Global thermal niche models of two European grasses show high invasion risks in Antarctica

The two non‐native grasses that have established long‐term populations in Antarctica (Poa pratensis and Poa annua) were studied from a global multidimensional thermal niche perspective to address the biological invasion risk to Antarctica. These two species exhibit contrasting introduction histories and reproductive strategies and represent two referential case studies of biological invasion processes. We used a multistep process with a range of species distribution modelling techniques (ecological niche factor analysis, multidimensional envelopes, distance/entropy algorithms) together with a suite of thermoclimatic variables, to characterize the potential ranges of these species. Their native bioclimatic thermal envelopes in Eurasia, together with the different naturalized populations across continents, were compared next. The potential niche of P. pratensis was wider at the cold extremes; however, P. annua life history attributes enable it to be a more successful colonizer. We observe that particularly cold summers are a key aspect of the unique Antarctic environment. In consequence, ruderals such as P. annua can quickly expand under such harsh conditions, whereas the more stress‐tolerant P. pratensis endures and persist through steady growth. Compiled data on human pressure at the Antarctic Peninsula allowed us to provide site‐specific biosecurity risk indicators. We conclude that several areas across the region are vulnerable to invasions from these and other similar species. This can only be visualized in species distribution models (SDMs) when accounting for founder populations that reveal nonanalogous conditions. Results reinforce the need for strict management practices to minimize introductions. Furthermore, our novel set of temperature‐based bioclimatic GIS layers for ice‐free terrestrial Antarctica provide a mechanism for regional and global species distribution models to be built for other potentially invasive species.     

Infection of cereals and grasses by isolates of Polymyxa graminis (Plasmodiophorales)

The host range of isolates of Polymyxa was tested in mono-fungal sand cultures. Fourteen isolates of P. graminis, obtained from barley, wheat, oats or Poa annua and from several different countries, all infected barley and all but one infected wheat. Rye was also a good host, whereas oats (nine cultivars), Lolium multiflorum and Poa pratensis became only slightly infected. Wheat cultivars differed in susceptibility, with Galahad much more resistant than Avalon. Several common weed and pasture grasses were not infected by the two isolates tested. A range of wild Hordeum spp. were mostly susceptible to P. graminis and/or barley mild mosaic virus, which it transmits. An isolate of P. betae, used for comparison, caused slight infection on oats but not on other cereals. The variation within and between species of Polymyxa needs more detailed investigation.     

Ecological Strategies of Soil Microbial Communities under Plants of Meadow Ecosystems

Dominant growth strategies of soil microbial communities of mown and unmown meadows were assessed with respect to the constants of saturation and maximal specific growth rate of microorganisms. The microbial community of mown-meadow soil was characterized by a greater biomass and activity due to prevalence of microorganisms with the r strategy, compared to the microbial community of unmown-meadow soil. In contrast to nonrhizosphere soil, rhizosphere soil was dominated by rapidly growing microorganisms with the r strategy. The dependence of the dominant ecological strategy of the rhizosphere microbial community on the vegetation stage of plants has been traced. Study of the effect of plant species on the growth strategies of rhizosphere microorganisms showed that the features of the K strategy are more pronounced in the following rhizosphere microbial communities of grasses at the same growth stage: r strategy–Bromopsis inermis L.–Poa pratensis L., P. compressa L.–Dactylis glomerata L.–Festuca pratensisL.–K strategy. In the absence of limitation by climatic factors, the growth strategies of rhizosphere microorganisms are determined by the competition between microorganisms and plants for nutrients.     

