Does phosphorus influence performance of a native hemiparasite and its impact on a native legume?

Phosphorus (P) is an essential plant nutrient and can become limiting in terrestrial ecosystems where parasitic plant:host associations occur. Yet little is known on how P availability influences parasite performance and its impact on hosts. We investigated the performance of the Australian native stem hemiparasite Cassytha pubescens and its impact on the native leguminous shrub Acacia paradoxa in high or low P conditions in a glasshouse experiment. Infected plants had significantly lower total, shoot, root and nodule biomass and shoot:root ratio than uninfected plants, regardless of P supply. The significant negative effect of infection on arbuscular mycorrhizal colonisation of host roots was more severe in the high P treatment. Infection significantly decreased predawn quantum yield of A. paradoxa in low P but not high P conditions. This finding may be due to the parasite-induced significant enrichment of aluminium in host foliage in low P but not high P treatments. A. paradoxa had significantly lower foliar phosphorus concentration [P] and nitrogen concentration in low P than high P conditions. Parasite biomass and photosynthetic performance were unaffected by P, whereas C. pubescens had significantly lower stem [P] in the low P than high P treatment. Parasite carbon isotope composition was significantly higher than that of the host, especially in low P conditions. Our results show that: (a) native parasite growth and its negative impact on growth of this native shrub was unaffected by P supply and (b) soil P conditions may have no influence on stem hemiparasite:host associations in nature.

The combined effects of water and nitrogen on the relationship between a native hemiparasite and its invasive host.

Stem hemiparasites are dependent on their hosts for water and nitrogen. Most studies, however, have assessed the influence of one factor on parasite : host associations, thus limiting our mechanistic understanding of their performance in nature. We investigated the combined effects of water and nitrogen (N) availability on both a host (Ulex europaeus) and its parasite (Cassytha pubescens). Parasite infection significantly decreased host shoot biomass and shoot : root ratio more severely in high water than low water, irrespective of N supply. Parasite stem [N] was significantly higher in high water than low water treatments, regardless of N supply, but parasite biomass did not vary among treatments. Irrespective of water and N supply, infected plants had significantly lower total, root and nodule biomass, predawn and midday quantum yields, maximum electron transport rates, water potentials and nitrogen concentration [N]. Parasite δ13 C was significantly higher than that of the host. Our results suggested that stem hemiparasites can better extract resources from hosts when water availability is high, resulting in a greater impact on the host under these conditions. When hemiparasitic plants are being investigated as a biocontrol for invasive weeds, they may be more effective in wetter habitats than in drier ones.

The impact of a native hemiparasite on a major invasive shrub is affected by host size at time of infection.

Many studies have investigated the effect of parasitic plants on their hosts; however, few have examined how parasite impact is affected by host size. In a glasshouse experiment, we investigated the impact of the Australian native hemiparasitic vine, Cassytha pubescens, on a major invasive shrub, Ulex europaeus, of different sizes. Infected plants had significantly lower total, shoot, and root biomass, but the parasite's impact was more severe on small than on large hosts. When infected, small but not large hosts had significantly lower nodule biomass. Irrespective of size, infection significantly decreased the host shoot/root ratio, pre-dawn and midday quantum yields, maximum electron transport rates, and carbon isotope composition, and the host nodule biomass per gram of root biomass significantly increased in response to infection. Infection did not affect host foliar nitrogen concentration or midday shoot water potential. Parasite biomass was significantly lower on small relative to large hosts, but was similar when expressed on a per gram of host total biomass basis. Parasite stem nitrogen, phosphorus, and potassium concentrations were significantly greater when C. pubescens was growing on small than on large hosts. Our results clearly show that C. pubescens strongly decreases performance of this major invasive shrub, especially when hosts are small. This suggests that C. pubescens could be used most effectively as a native biocontrol when deployed on smaller hosts.

A native parasitic plant affects the performance of an introduced host regardless of environmental variation across field sites.

