolates and other metabolites in the leaves of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> from natural populations and their effects on a generalist and a specialist herbivore

Because many secondary metabolites in plants act as defense against herbivores it has been postulated that these compounds have evolved under selective pressure by insect herbivores. One explanation for the within-species variation in metabolite patterns in a particular species is that different populations are under selection by different herbivores. We tested this hypothesis, using Arabidopsis thaliana plants that originated from dune and inland areas. We analyzed Arabidopsis thaliana leaves using NMR spectroscopy and multivariate data analysis. Major differences in chemical composition were found in water-methanol fractions and were due to higher concentrations of sinigrin and fumaric acid in dune plants. Inland plants showed lower levels of glucose. Quantitative analysis of glucosinolates was performed with HPLC. Individual plants and populations demonstrated differences in glucosinolate composition and concentration. In growth chamber experiments, the generalist herbivore, Spodoptera exigua grew significantly better on the inland plants, while the specialist herbivore Plutella xylostella performed equally well on plants of both origins. Aliphatic glucosinolate as well as total glucosinolate concentrations negatively correlated with larval mass of Spodoptera exigua. No significant correlations, however, were found between larval mass of Plutella xylostella and glucosinolates in the leaves. A specialist and a generalist herbivore were responding differently to plant secondary chemistry, as was also found in several other studies. This is an important indication that differences in glucosinolate concentrations among populations may result from differential selection by different guilds of herbivores.     

Herbivory by an introduced Asian weevil negatively affects population growth of an invasive Brazilian shrub in Florida

The enemy release hypothesis (ERH) is often cited to explain why some plants successfully invade natural communities while others do not. This hypothesis maintains that plant populations are regulated by coevolved enemies in their native range but are relieved of this pressure where their enemies have not been co-introduced. Some studies have shown that invasive plants sustain lower levels of herbivore damage when compared to native species, but how damage affects fitness and population dynamics remains unclear. We used a system of co-occurring native and invasive Eugenia congeners in south Florida (USA) to experimentally test the ERH, addressing deficiencies in our understanding of the role of natural enemies in plant invasion at the population level. Insecticide was used to experimentally exclude insect herbivores from invasive Eugenia uniflora and its native co-occurring congeners in the field for two years. Herbivore damage, plant growth, survival, and population growth rates for the three species were then compared for control and insecticide-treated plants. Our results contradict the ERH, indicating that E. uniflora sustains more herbivore damage than its native congeners and that this damage negatively impacts stem height, survival, and population growth. In addition, most damage to E. uniflora, a native of Brazil, is carried out by Myllocerus undatus, a recently introduced weevil from Sri Lanka, and M. undatus attacks a significantly greater proportion of E. uniflora leaves than those of its native congeners. This interaction is particularly interesting because M. undatus and E. uniflora share no coevolutionary history, having arisen on two separate continents and come into contact on a third. Our study is the first to document negative population-level effects for an invasive plant as a result of the introduction of a novel herbivore. Such inhibitory interactions are likely to become more prevalent as suites of previously noninteracting species continue to accumulate and new communities assemble worldwide.     

Species-specific responses of herbivores to within-plant and environmentally mediated between-plant variability in plant chemistry

