Metabolite changes with induction of Cuscuta haustorium and translocation from host plants

Cuscuta is a stem holoparasitic plant without leaves or roots, parasitizing various types of host plants and causing major problems for certain crops. Cuscuta is known as a generalist and, thus, must have unique parasite strategies to cope with different host plants. For elucidating metabolic responses and mechanisms of parasitization, metabolomic approaches using GC/MS were applied. We compared five stages of Cuscuta japonica: early stage seedlings, with far red light (FR) cue, with contact signal, haustorium induced seedlings by both signals and adult plant parasites on host plants. Sugars, amino acids, organic acids, nucleic acids, and polyols were identified from the polar phase fraction. The apical part contained metabolite profiles different from the haustorium induced part or the basal part. Amino acid and some organic acids were up-regulated for haustorium induction but decreased after parasitization. After attachment to different host plants, metabolite profiles of Cuscuta japonica changed dramatically due to the absorption of specific host plant metabolites such as pinitol. Cuscuta seedlings attached to pinitol rich host plants contained more pinitol and showed different profiles from those attached to plants having less or lacking pinitol.     

The parasitic mechanism of the holostemparasitic plant Cuscuta

Cuscuta is a stem holoparasitic plant without leaves or roots, which develops a haustorium and sucks nutrients from host plants. The genus Cuscuta comprises about 200 species, many of which can cause severe problems for certain crops. The parasitic process in Cuscuta begins in finding and attaching to a host plant and then developing a haustorium. The process does not always require any chemical signal, but does require a light signal. Finding a host involves detecting the lower red light:far-red light ratio near a potential host plant by phytochrome. A contact signal is also necessary for haustorium induction. Apparently, cytokinin increase is downstream of the light and contact signal and is critical for haustorium induction. This pathway, however, appears to be slightly different from a standard pathway. The direct connection between Cuscuta and its host involves both the xylem and phloem, and mRNA and proteins can translocate. Several features indicate that Cuscuta is a useful model plant for parasite plant research as well as plant–plant interaction research. These include the simple anatomical structure and seedling development, no chemical requirement for haustorium induction, and the wide range of host plants.     

Specificity of systemically released rice stem volatiles on egg parasitoid,Trichogramma japonicum Ashmead behaviour

The yellow stem borer, Scirpophaga incertulas Walker (YSB), infested rice plants emit chemicals through the surface of their infested stems. These induce attractant activity and cause arrestment responses and ovipositional stimulation in its egg parasitoid, Trichogramma japonicum Ashmead. Laboratory experiments on short‐range host searching and oviposition were performed to assess the how these crude stem extracts and their fractions influence the biological control efficiency of Trichogramma. The activity of these chemicals as long‐range attractants was confirmed through wind tunnel bioassays. Stem borer‐infested plant extracts had enhanced the parasitization rate of T. japonicum, whereas host eggs treated with the extract from undamaged stems or solvent‐treated control failed to evoke changes in the parasitoid’s behaviour. A preliminary GC‐MASS analysis indicated the presence of several hydrocarbon compounds. The analysis also revealed qualitative and quantitative differences between the chemical profiles of the infested and non‐infested plants. We hypothesized that herbivore‐induced plant chemicals are released through the stem surfaces and attract T. japonicum, even over long distances. These cues elicit parasitoid arrestment on pest‐damaged plants and subsequently lead to the successful parasitization of the stem borer.     