Nitrogen effects on rhizosphere processes of range grasses from different successional seres

Nitrogen and rhizosphere microorganism effects on nitrogen and carbon dynamics of Sitanion hystrix (early successional species), Stipa comata and Poa secundu which are (mid-successional species), and Agropyron spicatum (late successional species) were evaluated in a growth chamber study. Rhizosphere inocula resulted in increased nitrogen in both root and shoot tissue, and also of water-extractable carbon in the rhizosphere. Plant species, rhizosphere inocula and nitrogen level showed a three-way significant interaction for total and plant-available nitrogen. Rhizosphere microbe presence resulted in higher plant-available nitrogen in the rhizosphere of S. hystrix and less with A. spicatum, suggesting nitrogen immobilization with the later successional grass. Higher nitrogen resulted in decreased active bacteria in the rhizosphere of all plants tested, and decreased fungal hyphal lengths in the rhizosphere of the later successional P. secunda and A. spicutum. Exudate carbon in the rhizosphere of the late successional species A. spicatum, was more recalcitrant, which also may contribute to nitrogen immobilization. These differential responses of early- and late-successional grasses may be important factors contributing to plant succession.     

Studies on the Host Range,Biology, and Pathogenicity of Punctodera punctata Infecting Turfgrasses

Punctodera punctata completed its life cycle on Poa annua (annual bluegrass), P. pratensis (Merion Kentucky bluegrass), Lolium perenne (perennial ryegrass), and Festuca rubra rubra (spreading fescue). Minimum time for completion of a life cycle from second-stage juvenile to mature brown cyst was 40 days at 22-28 C. Inoculation by single juveniles indicated that reproduction was most likely by amphimixis. Infestation levels of 50 or 500 juveniles/250 cm³ soil did not affect top dry weight, root dry weight, or total dry weight of Poa annua.     

Compartmentation and fluxes of carbon in leaf blades and leaf sheaths of Poa annua L. and Poa x jemtlandica (Almq.) Richt

Abstract The allocation of photosynthetically fixed carbon in the leaf blades and sheaths of Poa annua (a ruderal grass) and Poa x jemtlandica (a sub-arctic grass) was followed over a light-dark cycle. Labelling with ¹⁴Carbon and gas exchange measurements provide data for an eight-compartment model describing the partitioning of carbon between spatially and chemically separated pools and their rates of turnover. Soluble sugars and fructans were turned over rapidly in the leaf blades of both species. The flux of carbon through pools of storage carbohydrates was higher in the leaves of P. x jemtlandica than in P. annua. The exchange of carbon between pools was slower in the sheath than the blade. Carbohydrates stored in the sheath appeared to have no significant role in metabolism over the light-dark cycle studied here.     

Frankia in the rhizosphere of nonhost plants: A comparison with root-associated N2-fixing Enterobacter,Klebsiella and Pseudomonas

Bacterial growth in the rhizosphere and resulting changes in plant growth parameters were studied in small aseptic seedlings of birch (Betula pendula and B. pubescens) and grasses (Poa pratensis and Festuca rubra). The seedlings were inoculated with three Frankia strains (Ai1a and Ag5b isolated from native Alnus root nodules and Ai17 from a root nodule induced by soil originating from a Betula pendula stand), and three associative N₂-fixing bacteria (Enterobacter agglomerans, Klebsiella pneumoniae and Pseudomonas sp., isolated from grass roots). Microscopic observations showed that all the Frankia strains were able to colonize and grow on the root surface of the plants tested without addition of an exogenous carbon source. No net growth of the associative N₂-fixers was observed in the rhizosphere, although inoculum viable counts were maintained over the experimental period. Changes in both the biomass and morphology of plant seedlings in response to bacterial inoculation were recorded, which were more dependent on the plant species than on the bacterial strain.     