Increasing evidence from glasshouse studies shows that native hemiparasitic plants can significantly impact the performance and growth of introduced host plants. We investigated the effect of the native Australian hemiparasite Cassytha pubescens R.Br. on the introduced shrub Ulex europaeus L. at three field sites in South Australia. Parasite infection significantly decreased midday PSII efficiency (ΦPSII) and the maximum electron transport rates (ETRmax) of U. europaeus across sites. The impact of C. pubescens on the photosynthetic performance of U. europaeus may have been caused by infected plants having significantly lower N and K, but higher Fe and Al than uninfected plants at all sites. Significant Al and Fe enrichment in infected plants may be possibly due to the parasite indirectly inducing rhizosphere acidification. At two sites, C. pubescens significantly affected host Fv/Fm, indicating chronic photoinhibition in response to infection. The impact of infection on Fv/Fm was greatest at the wettest site, in line with an experiment where C. pubescens had more impact under high water availability. At this site, infected plants also had the highest foliar Fe and Al. The C isotope (δ13C) of infected plants was significantly lower than that of uninfected plants at only one site. Unusually, the δ13C of the parasite was the same as or significantly higher than that of the hosts. There were no site effects on parasite Fv/Fm or ΦPSII; however, ETRmax and δ13C varied across sites. The results suggest that this native parasite has negative effects on U. europaeus in the field, as was found for glasshouse studies. The abundance of this introduced weed in Australia could be negatively affected by C. pubescens infection.

Does nitrogen affect the interaction between a native hemiparasite and its native or introduced leguminous hosts?

Associations between plants and nitrogen (N)-fixing rhizobia intensify with decreasing N supply and come at a carbon cost to the host. However, what additional impact parasitic plants have on their leguminous hosts' carbon budget in terms of effects on host physiology and growth is unknown. Under glasshouse conditions, Ulex europaeus and Acacia paradoxa either uninfected or infected with the hemiparasite Cassytha pubescens were supplied (high nitrogen (HN)) or not (low nitrogen (LN)) with extra N. The photosynthetic performance and growth of the association were measured. Cassytha pubescens significantly reduced the maximum electron transport rates and total biomass of U. europaeus but not those of A. paradoxa, regardless of N. Infection significantly decreased the root biomass of A. paradoxa only at LN, while the significant negative effect of infection on roots of U. europaeus was less severe at LN. Infection had a significant negative impact on host nodule biomass. Ulex europaeus supported significantly greater parasite biomass (also per unit host biomass) than A. paradoxa, regardless of N. We concluded that rhizobia do not influence the effect of a native parasite on overall growth of leguminous hosts. Our results suggest that C. pubescens will have a strong impact on U. europaeus but not A. paradoxa, regardless of N in the field.

High water availability increases the negative impact of a native hemiparasite on its non-native host.

Environmental factors alter the impacts of parasitic plants on their hosts. However, there have been no controlled studies on how water availability modulates stem hemiparasites' effects on hosts. A glasshouse experiment was conducted to investigate the association between the Australian native stem hemiparasite Cassytha pubescens and the introduced host Ulex europaeus under high (HW) and low (LW) water supply. Cassytha pubescens had a significant, negative effect on the total biomass of U. europaeus, which was more severe in HW than LW. Regardless of watering treatment, infection significantly decreased shoot and root biomass, nodule biomass, nodule biomass per unit root biomass, F v/F m, and nitrogen concentration of U. europaeus. Host spine sodium concentration significantly increased in response to infection in LW but not HW conditions. Host water potential was significantly higher in HW than in LW, which may have allowed the parasite to maintain higher stomatal conductances in HW. In support of this, the δ(13)C of the parasite was significantly lower in HW than in LW (and significantly higher than the host). C. pubescens also had significantly higher F v/F m and 66% higher biomass per unit host in the HW compared with the LW treatment. The data suggest that the enhanced performance of C. pubescens in HW resulted in higher parasite growth rates and thus a larger demand for resources from the host, leading to poorer host performance in HW compared with LW. C. pubescens should more negatively affect U. europaeus growth under wet conditions rather than under dry conditions in the field.

Native hemiparasite and light effects on photoprotection and photodamage in a native host.