玉米瘤黑粉菌的寄生策略及其调控机制

玉米瘤黑粉病是由担子菌Ustilago maydis对玉米的活体寄生所引起的真菌病害。该病原菌为双相型真菌,需要寄生于玉米植株来完成其有性生殖过程。综合相关研究报道,本文把U.maydis对寄主植物的寄生过程划分为7个阶段,包括形成致病性双核菌丝体、附着寄主植物表面、穿透寄主表皮、消减寄主防御反应、在寄主体内菌丝增殖、使寄主瘤变和生成厚垣孢子等。围绕寄生进程特点和关键基因,分别阐述了各个阶段的相关调控机制以及对寄主植物的致病性;展现了U.maydis为达到有性生殖目的而实施步步为营的寄生策略。本文对U.maydis寄生过程的阶段划分,有助于人们深入了解U.maydis与寄主植物之间互作机制、提供相关病害防控新思路。     

Tocochromanol content and composition in different species of the parasitic flowering plant genus Cuscuta

The holoparasitic plant genus Cuscuta is comprised of species with various degrees of plastid functionality and significant differences in photosynthetic capacity, ranging from moderate to no photosynthetic carbon fixation. In the present study, several Cuscuta species were analyzed with respect to the overall contents of tocochromanols and plastoquinone and the levels of the individual tocochromanols. No correlations among photosynthetic capacity, the amount of carotenoids, of plastoquinone and of tocochromanols were observed. On the contrary, wide variation in the composition of the tocochromanol fraction was observed among different species, as well as in stems of the same species in response to starvation conditions. The implications of these findings are discussed.     

Diversity,evolution, and function of stomata bearing structures in Cuscuta (dodders,Convolvulaceae): From extrafloral nectar secretion to transpiration in arid conditions

Cuscuta includes ca. 200 species of functionally holoparasitic plants grouped in four subgenera: Monogynella, Cuscuta, Pachystigma, and Grammica. Multicellular structures with stomata in Cuscuta are represented by extrafloral nectaries (ENs), reported from the stems of one Monogynella species, and stomatiferous protuberances (SPs), which are non-secretory. These latter structures had been noted on the stems of three Grammica species more than a century ago but entirely forgotten until recently when similar, non-secretory SPs were reported on the flowers of several new Grammica species. Here we study for the first time: (1) the extent of occurrence, diversity and evolution of secretory (ENs) and non-secretory (SPs) multicellular structures in Cuscuta, and (2) the function of SPs. We undertook a character evolution study of ENs and SPs on the stems and flowers of 136 Cuscuta taxa, and examined the structure/ultrastructure of SPs. ENs are inferred as primitive and characterize subg. Monogynella. SPs are derived in the remaining subgenera; they are ubiquitous on the flowers of Cuscuta and Pachystigma, but absent on their stems. Subgenus Grammica species develop two functional types of stems during their life cycle: vegetative, exploratory stems with very low stomatal densities (and no SPs), and reproductive, haustorial stems with numerous SPs. Moreover, 24 species from nine clades of subg. Grammica have evolved morphologically diverse floral SPs with systematic significance. To preliminarily ascertain SP function, we determined in the field the water uptake of Tithonia tubiformis plants parasitized or not by Cuscuta costaricensis, a species with both stem and floral SPs, and the stomatal conductance of dodder stems and flowers, as well as host leaves. Water uptake of parasitized hosts was significantly higher compared to non-parasitized plants, even after host leaves were removed, both during the day and night. The increased water uptake of parasitized hosts and stomatal conductance values suggest a transpiration role for the SPs, which is also confirmed by their lacunar structure. Grammica species with floral SPs grow in arid areas or characterized by a pronounced dry season during flowering/fruiting, which suggests that SPs may have evolved to stimulate the host water uptake during these phenophases.     