Comparative nutritional ecology of grass-feeding in a sub-Antarctic beetle: the impact of introduced species on Hydromedion sparsutum from South Georgia

South Georgia has many introduced plant and animal species, a consequence of its long history of human habitation. Introduced reindeer have a strong effect on the vegetation of the Stromness Bay area by causing the replacement of indigenous species by grazing-tolerant grasses such as the exotic Poa annua, and in certain circumstances, the indigenous Festuca contracta. Recently it has been argued that an introduced predatory carabid has contributed to declines in the abundance and an increase in the body size of adults of the indigenous perimylopid beetle Hydromedion sparsutum. However, it also appears that body size of these beetles is smaller in areas where exotic grasses predominate compared to undisturbed areas. Here we test the hypothesis that by causing the spread of poorer quality grasses, especially the exotic Poa annua, reindeer may be having an indirect effect on H. sparsutum. To do this we examined the nutritional ecology of H. sparsutum larvae on four grass species which form a major part of its diet, viz. the indigenous Parodiochloa flabellata, Phleum alpinum and Festuca contracta, and the exotic Poa annua. Larvae showed the highest growth rate on Parodiochloa flabellata, followed by Phleum alpinum, F. contracta and Poa annua. These differences are due to poorer absorption of the exotic grass, and poorer utilization of the absorbed material in the case of F. contracta. Poor growth of larvae on F. contracta appears to be due to its low water and nitrogen contents, whereas in the case of P. annua a combination of low water content and high nitrogen content may be responsible for low growth rates. Low growth rates associated with poor-quality food may lead either to a prolongation of the life cycle or of the length of feeding bouts of an insect. Neither option appears to be feasible for H. sparsutum, and this means that the outcome of feeding on poorer-quality foods would be a reduction in final adult size. This has fitness consequences for the beetle. Hence it appears that by causing the spread of grasses that are unsuitable for growth of H. sparsutum, reindeer may be having an indirect effect on this beetle species. Received: 18 August 1996 / Accepted: 7 February 1997     

Nitrate production in the rhizosphere of Plantago species

Summary The production of nitrate in an old established dune grassland soil and its uptake by plants was studied by comparing amounts of mineral nitrogen and numbers of nitrifying bacteria in the rhizosphere on the one hand, and on the other accumulated nitrate and levels of nitrate reductase (NaR) of individual plants of three Plantago species,i. e., P. major, P. lanceolata andP. coronopus. For these three Plantago species andP. media basal levels of NaR in the absence of nitrate were determined in plants grown in culture solutions. The basal NaR levels ofP. major andP. media (species occurring on nutrient-rich soils) were significantly higher than those ofP. lanceolata andP. coronopus (species found on nutrient-poor soils). NaR activity increased in the presence of nitrate and was suppressed by ammonium.From the numbers of nitrifying bacteria in the rhizosphere and NaR activity in the leaves it was concluded that nitrate was produced in the root environments of the three Plantago species and that the compound was taken up by the plants. NaR activities and numbers of nitrifying bacteria were higher for individuals ofP. major than for those ofP. lanceolata andP. coronopus. No correlation was found between the ammonium levels and the numbers of nitrifying bacteria in the soil, and no indications of inhibition of nitrifying bacteria in the rhizosphere were obtained. For individuals ofP. lanceolata a correlation was found between the numbers of nitrifying bacteria in the soil and NaR activity in the leaves. The results are discussed in relation to the ecological habitats of the three species.Grassland Species Research Group Publication No.38.     

Alleviation of salt-induced stress on seed emergence using soil additives in a greenhouse

Use of sodium chloride (NaCl) as a deicing salt results in high concentrations of ions in roadside soils, which decreases seedling emergence in these areas. Greenhouse experiments performed in soil culture tested the efficacy of three soil fertilizers, gypsum (CaSO₄), potash (potassium chloride, KCl) and potassium nitrate (KNO₃), in alleviating NaCl stress on seedling emergence of three grass species exhibiting a range of salt tolerance, Poa pratensis (Kentucky bluegrass), Bouteloua gracilis (blue grama), and Puccinellia distans (alkali grass). Two-factor factorial designs were utilized for each species-fertilizer combination. Treatments of 5000 mg/L (0.086 M) NaCl with or without fertilizer, in concentrations that were equal to 0.5, 1, and 2 times the molar equivalent of 5000 mg/L NaCl were applied biweekly. Salt stress on Poa pratensis emergence was alleviated by all fertilizers with CaSO₄ having the greatest effect in alleviating NaCl stress and potash and potassium nitrate alleviating stress at lower treatment levels. Emergence of Bouteloua gracilis and Puccinellia distans was in most cases negatively effected by soil amendments.     