Allocation of resources to growth and defense against herbivores crucially affects plant competitiveness and survival, resulting in a specific distribution of assimilates and defense compounds within plant individuals. Additionally, plants rarely experience stable environmental conditions, and adaptations to abiotic and biotic stresses may involve shifts in resistance to herbivores. We studied the allocation of phytochemicals in Brassica oleracea (Brussels sprouts) due to leaf age, drought stress and herbivore damage and assessed effects on two lepidopteran herbivores differing in diet breadth: the generalist Spodoptera littoralis and the specialist Pieris brassicae. Glucosinolates as secondary defense compounds and total nitrogen and carbon were quantified and linked to plant palatability, i.e., herbivore feeding preference. Herbivore responses were highly species-specific and partially related to changes in phytochemicals. Spodoptera littoralis preferred middle-aged leaves with intermediate levels of glucosinolates and nitrogen over young, glucosinolate and nitrogen rich leaves, as well as over old leaves, poor in glucosinolates and nitrogen. In contrast, P. brassicae preferred young leaves. Both species preferred severely drought-stressed plants to the well-watered control, although analyzed glucosinolate concentrations did not differ. Both S. littoralis and P. brassicae feeding induced an increase of indole glucosinolate levels, which may explain a reduced consumption of damaged plants detected for S. littoralis but not for P. brassicae. By revealing distinct, sometimes contrasting responses of two insect herbivores to within-plant and stress-mediated intraspecific variation in phytochemistry of B. oleracea, this study emphasizes the need to consider specific herbivore responses to understand and predict the interactions between herbivores and variable plants.     

Do plant density, nutrient availability, and herbivore grazing interact to affect phlorotannin plasticity in the brown seaweed Ascophyllum nodosum

Plants have different strategies to cope with herbivory, including induction of chemical defences and compensatory growth. The most favourable strategy for an individual plant may depend on the density at which the plants are growing and on the availability of nutrients, but this has not been tested previously for marine plant–herbivore interactions. We investigated the separate and interactive effects of plant density, nutrient availability, and herbivore grazing on the phlorotannin (polyphenolic) production in the brown seaweed Ascophyllum nodosum. Seaweed plants grown at low or high densities were exposed either to nutrient enrichment, herbivorous littorinid gastropods (Littorina obtusata), or a combination of nutrients and herbivores in an outdoor mesocosm experiment for 2 weeks. Seaweeds grown at a low density tended to have higher tissue nitrogen content compared to plants grown at a high density when exposed to elevated nutrient levels, indicating that there was a density dependent competition for nitrogen. Herbivore grazing induced a higher phlorotannin content in plants grown under ambient, but not enriched, nutrient levels, indicting either that phlorotannin plasticity is more costly when nutrients are abundant or that plants responded to herbivory by compensatory growth. However, there were no significant interactive or main effects of plant density on the seaweed phlorotannin content. The results indicate that plants in both high and low densities induce chemical defence, and that eutrophication may have indirect effects on marine plant–herbivore interactions through alterations of plant chemical defence allocation.     

Genetics, environment, and their interaction determine efficacy of chemical defense in trembling aspen

Optimal defense theories suggest that a trade-off between defense costs and benefits maintains genetic variation within plant populations. This study assessed the independent and interactive effects of genetic- and environment-based variation in aspen leaf chemistry on insect performance, preference, and defoliation. Gypsy moth larvae were released into screenhouses containing eight aspen genotypes growing with high and low levels of nutrient availability. Plant chemistry, defoliation, and larval growth rates varied in response to genotype, nutrient availability, and their interaction. Total phenolic glycoside concentrations were inversely correlated with patterns of larval preference and were the best predictor of larval performance and defoliation among genotypes. Low-nutrient trees were less heavily defoliated and afforded decreased larval growth rates compared with high-nutrient trees. Nutrient availability mediated the defense benefits of phenolic glycosides, as plant chemistry explained significantly less variation in defoliation in low- compared with high-nutrient trees (7% vs. 44% of variation explained). These results suggest that spatial and temporal variation in resource availability may influence the relative magnitude of defense benefits in plants. Environmental mediation of the defense costs and benefits likely leads to diversifying selection and may maintain genetic polymorphisms in chemical defense traits in plant populations.     