Connectivity counts: disentangling effects of vegetation structure elements on the searching movement of a parasitoid

1. A heterogeneous habitat structure can have a profound impact on foraging carnivorous arthropods. In the present study, we examined which elements of complex vegetation structure influence the searching movement of a parasitoid model organism. 2. Previous field work showed that tall and dense vegetation reduces the parasitism success of the eulophid egg parasitoid Oomyzus galerucivorus while the probability of host egg deposition increased close to plant tips. 3. In laboratory bioassays, dried grass stems were arranged according to the natural situation in different setups. The wasps' walking time on stems increased with increasing stem height and density. High stem density decreased the walking time of the parasitoids on the ground and an increased stem height reduced the propensity to fly to the ground. Connectivity had a minor positive effect on the number of stem contacts, but considerably reduced the number of wasps reaching the upper part of grass stems by two‐thirds. 4. Thus, although enhanced vegetation complexity enhances walking activity of the parasitoids in the vegetation, laying eggs at the tip of long grass stems in dense vegetation can be an adaptive strategy for the host, as it maximises the number of connections between plant parts to cross by parasitoids before reaching the host. The connection points disorient the wasps, which lose time, reverse their direction or fly away.     

Larval host plants of two lizard beetles in the genus Tetraphala (Coleoptera,Erotylidae, Languriinae,Languriini) from Taiwan,with a host plant list of Languriini

Lizard beetles (Erotylidae, Languriinae, Languriini) are known as stem borers of plants and contain agricultural pests and endangered species, but their species–host plant associations have been poorly documented. Here we investigated the larval host plants of two species of the genus Tetraphala Strum, T. collaris (Crotch) and Tetraphala sp. occurring in Taiwan. Females of T. collaris excavated living leafstalks and stems of the herbaceous dicot, Sambucus chinensis (Adoxaceae) using their mandibles for oviposition. We observed the eggs and early-instar larvae inside and nearby oviposition holes and late-instar larvae inside stems, suggesting that T. collaris uses living leafstalks and stems of S. chinensis as oviposition substrate and the larvae tunnel into stems with feeding on the tissues. Similarly, females of Tetraphala sp. excavated living leafstalks of the fern, Pteris wallichiana (Pteridaceae) using their mandibles for oviposition. We observed the eggs and early-instar larvae inside and nearby oviposition holes. When reared in laboratory, a larva reached adulthood inside the leafstalk. These results indicated that Tetraphala sp. uses living leafstalks of Pt. wallichiana as oviposition substrate and the larvae complete their development within. This study revealed that the genus Tetraphala contains both fern- and dicot-users during larval period. Further study is needed to clarify the evolutionary process of host plant use of languriines. Additionally, the host plant list of Languriini is provided.     

Gypsy moth herbivory induced volatiles and reduced parasite attachment to cranberry hosts

Interactions between species can have cascading effects that shape subsequent interactions. For example, herbivory can induce plant defenses that affect subsequent interactions with herbivores, pathogens, mycorrhizae, and pollinators. Parasitic plants are present in most ecosystems, and play important roles in structuring communities. However, the effects of host herbivory on parasitic plants, and the potential mechanisms underlying such effects, are not well known. We conducted a greenhouse study to ask whether gypsy moth (Lymantria dispar) damage, host cultivar, and their interaction affected preference of the stem parasite dodder (Cuscuta spp.) on cranberry hosts (Vaccinium macrocarpum). We then assessed the mechanisms that could underlie such effects by measuring induced changes in phytohormones and secondary compounds. We found that damage by gypsy moths delayed dodder attachment by approximately 0.3 days when dodder stems were added 2 days after damage, and reduced attachment by more than 50% when dodder stems were added 1 week after host plant damage. Gypsy moth damage significantly increased jasmonic acid (JA) levels, total volatile emissions, and the flavonol, quercetin aglycone, suggesting possible mechanisms underlying variation in dodder ability to locate or attach to hosts. Dodder preference also differed between cranberry cultivars, with the highest attachment on the cultivar that had significantly lower levels of total volatile emissions and total phenolic acids, suggesting that volatile composition and phenolics may mediate dodder preference. Our results indicate that herbivory can reduce subsequent attachment by a highly damaging parasitic plant, demonstrating the potential importance of early damage for shaping subsequent species interactions.     

Effect of insecticide application and host plant resistance on parasitization of sorghum midge,Contarinia sorghicola coq.