Effects of temperature, light, nitrate and pre chilling on seed germination of grassland plants

The effects of factorial combinations of alternating (20/10 ^oC) and constant temperature (20 ^oC), of light (intermittent) and dark, and of distilled water and nitrate on the germination of a range of species of indigenous grassland were investigated in 1979, and in 1980 when the effects of pre-chilling were also studied. Species differed greatly in their response to the eight sets of environmental conditions in 1979. With the exception of Lolium perenne ssp. perenne and Cynosurus cristatus, only a small percentage of seeds of most species was able to germinate in constant temperature in the dark. However, when light was supplied there was moderate germination of Anthoxanthum odoratum, Cerastium fontanum ssp. glabrescens, Festuca rubra, Holcus lanatus and Poa trivialis. Alternating temperature greatly increased the germination of most species and nitrate further increased germination of Agrostis capillaris, Deschampsia caespitosa, H. lanatus, Poa annua and P. trivialis. In alternating temperatures, light increased germination of these species even more than did nitrate. A. capillaris, D. caespitosa and P. annua required all three factors for maximum germination, and another 7% of seeds of A. capillaris also required gibberellic acid. Germination responses following sequential application of factors often differed from those resulting from simultaneous treatment: in particular, the germination of Ranunculus species was greatly enhanced. Although seeds of the species tested in 1980 were more germinable than those of the same species in 1979, they responded similarly to the different factors. Light increased the germination of both Taraxacum officinale and Plantago lanceolata in constant but not in alternating temperature, while nitrate was much more stimulatory to the latter species. About half the seeds of P. lanceolata were dormant. Pre-chilling at 4^oC for 7 days increased subsequent germination of all species when followed by constant temperature, except of A. capillaris in the dark and C. cristatus (already maximal) in the light. When followed by alternating temperature in the dark, pre-chilling greatly decreased germination of A. capillaris. In the light, where germination of most species was maximal, there was little effect of pre-chilling. Longer durations (21 and 42 days) of chilling of older seed gave similar results to the 7 day pre-chilling.     

Introduced plants on Kilimanjaro: tourism and its impact

Kilimanjaro, a world heritage site and an icon of global change, not only suffers from climatic alterations but also is undergoing a drastic socio-economic upheaval. A strong increase of tourism enhances the risk of introducing alien plant species in particular in the upper zones of Kilimanjaro. One such species is Poa annua L., a cosmopolitan weed of European origin on roadsides and pastures. The aim of this study is to document its distribution, the speed of its propagation and risks for the indigenous vegetation of Kilimanjaro, and to compare the findings with other introduced species on this mountain. Based on a complete survey of the vegetation of Kilimanjaro with about 1,500 vegetation plots, plant communities invaded by Poa annua are determined. As with most of the other neophytes on Kilimanjaro, Poa annua invades only anthropogenic vegetation but not undisturbed natural vegetation. Similar to the situation in middle Europe, this neophyte is on Kilimanjaro a constituent of the vegetation of trampled ground, occurring between about 1,600 and 4,000 m asl along climbing routes or their vicinity. On a newly opened climbing route a rapid invasion (5.6 km in 3 months) was observed, which makes it likely that Poa annua spread on Kilimanjaro during the last 30 years in parallel to the increase of the climbing tourism. Although Poa annua is still in the stage of propagation, an invasion of natural vegetation types seems to be unlikely.     