Plants infected with hemiparasites often have lowered rates of photosynthesis, which could make them more susceptible to photodamage. However, it is also possible that infected plants increase their photoprotective capacity by changing their pigment content and/or engagement of the xanthophyll cycle. There are no published studies investigating infection effects on host pigment dynamics and how this relates to host susceptibility to photodamage whether in high (HL) or low light (LL). A glasshouse experiment was conducted where Leptospermum myrsinoides Schltdl. either uninfected or infected with Cassytha pubescens R.Br. was grown in HL or LL and pigment content of both host and parasite were assessed. Infection with C. pubescens significantly decreased all foliar pigment concentrations (except chlorophyll b) in L. myrsinoides in both HL and LL. Xanthophyll cycle (violaxanthin, antheraxanthin, zeaxanthin; VAZ) and chlorophyll (Chl) pigments decreased in parallel in response to infection, hence, VAZ/Chl of the host was unaffected by C. pubescens in either HL or LL. Pre-dawn and midday de-epoxidation state [(A+Z)/(V+A+Z)] of L. myrsinoides was also unaffected by infection in both HL and LL. Thus, L. myrsinoides infected with C. pubescens maintained similar photoprotective capacity per unit chlorophyll and engagement of the xanthophyll cycle as uninfected plants. Even though midday quantum yield (ΦPSII) of HL plants was affected by infection, pre-dawn maximum quantum yields (Fv/Fm) of hosts were the same as uninfected plants whether in HL or LL. This ability of L. myrsinoides to maintain photoprotective capacity/engagement when infected by C. pubescens thereby preventing photodamage could explain this host's tolerance to hemiparasite infection.

Do wide crowns in arid woodland trees reflect hydraulic limitation and reduction of self-shading?

In arid regions many tree species develop broad crowns. A number of hypotheses involve trade-offs between growth in height and horizontal spreading, but there is no explanation for the switch from vertical to horizontal growth during development. Using Acacia papyrocarpa Benth as a model, we measured tree height and crown shape across different sites and topographic positions. We also measured δ13C of phyllodes from crown tops and lateral spreading branches. Trees were significantly taller at the base of a hill, where water availability is typically greater, than on the adjacent steep hillslope. In contrast, δ13C from the treetops was not significantly different across this topographic gradient, despite variation in tree height. In addition, δ13C was higher at treetops than in lower, lateral branches. These observations are consistent with hydraulic limitation to tree height. The shape of mature and young crowns in open environments was not symmetrical. At all sites, branches were shortest, but tree crowns tallest, on south-facing (i.e. shadiest) aspect of crowns. This suggests that light limitation may also affect crown development. If upper branches become water-limited and lower branches light-limited, then middle lateral branches become the less-stressed part of the crown and may grow more, producing a broad crown.

Global sampling of plant roots expands the described molecular diversity of arbuscular mycorrhizal fungi.

We aimed to enhance understanding of the molecular diversity of arbuscular mycorrhizal fungi (AMF) by building a new global dataset targeting previously unstudied geographical areas. In total, we sampled 96 plant species from 25 sites that encompassed all continents except Antarctica. AMF in plant roots were detected by sequencing the nuclear SSU rRNA gene fragment using either cloning followed by Sanger sequencing or 454-sequencing. A total of 204 AMF phylogroups (virtual taxa, VT) were recorded, increasing the described number of Glomeromycota VT from 308 to 341 globally. Novel VT were detected from 21 sites; three novel but nevertheless widespread VT (Glomus spp. MO-G52, MO-G53, MO-G57) were recorded from six continents. The largest increases in regional VT number were recorded in previously little-studied Oceania and in the boreal and polar climatic zones - this study providing the first molecular data from the latter. Ordination revealed differences in AM fungal communities between different continents and climatic zones, suggesting that both biogeographic history and environmental conditions underlie the global variation of those communities. Our results show that a considerable proportion of Glomeromycota diversity has been recorded in many regions, though further large increases in richness can be expected in remaining unstudied areas.

Correlations between physical and chemical defences in plants: tradeoffs, syndromes, or just many different ways to skin a herbivorous cat?