Sorghum midge (Contarinia sorghicola Coq.) (Diptera: Cecidomyiidae) is one of the most important pests of grain sorghum. The effects of different insecticides applied at the complete‐an‐thesis stage for insect control, and host plant resistance on the parasitization of sorghum midge, were studied. Tetrastichus spp. and Eupelmus popa are the major parasites of sorghum midge larvae. Of the several insecticides tested, monocrotophos reduced midge parasitization by half compared with the untreated control. Midge parasitization was lower on midge‐resistant genotypes. Amongst the lines resistant to sorghum midge, the levels of parasitization were higher on the moderately resistant lines IS 10132 and PM 9760 compared to those on IS 3461, IS 7005, IS 9807, IS 19512 and AF 28, etc. Parasite activity closely followed the midge population density across sowing dates. The role of these factors on the effectiveness of midge parasites in integrated pest management are discussed.     

Diet mixing in a parasitic plant: adaptation or constraint?

Some herbivores deliberately consume a mixed diet, either to obtain a superior mix of nutrients or to avoid consuming too much of any one toxin. Few studies have examined diet mixing in parasitic plants, which typically have very broad host ranges. We offered the parasitic plant Cuscuta indecora (dodder), a range of mixtures of two hosts (Iva frutescens and Borrichia frutescens) in the greenhouse, and observed correlations between the host community and Cuscuta infection in the field. In the greenhouse, Cuscuta performed better on mixtures with a higher relative abundance of Iva. Cuscuta selectively foraged on whichever host was more abundant (diet switching), the exact opposite of the behavior that would be expected if diet mixing was advantageous. In the field, the intensity of Cuscuta infections was decreased by the presence of non-hosts (grasses), not strongly affected by the presence of intermediate hosts, and increased by the presence of Borrichia. We conclude that Cuscuta does not obtain nutritional benefits from a broad diet, but instead is constrained by its relative lack of mobility to attack hosts of intermediate value. In general, the lack of mobility of parasitic plants compared to herbivores probably selects for broad host ranges in parasitic plants.     

Diversity and evolution of pollen-ovule production in <Emphasis Type="Italic">Cuscuta</Emphasis> (dodders,Convolvulaceae) in relation to floral morphology

Cuscuta (dodder, Convolvulaceae) is a genus of about 200 species of obligate stem parasites with subcosmopolitan distribution. The diversity of pollen and ovule production was surveyed in 128 species and ten varieties. Taxa were assigned to Cruden’s mating system categories based on their pollen-ovule ratios. Variation and correlations among floral characters were analyzed using regression and ANOVA, while the mating system categories were subjected to a linear discriminant and canonical variates analysis to assess their cohesiveness. Our data strongly suggest that most Cuscuta species possess a wide range of mixed-mating systems. Whereas four ovules develop in each flower, pollen production varies over three orders of magnitude. Several Cuscuta taxa are highly outcrossing, but no species could be identified that are exclusively selfing. The transition from the one-style flowers of subg. Monogynella to the two-style flowers of subgenera Cuscuta and Grammica, and from simultaneous to sequential maturation of the two stigmas in the latter subgenus, has decreased the role of herkogamy as a facilitator of outcrossing. These evolutionary changes are associated with an increase of species richness in subgenus Cuscuta, and especially in subgenus Grammica. Morphological features were not individually found to have a strong correlation to the mating system, but in general, larger corollas and stigmas were associated with greater pollen-ovule ratios. Cuscuta presents some puzzling results when considered in light of the sex allocation theory, as only some infrageneric lineages demonstrate the predicted pollen size-number tradeoff, while Cuscuta gracillima complex (in subgenus Grammica) displays an unexpected negative relationship between pollen size and style length. The relationship between host range and mating system is discussed, prompting further research into the co-evolution of pollination systems and life history traits between parasites and their host species.     