Mode of reproduction is detected by Parth1 and Sex1 SCAR markers in a wide range of facultative apomictic Kentucky bluegrass varieties

Gametophytic apomixis in Kentucky bluegrass (Poa pratensis L.) involves the parthenogenetic development of unreduced eggs from aposporic embryo sacs. Marker-assisted selection for the mode of reproduction in P. pratensis would avoid costly and time-consuming phenotypic progeny tests. We developed and tested two SCAR primer pairs that are associated with the mode of reproduction in P. pratensis. The SCAR primers identified the apomictic and sexual genotypes among progenies of sexual x apomictic crosses with very low bias. Furthermore, when tested on a wide range of Italian and exotic P. pratensis germplasm, they were able to unequivocally distinguish sexual from apomictic genotypes. This system should, therefore, allow new selection models to be set up in this species.     

Ecological strategy for soil contaminated with mercury

Aims

The paper presents results from plot experiments aimed at the development of an ecological strategy for soil contaminated with mercury. Meadow grass (Poa pratensis) was tested on mercury contaminated soil in a former chlor-alkali plant (CAP) in southern Poland for its phytoremediation potential.

Methods

The stabilisation potential of the plants was investigated on plots without additives and after the addition of granular sulphur. Biomass production, uptake and distribution of mercury by plants, as well as leachates and rhizosphere microorganisms were investigated, along with the growth and vitality of plants during one growing season.

Results

The analysed plants grew easily on mercury contaminated soil, accumulating lower amounts of mercury, especially in the roots, from soil with additive of granular sulphur (0.5 % w/w) and sustained a rich microbial population in the rhizosphere. After amendment application the reduction of Hg evaporation was observed.

Conclusions

The obtained results demonstrate the potential of using Poa pratensis and sulphur for remediation of mercury contaminated soil and reduction of the Hg evaporation from soil. In the presented study, methods of Hg reduction on “hot spots” were proposed, with a special focus on environmental protection. This approach provides a simple remediation tool for large areas heavily contaminated with mercury.

     

Effects of grazing on plant communities and successional processes vary along an aridity gradient at a northern temperate grassland

The intermountain grasslands of North America reach their most northern geographic extent in interior British Columbia’s Cariboo-Chilcotin region. Here, this study examined the long-term effects of livestock grazing exclusion and reductions in grazing severity on plant community characteristics including plant and litter cover, species richness and abundance of leading species of 33 grassland sites across a broad aridity and soil property gradient. Across the aridity gradient, grazing reduced species richness, plant cover, and litter cover. However, the effects of grazing on dominant species varied across the gradient. In more arid grasslands, historical grazing substantially reduced cover of late-seral native bunchgrass Psuedoroegnaria spicata, and repeated measurements indicate that very long time periods are necessary for successional processes associated with recovery of native bunchgrasses. At the cool-wet end of the aridity gradient, successional processes are more rapid but dominated by exotic species Poa pratensis and Tragopogon pratensis. Recent (past 20 years) light grazing and rest-rotation have favored Poa pratensis at the expense of native needlegrasses (Achnatherum spp. and Hesperostipa spp.). We suggest that absence of a dominant large-stature native bunchgrass for mesic grasslands was a key factor in the invasion and dominance of Poa pratensis.

     

Using structural equation modelling to test the passenger,driver and opportunist concepts in a Poa pratensis invasion

The passenger, driver, and opportunist models are conceptual models of the invasion process used to describe alternative invasion scenarios. In the passenger model, both the invasive species and native community respond independently to environmental changes. In the driver model, changes to the native community are driven by the invasive species, while in the opportunist model invasion occurs in response to changes in the native community. In any given invasion scenario, however, it is possible that the relationships between the invasive, the native community, and the environment correspond to some combination of these invasion models acting simultaneously. We study invasion by Poa pratensis in a grassland in Alberta, Canada. Poa pratensis is a non‐native plant implicated with loss of plant diversity in the region. In a three year field experiment, we manipulate the environment though defoliation, water addition, and nitrogen addition, and measure responses of P. pratensis cover, and cover and richness of the native community. We use structural equation modelling to describe the relationships between the invasive, the native community, and the environmental changes, and then interpret these relationships using the three invasion models. We found that P. pratensis predominantly invaded via the driver model, with subsequent reductions in native plant cover, but not in species richness. Positive effects of the environmental changes on P. pratensis also aided its ability to drive native cover. As well, we found some involvement of the opportunist model, through a negative relationship between the native community and the invasive. As invasion mainly proceeded via the driver model, management actions to limit invasion should focus on efforts to control abundance of P. pratensis itself.     