二氧化碳浓度升高对植物入侵的影响

从入侵植物和入侵植物群落两个方面,综述了大气二氧化碳浓度升高对植物入侵的影响。二氧化碳浓度升高,可以增加C3植物的入侵性,提高入侵植物的生物量、资源利用率以及繁殖能力,直接影响植物入侵;还可以通过改变土壤水分、氮循环、干扰体系等其它环境因子间接地影响植物入侵。此外,二氧化碳浓度升高,对入侵群落的初级生产量、组成与结构以及群落动态产生重要影响,改变群落的可入侵性。今后应当着重从群落水平,结合其它全球变化因子的共同作用研究二氧化碳浓度升高对植物入侵的影响,同时深入探讨其作用机制以及不同植物类群对二氧化碳的响应,为入侵种的预防和控制提供理论指导。     

Allocation of nitrogen and carbon in an estuarine salt marsh in Portugal

Above and below-ground biomass and nitrogen and carbon composition ofSpartina maritima, Halimione portulacoides andArthrocnemum perenne, dominating species in plant communities of the lower, middle and higher salt marsh, respectively, were compared in an estuarine salt marsh in Portugal. Plant and soil nitrogen and carbon pools were estimated. For all three species root biomass was significantly higher (70–92% of total biomass) than above-ground biomass. The percentage of root biomass was related to the location of the plants in the marsh: higher values were found in plants growing in the lower salt marsh where the sediment was more unstable and subject to tidal action, which stresses the role of the roots as an anchor. For all three species nitrogen concentrations were highest in leaves, reflecting the photosynthetic role of the tissue. For carbon higher concentrations were found in the stems, with the exception ofS. maritima. In general, lower nitrogen concentrations were found in summer, which can be explained by dilution processes due to plant growth. For both nitrogen and carbon, higher concentrations were found in the soil surface layers. Higher soil nitrogen and carbon levels were associated with higher organic matter contents. Most of the nitrogen in the salt marsh occurred in the sediments (0–40 cm) and only ca. 5.7–13.3% of the total was found in the plants. The greater portion (76.5%–86%) of carbon was found in the sediment.     

大气CO2体积分数升高对植物N素吸收的影响

从影响植物N素吸收的因素来看，大气CO2体积分数升高条件下植物净光合作用增强，碳同化产物增多，利于改善N素吸收的能量和物质基础：植物根系生长增强，生物量增多且空间分布加大，有利于N素吸收；但土壤有效N供应能力的变化存在增强和减弱两种观点。从植物N素吸收的实际情况来看，大气CO2体积分数升高条件下植物N吸收总量并末增加，植物体内N质量分数普遍降低，某些种类植物N吸收形态也发生了改变。因此要阐明大气CO2体积分数升高对植物N素吸收的影响机制，必须探明土壤有效N供应能力的变化：CO2体积分数升高条件下N矿化作用是否增强，微生物和植物间是否存在对有效N的竞争，此外，CO2体积分数升高条件下植物根系形态特征变化和N素吸收(包括主动和被动吸收)的生理机制及其与环境因素的关系也值得进一步研究。     

酸雨对外来植物入侵的影响

酸雨和外来种入侵都是全球关注的问题。结合外来入侵植物的生态适应特性以及酸雨的危害特征,系统分析了酸雨对外来植物入侵产生的影响。酸雨对外来植物入侵的影响是复杂多样的。酸雨导致群落冠层稀疏,群落透光率增加,加之氮沉降后土壤、水体氮素的增加,有利于生长力强的外来喜阳植物入侵;酸雨加速土壤酸化,促使基本离子淋失以及A1毒等危害植物的生长发育,植物的内源激素以及化感作用发生改变,适应力和耐受力强的外来植物在与本地植物竞争中处于相对优势而成为入侵种;酸雨以及外来植物入侵改变了土壤微生物群落结构,影响本地植物的生长而促使外来植物的入侵。     

Asymmetric competition via induced resistance: specialist herbivores indirectly suppress generalist preference and populations