The influence of plant species on attraction and host acceptance inCotesia glomerata (Hymenoptera: Braconidae)

Females of the larval parasitoidCotesia glomerata (L.) use plant-associated cues to locate their lepidopteran host,Pieris rapae L. In this study we investigated the influence of four host plant species,Brassica oleracea var.acephala (‘Vates’ kale),Tropaeolum majus (nasturtium),Lunaria annua (honesty), andCleome spinosa (spider flower), on two components of the host selection process inC. glomerata, namely, attraction and host acceptance. Choice tests in a flight tunnel showed that parasitoids were attracted to some host plant species more than to others in the absence of host larvae.B. oleracea was the most attractive plant species, followed byL. annua, T. majus, andC. spinosa. In previous studies it was shown thatB. oleracea carries highly suitable hosts forC. glomerata and that, in the field, parasitization rates on this plant were the highest. When host larvae were reared on the four host plant species and then transferred to a common substrate (B. oleracea var.capitata, cabbage), plant species that had served as diet for the hosts did not have a significant effect on acceptance for parasitization. Thus, parasitoids were attracted to host plant species differentially, but they did not discriminate among host larvae based on the dietary history of their hosts. ForC. glomerata, it appears that phytochemistry mediates host selection more by influencing parasitoid attraction than it does by affecting host acceptance.     

空心莲子草响应南方菟丝子寄生的生长-防御权衡

为探讨全寄生植物南方菟丝子(Cuscuta australis)防治入侵植物空心莲子草(Alternanthera philoxeroides)的可行性,以二者野外天然生长的种群为研究对象,分析南方菟丝子寄生对空心莲子草生长及防御的影响,阐明空心莲子草在受到寄生胁迫时如何权衡自身生长与防御的关系,进而发展出一套应对南方菟丝子寄生的生长-防御策略。结果显示:(1)南方菟丝子寄生显著改变空心莲子草茎的形态,茎直径和平均节间长均增加,茎直径变化极显著(P0.01);(2)南方菟丝子寄生显著减少空心莲子草叶片数,但同时显著增加后者茎的分枝数,而茎上的节是潜在的无性繁殖体,故有利于空心莲子草的克隆繁殖;此外,南方菟丝子寄生显著降低了空心莲子草的根、茎、叶生物量和总生物量,抑制空心莲子草的生长;(3)南方菟丝子寄生显著增加空心莲子草茎的单宁、总酚、三萜皂苷含量,增强其防御能力;(4)南方菟丝子寄生的空心莲子草的生物量与茎部木质素、三萜皂苷、单宁和总酚含量均呈现显著负相关性(P0.01),对照组则不存在相关性;且寄生组较对照组相比,生物量的相对百分比显著低于对照组(P0.01),而用于防御的次生代谢产物总含量的相对百分比显著高于对照组(P0.01)。以上结果表明,受到南方菟丝子寄生胁迫后,空心莲子草改变自身的生长-防御策略,减少营养生长投入而将更多的资源投向克隆繁殖,同时增强对"防御"物质的投入,增强其防御能力,以利于后代生存和繁衍。     

Structural refuges in two stem-boring weevils on Rumex crispus

Abstract.

• 1 Differences in feeding positions, parasitoid complexes and parasitization rates of the two dominant insect species in stems of Rumex crispus were investigated. Apion miniatum (inhabiting the lower 10% of the stem) and Apion violaceum (occurring all along the stem) exploited distinctly different feeding positions, partly due to differences in time of oviposition.
• 2 A.miniatum suffered less parasitization thanA.violaceum, measured both by number of parasitoid species and parasitization rates. Its feeding position constitutes a refuge.
• 3 Parasitization of A.violaceum was least in individuals feeding in basal, thick parts of the stem. There is a trend that the developed refuge increases with increasing stem diameter.
• 4 The refuge of A.violaceum in basal positions only affected idiobiont ectoparasitoids, because koinobiont endoparasitoids attack the weevils before they enter the stem. Moreover, the koinobiont did benefit from the refuge itself by avoiding hyperparasitism.
• 5 Densities of A.violaceum increased with plant size, whereas percentage parasitism did not differ. Its three most abundant parasitoid species showed small-scale density-independent patterns of attack over all plants. Therefore a constant proportion of weevil larvae mine within the refuge independent of its potential size.
• 6 Synchronization between weevils and plants, as well as between parasitoids and hosts, appears to be the dominant factor for the spatial community structure of the stem borers of Rumex crispus and their enemies.