矮嵩草草甸主要植物不同器官对氮素的吸收及分配特征研究

以高寒矮嵩草(Kobresia humilis)草甸7个主要植物种为研究对象,利用15 N同位素标记技术,通过分析不同器官对氮素的吸收及分配特征,揭示主要植物种在群落中的生态适应性、竞争力和地位。结果显示:(1)矮嵩草的叶和茎、垂穗披碱草(Elymus nutans)的叶,以及双柱头藨草(Scirpus distigmaticus)和鹅绒委陵菜(Potentilla anserina)的叶、茎、根均偏好累积硝态氮,早熟禾(Poa annua)的穗和叶以及甘肃马先蒿(Pedicularis kansuensis)和短穗兔耳草(Lagotis brachystachya)的根均偏好积累铵态氮。(2)矮嵩草对吸收的甘氨酸和硝态氮主要分配于叶中,铵态氮分配于茎中;双柱头藨草对吸收的甘氨酸和硝态氮主要分配于茎中,铵态氮分配于叶中;垂穗披碱草和早熟禾对吸收的硝态氮和铵态氮主要分配于叶中;垂穗披碱草对吸收的甘氨酸主要分配于根中,而早熟禾将较多的甘氨酸分配到穗中;甘肃马先蒿对吸收的硝态氮主要分配于叶中,铵态氮分配于根中;鹅绒委陵菜对吸收的甘氨酸、硝态氮和铵态氮主要分配于叶中;短穗兔耳草对吸收的甘氨酸主要分配于叶中,硝态氮和铵态氮主要分配于根中。(3)在牧草生长盛期,矮嵩草草甸土壤的有机氮和无机氮主要贡献于甘肃马先蒿的花、早熟禾的穗、垂穗披碱草的根和鹅绒委陵菜的茎叶。研究表明,高寒矮嵩草草甸主要植物不同器官对氮素的吸收及分配呈现多元化特征,因不同植物种的生物学特性和生态适应习性而异。     

Effects of nitrogen supply on the anatomy and chemical composition of leaves of four grass species belonging to the genus Poa, as determined by image-processing analysis and pyrolysis–mass spectrometry

Previous experiments have shown that the anatomy and chemical composition of leaves of inherently fast- and slow-growing grass species, grown at non-limiting nitrogen supply, differ systematically. The present experiment was carried out to investigate whether these differences persist when the plants are grown at an intermediate or a very low nitrogen supply. To this end, the inherently fast-growing Poa annua L. and Poa trivialis L., and the inherently slow-growing Poa compressa L. and Poa pratensis (L.) Schreb. were grown hydroponically at three levels of nitrate supply: at optimum (RGR_max) and at relative addition rates of 100 and 50 mmol N (mol N)^?1 d^?1 (RAR₁₀₀ and RAR₅₀), respectively. As expected, at the lowest N supply, the potentially fast-growing species grew at the same rate as the inherently slow-growing ones. Similarly, the differences in leaf area ratio (LAR, leaf area:total dry mass), specific leaf area (SLA, leaf arear:leaf dry mass) and leaf mass ratio (LMR, leaf dry mass:total dry mass) disappeared. Under optimal conditions, the fast-growing species differed from the slow-growing ones in that they had a higher N concentration. There were no significant differences in C concentration. With decreasing N supply, the total N concentration decreased and the differences between the species disappeared. The total C concentration increased for the fast-growing species and decreased for the slow-growing ones, i.e. the small, but insignificant, difference in C concentration between the species at RGR_max increased with decreasing N supply. The chemical composition of the leaves at low N supply, analysed in more detail by pyrolysis–mass spectrometry, showed an increase in the relative amounts of guaiacyl lignin, cellulose and hemicellulose, whereas those of syringyl lignin and protein decreased. The anatomy and morphology of the leaves of the four grass species differing in RGR_max were analysed by image-processing analysis. The proportion of the total volume occupied by mesophyll plus intercellular spaces and epidermis did not correlate with the amount of leaf mass per unit leaf area (specific leaf mass, SLM) at different N supply. The higher SLM at low N supply was caused partly by a high proportion of non-veinal sclerenchymatic cells per cross-section and partly by the smaller volume of epidermal cells. We conclude that the decrease in relative growth rate (and increase in SLM) at decreasing N supply is partly due to chemical and anatomical changes. The differences between the fast- and slow-growing grass species at an optimum nutrient supply diminished when plants were growing at a limiting nitrogen supply.     