Species may compete indirectly by altering the traits of a shared resource. For example, herbivore-induced responses in plants may make plants more resistant or susceptible to additional herbivorous insect species. Herbivore-induced plant responses can significantly affect interspecific competition and herbivore population dynamics. These herbivore-herbivore indirect interactions have been overlooked in aquatic ecosystems where previous studies used the same herbivore species to induce changes and to assess the effects of these changes. We asked whether seaweed grazing by one of two herbivorous, congeneric snail species (Littorina obtusata or Littorina littorea) with different feeding strategies and preferences would affect subsequent feeding preferences of three herbivore species (both snails and the isopod Idotea baltica) and population densities of three herbivore species (both snails and a third periwinkle snail, Lacuna vincta). In addition, we measured phlorotannin concentrations to test the hypothesis that these metabolites function as induced defenses in the Phaeophyceae. Snail herbivory induced cue-specific responses in apical tissues of the seaweed Fucus vesiculosus that affected the three herbivore species similarly. When compared to ungrazed controls, direct grazing by Littorina obtusata reduced seaweed palatability by at least 52% for both snail species and the isopod species. In contrast, direct grazing by L. littorea did not decrease seaweed palatability for any herbivore, indicating herbivore-specific responses. Previous grazing by L. obtusata reduced populations of L. littorea on outplanted seaweeds by 46% but had no effect on L. obtusata populations. Phlorotannins, a potential class of inducible chemicals in brown algae, were not more concentrated in grazed seaweed tissues, suggesting that some other trait was responsible for the induced resistance. Our results indicate that marine herbivores may compete via inducible responses in shared seaweeds. These plant-mediated interactions were asymmetric with a specialist (L. obtusata) competitively superior to a generalist (L. littorea).     

Leaf quality, predators, and stochastic processes in the assembly of a diverse herbivore community

Ecological communities are structured by both deterministic, niche-based processes and stochastic processes such as dispersal. A pressing issue in ecology is to determine when and for which organisms each of these types of processes is important in community assembly. The roles of deterministic and stochastic processes have been studied for a variety of communities, but very few researchers have addressed their contribution to insect herbivore community structure. Insect herbivore niches are often described as largely shaped by the antagonistic pressures of predation and host plant defenses. However host plants are frequently discrete patches of habitat, and their spatial arrangement can affect herbivore dispersal patterns. We studied the roles of predation, host plant quality, and host spatial proximity for the assembly of a diverse insect herbivore community on Quercus alba (white oak) across two growing seasons. We examined abundances of feeding guilds to determine if ecologically similar species responded similarly to variation in niches. Most guilds responded similarly to leaf quality, preferring high-nitrogen, low-tannin host plants, particularly late in the growing season, while bird predation had little impact on herbivore abundance. The communities on the high-quality plants tended to be larger and, in some cases, have greater species richness. We analyzed community composition by correlating indices of community similarity with predator presence, leaf quality similarity, and host plant proximity. Birds did not affect community composition. Community similarity was significantly associated with distance between host plants and uncorrelated with leaf quality similarity. Thus although leaf quality significantly affected the total abundance of herbivores on a host plant, in some cases leading to increased species richness, dispersal limitation may weaken this relationship. The species composition of these communities may be driven by stochastic processes rather than variation in host plant characteristics or differential predation by insectivorous birds.     

2种入侵植物对根际土壤微生物种群及代谢的影响

选取2种入侵植物（一年蓬Annual Fleabane及加拿大蓬Erigeron Canadensis）及本土植物（艾蒿Artemisia argyi）的根际土壤微生物种群为研究对象,以分析不同根际土壤微生物种群的数量及测定根际土壤微生物酶活的活性为切入点来探析入侵植物对土壤微生物的影响及其响应机制。结果显示：2种入侵植物显著增加了其根际土壤中的细菌的数量,而显著抑制了真菌与放线菌的数量。另外,入侵植物一年蓬显著抑制其根际土壤中纤维素酶的活性,而3种植物的根际土壤硝酸还原酶活性无显著差异。入侵植物对其他3种土壤酶（即转化酶、脲酶及酸性磷酸酶）活性的影响却呈现出截然相反的影响,即一年蓬显著增加了3种根际土壤酶的活性,而加拿大蓬却显著减少了3种根际土壤酶的活性。导致这种现象的原因可能是不同入侵植物的根系释放不同的化学物质进而对土壤微生物的数量和活性造成不同的影响。     