     

Host-produced ethylene is required for marked cell expansion and endoreduplication in dodder search hyphae

The genus Cuscuta comprises stem holoparasitic plant species with wide geographic distribution. Cuscuta spp. obtain water, nutrients, proteins, and mRNA from their host plants via a parasitic organ called the haustorium. As the haustorium penetrates into the host tissue, search hyphae elongate within the host tissue and finally connect with the host’s vascular system. Invasion by Cuscuta spp. evokes various reactions within the host plant’s tissues. Here, we show that, when Arabidopsis (Arabidopsis thaliana) is invaded by Cuscuta campestris, ethylene biosynthesis by the host plant promotes elongation of the parasite’s search hyphae. The expression of genes encoding 1-aminocylclopropane-1-carboxylic acid (ACC) synthases, ACC SYNTHASE2 (AtACS2) and ACC SYNTHASE6 (AtACS6), was activated in the stem of Arabidopsis plants upon invasion by C. campestris. When the ethylene-deficient Arabidopsis acs octuple mutant was invaded by C. campestris, cell elongation and endoreduplication of the search hyphae were significantly reduced, and the inhibition of search hyphae growth was complemented by exogenous application of ACC. In contrast, in the C. campestris-infected Arabidopsis ethylene-insensitive mutant etr1-3, no growth inhibition of search hyphae was observed, indicating that ETHYLENE RESPONSE1-mediated ethylene signaling in the host plant is not essential for parasitism by C. campestris. Overall, our results suggest that C. campestris recognizes host-produced ethylene as a stimulatory signal for successful invasion.

Growth of Cuscuta campestris search hyphae is inhibited in ethylene-deficient Arabidopsis mutants, suggesting that host-derived ethylene acts as a stimulatory signal for parasitism by Cuscuta spp.     

<Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> is a susceptible host plant for the holoparasite <Emphasis Type="Italic">Cuscuta </Emphasis>spec

Arabidopsis thaliana and Cuscuta spec. represent a compatible host–parasite combination. Cuscuta produces a haustorium that penetrates the host tissue. In early stages of development the searching hyphae on the tip of the haustorial cone are connected to the host tissue by interspecific plasmodesmata. Ten days after infection, translocation of the fluorescent dyes, Texas Red (TR) and 5,6-carboxyfluorescein (CF), demonstrates the existence of a continuous connection between xylem and phloem of the host and parasite. Cuscuta becomes the dominant sink in this host–parasite system. Transgenic Arabidopsis plants expressing genes encoding the green fluorescent protein (GFP; 27 kDa) or a GFP–ubiquitin fusion (36 kDa), respectively, under the companion cell (CC)-specific AtSUC2 promoter were used to monitor the transfer of these proteins from the host sieve elements to those of Cuscuta. Although GFP is transferred unimpedly to the parasite, the GFP–ubiquitin fusion could not be detected in Cuscuta. A translocation of the GFP–ubiquitin fusion protein was found to be restricted to the phloem of the host, although a functional symplastic pathway exists between the host and parasite, as demonstrated by the transport of CF. These results indicate a peripheral size exclusion limit (SEL) between 27 and 36 kDa for the symplastic connections between host and Cuscuta sieve elements. Forty-six accessions of A. thaliana covering the entire range of its genetic diversity, as well as Arabidopsis halleri, were found to be susceptible towards Cuscuta reflexa.     