Biosynthesis of plant phenolic compounds in elevated atmospheric CO2

Experiments were carried out to determine the effects of elevated atmospheric carbon dioxide (CO₂) on phenolic biosynthesis in four plant species growing over three generations for nine months in a model plant community. Results were compared to those obtained when the same species were grown individually in pots in the same soils and controlled environment. In the model herbaceous plant community, only two of the four species showed any increase in biomass under elevated CO₂, but this occurred only in the first generation for Spergula arvensis and in the second generation for Poa annua. Thus, the effects of CO₂ on plant biomass and carbon and nitrogen content were species‐ and generation‐specific. The activity of the principle phenolic biosynthetic enzyme, phenylalanine ammonia lyase (PAL), increased under elevated CO₂ in Senecio vulgaris only in Generation 1, but increased in three of the four plant species in Generation 2. There were no changes in the total phenolic content of the plants, except for P. annua in Generation 1. Lignin content decreased under elevated CO₂ in Cardamine hirsuta in Generation 1, but increased in Generation 2, whilst the lignin content of P. annua showed no change, decreased, then increased in response to elevated CO₂ over the three generations. When the species were grown alone in pots, elevated CO₂ increased PAL activity in plants grown in soil taken from the Ecotron community after nine months of plant growth, but not in plants grown in the soil used at the start of the experiment (‘initial' soil). In P. annua, phenolic biosynthesis decreased under elevated CO₂ in initial soil, and in both P. annua and S. vulgaris there was a significant interaction between effects of soil type and CO₂ level on PAL activity. In this study, plant chemical composition altered more in response to environmental factors such as soil type than in response to carbon supply. Results were species‐specific and changed markedly between generations.     

Impact of chemical polymorphism of Thymus pulegioides on some associated plant species under natural and laboratory conditions

Abstract

Allelopathic potential of Thymus pulegioides L. chemical polymorphism was investigated under natural and laboratory conditions. A field analysis of 127 natural habitats hosting chemotypes of T. pulegioides with different ratios of phenolics, geraniol, and ɑ-terpinyl acetate was conducted. Effects of chemotypes, and their main compounds on seed germination and radicle growth of Trifolium pratense L. and Poa pratensis L. were conducted under laboratory conditions. Field analysis showed that Poa species were more plentiful in comparison with Trifolium species, independent of the chemotypical composition of T. pulegioides habitats. Laboratory tests with plant-acceptors showed a stronger inhibitory effect of essential oils on the germination and radicle growth of P. pratensis but in some instances germination was stimulated. Dissimilar effects were observed for the same allelochemical through air and water on the same plant-acceptor. Significantly, different effects of essential oils on radicle growth occurred in T. pratense and P. pratensis: with sensitivity to the phenolic chemotype via air and the ɑ-terpinyl acetate chemotype through water. This demonstrates that chemical polymorphism can expand communication opportunities of T. pulegioides with associated plant species. Combining investigations in natural habitats with laboratory experiments can help understand the effect of chemical polymorphism on plant-plant ecological interactions.