Nutrient availability and accumulation of phenolic compounds in the brown alga Fucus vesiculosus

Plant phenols tend to accumulate under conditions where plants have excess carbon above the level which can be used for growth, and where phenylalanine, the substrate of phenylpropanoid synthesis, accumulates due to suppressed protein synthesis. These internal balances imply an accumulation of phenols as a consequence of nitrogen deficiency suppressing plant primary metabolism. In three sublittoral populations of the brown alga Fucus vesiculosus (L.) collected from the northern Baltic Sea between May and September 1982, the accumulation of phenolic compounds correlated inversely with nitrogen content of thallus; higher phenolic contents were on average found under nitrogen deficiency. Phenolic content did not correlate with carbon content of thallus as such, while a significant negative correlation was found with the nitrogen: carbon ratio. Phenolic compounds, although having possibly defensive functions in plants, may thus partially vary as a function of resource availability rather than as a result of an active allocation into plant defences.     

Intraspecific Chromosome Number Variation: a Neglected Threat to the Conservation of Rare Plants

Abstract: The effectiveness of rare plant conservation will increase when life history, demographic, and genetic data are considered simultaneously. Inbreeding depression is a widely recognized genetic concern in rare plant conservation, and the mixing of genetically diverse populations in restoration efforts is a common remedy. Nevertheless, if populations with unrecognized intraspecific chromosome variation are crossed, progeny fitness losses will range from partial to complete sterility, and reintroductions and population augmentation of rare plants may fail. To assess the current state of cytological knowledge of threatened and endangered plants in the continental United States, we searched available resources for chromosome counts. We also reviewed recovery plans to discern whether recovery criteria potentially place listed species at risk by requiring reintroductions or population augmentation in the absence of cytological information. Over half the plants lacked a chromosome count, and when a taxon did have a count it generally originated from a sampling intensity too limited to detect intraspecific chromosome variation. Despite limited past cytological sampling, we found 11 plants with documented intraspecific cytological variation, while 8 others were ambiguous for intraspecific chromosome variation. Nevertheless, only one recovery plan addressed the chromosome differences. Inadequate within‐species cytological characterization, incomplete sampling among listed taxa, and the prevalence of interspecific and intraspecific chromosome variation in listed genera, suggests that other rare plants are likely to have intraspecific chromosome variation. Nearly 90% of all recovery plans called for reintroductions or population augmentation as part of recovery criteria despite the dearth of cytological knowledge. We recommend screening rare plants for intraspecific chromosome variation before reintroductions or population augmentation projects are undertaken to safeguard against inadvertent mixtures of incompatible cytotypes.     

Soil characteristics influence the radionuclide uptake of different plant species

The key point of food plant agriculture is how to regulate the harmonious relationship between the soil and the plant environment. This study deals with radionuclide uptake by two food plant and two fruit tree species in relation to the geochemical characteristics of the soil. Uranium and thorium content was determined in coastal black sand and inland cultivated soils. Four commonly cultivated species Eruca sativa, Lycopersicon esculentum, Psidium guajava and Mangifera indica were investigated. Physical and chemical properties of the soil were analysed in relation to uranium and thorium uptake by plants. The results revealed the ability of plants to accumulate uranium and thorium in their edible portions. The absorbed radionuclides were positively correlated with their concentrations in the soil and the geochemical characteristics of the soil. The transfer of radioactive elements from soil to plant is a complex process that can be regulated by controlling the geochemical characteristics of the soil, including pH, clay, silt and organic matter content that reduce the bioavailability of soil radionuclides to plants, and in turn reduce the risks of biota and human exposure to radionuclide contamination.