Facing herbivory on the climb up: Lost opportunities as the main cost of herbivory in the wild yam Dioscorea praehensilis

Plants with simple architecture and strong constraints on their growth may offer critical insights into how growth strategies affect the tolerance of plants to herbivory. Although Dioscorea praehensilis, a wild yam of African forests, is perennial, both aerial apparatus and tuber are annually renewed. Each year, the tuber produces a single stem that climbs from the ground to the forest canopy. This stem bears no leaves and no branches until it reaches optimal light conditions. Once in the canopy, the plant's production fuels the filling of a new tuber before the plant dies back to the ground. We hypothesized that if deprived of ant defense, the leafless growth phase is a vulnerable part of the cycle, during which a small amount of herbivory entails a high cost in terms of loss of opportunity. We compared the growth of stems bearing ants or not as well as of intact stems and stems subjected to simulated or natural herbivory. Ants reduce herbivory; herbivory delays arrival to the canopy and shortens the season of production. Artificially prolonging the stem growth to the canopy increased plant mortality in the following year and, in surviving plants, reduced the stem diameter and likely the underground reserves produced. Tuber size is a key variable in plant performance as it affects both the size of the aerial apparatus and the duration of its single season of production. Aerial apparatus and tuber are thus locked into a cycle of reciprocal annual renewal. Costs due to loss of opportunity may play a major role in plant tolerance to herbivory, especially when architectural constraints interact with ecological conditions to shape the plant's growth strategy.     

基质养分对寄生植物南方菟丝子生长的影响

群落中各营养级的相互作用在群落结构形成中起了重要作用.以南方菟丝子(Cuscuta australis R.Br.)和三叶鬼针草(Biden pilosa L.)为研究对象,采用完全随机区组实验设计方法,测定并分析基质养分(不施肥与施肥)对寄生植物生长的影响,探讨寄生植物生物量与寄主生长特性、生物量和光源捕获能力的相关性.结果表明,施肥显著增加寄生植物南方菟丝子的吸器数量、缠绕圈数、相对盖度、营养器官生物量、生殖器官生物量和总生物量,但对生殖器官的生物量比无显著影响.施肥显著增加寄主植物的根、茎、叶生物量和总生物量、叶生物量比、比叶面积和叶绿素含量,但显著降低根冠比与根生物量比.南方菟丝子生物量与三叶鬼针草生物量、叶生物量比、比叶面积以及相对叶绿素含量之间均存在显著正相关,与根生物量比和根冠比存在显著负相关.研究结果表明施肥可以提高寄主植物的光资源捕获能力,将更多地生物量分配至叶等光合机构上,从而促进寄主植物(生产者)的生长,并间接促进寄生植物(初级消费者)的生长.     

Like herbivores,parasitic plants are limited by host nitrogen content

Herbivores generally benefit from increased plant nitrogen content, because the nitrogen content of animals is much higher than that of plants. Consequently, high plant nitrogen alleviates the profound stoichiometric imbalance that herbivores face in their diets. Parasitic plants provide the opportunity to test this generalization for consumers across kingdoms. We fertilized two microhabitats in a California salt marsh that were dominated by Salicornia virginica or a mixture of S. virginica and Jaumea carnosa. The nitrogen content of both host plants and of the holoparasite Cuscuta salina (dodder) increased in fertilized plots in both microhabitats. Cuscuta preferred to attack Jaumea, although Jaumea had lower nitrogen content than Salicornia. When host nitrogen content was altered by fertilizing plots, however, the percent cover of the parasite doubled. Although parasitic plants and their hosts have similar tissue nitrogen contents, suggesting no stoichiometric imbalance between host and consumer, parasitic plants do not feed on host tissue, but on host xylem and phloem, which are very low in nitrogen. Consequently, parasitic plants face the same dietary stoichiometric constraints as do herbivores, and both herbivores and holoparasitic plants may respond positively to increases in host nitrogen status.