Most plant species have a range of traits that deter herbivores. However, understanding of how different defences are related to one another is surprisingly weak. Many authors argue that defence traits trade off against one another, while others argue that they form coordinated defence syndromes. We collected a dataset of unprecedented taxonomic and geographic scope (261 species spanning 80 families, from 75 sites across the globe) to investigate relationships among four chemical and six physical defences. Five of the 45 pairwise correlations between defence traits were significant and three of these were tradeoffs. The relationship between species' overall chemical and physical defence levels was marginally nonsignificant (P = 0.08), and remained nonsignificant after accounting for phylogeny, growth form and abundance. Neither categorical principal component analysis (PCA) nor hierarchical cluster analysis supported the idea that species displayed defence syndromes. Our results do not support arguments for tradeoffs or for coordinated defence syndromes. Rather, plants display a range of combinations of defence traits. We suggest this lack of consistent defence syndromes may be adaptive, resulting from selective pressure to deploy a different combination of defences to coexisting species.

Changes in seed dispersal processes and the potential for between-patch connectivity for an arid land daisy.

Dispersal is a major and critical process in population biology that has been particularly challenging to study. Animals can have major roles in seed dispersal even in species that do not appear specifically adapted to animal-aided dispersal. This can occur by two processes: direct movement of diaspores by animals and modification of landscape characteristics by animals in ways that greatly influence dispersal. We exploited the production of large, persistent dispersal structures (seed heads, henceforth) by Erodiophyllum elderi (Asteraceae), a daisy from arid Australia, to further understand secondary dispersal. Seed head dispersal on and off animal tracks in eight E. elderi patches was monitored for 9.5 months by periodically recording the location of marked seed heads. Sites were located inside a reserve that excludes sheep but not kangaroos, and in a nearby area with both kangaroos and sheep. The distance moved and likelihood of seed head movement was higher in areas with sheep, and especially along animal tracks. There was clear evidence that seed heads were channeled down animal tracks during large rainfall events. Seed head dispersal away from patches occurred to a limited extent via their physical contact with sheep and potentially via wind dispersal. Thus, the advantages of this study system allowed us to demonstrate the two postulated effects of herbivores on dispersal via direct movement of seed heads, and two distinct indirect effects through landscape modification by herbivores from the creation of animal tracks and the denudation of vegetation.

Putting plant resistance traits on the map: a test of the idea that plants are better defended at lower latitudes.

• It has long been believed that plant species from the tropics have higher levels of traits associated with resistance to herbivores than do species from higher latitudes. A meta-analysis recently showed that the published literature does not support this theory. However, the idea has never been tested using data gathered with consistent methods from a wide range of latitudes. • We quantified the relationship between latitude and a broad range of chemical and physical traits across 301 species from 75 sites world-wide. • Six putative resistance traits, including tannins, the concentration of lipids (an indicator of oils, waxes and resins), and leaf toughness were greater in high-latitude species. Six traits, including cyanide production and the presence of spines, were unrelated to latitude. Only ash content (an indicator of inorganic substances such as calcium oxalates and phytoliths) and the properties of species with delayed greening were higher in the tropics. • Our results do not support the hypothesis that tropical plants have higher levels of resistance traits than do plants from higher latitudes. If anything, plants have higher resistance toward the poles. The greater resistance traits of high-latitude species might be explained by the greater cost of losing a given amount of leaf tissue in low-productivity environments.

Rooting theories of plant community ecology in microbial interactions.

Predominant frameworks for understanding plant ecology have an aboveground bias that neglects soil micro-organisms. This is inconsistent with recent work illustrating the importance of soil microbes in terrestrial ecology. Microbial effects have been incorporated into plant community dynamics using ideas of niche modification and plant-soil community feedbacks. Here, we expand and integrate qualitative conceptual models of plant niche and feedback to explore implications of microbial interactions for understanding plant community ecology. At the same time we review the empirical evidence for these processes. We also consider common mycorrhizal networks, and propose that these are best interpreted within the feedback framework. Finally, we apply our integrated model of niche and feedback to understanding plant coexistence, monodominance and invasion ecology.

Underground friends or enemies: model plants help to unravel direct and indirect effects of arbuscular mycorrhizal fungi on plant competition.

*We studied the effects of two arbuscular mycorrhizal (AM) fungi, singly or together, on the outcome of competition between a host (tomato cultivar, wild-type (WT)) and a surrogate nonhost (rmc, a mycorrhiza-defective mutant of WT) as influenced by the contributions of the direct and AM phosphorus (P) uptake pathways to plant P. *We grew plants singly or in pairs of the same or different genotypes (inoculated or not) in pots containing a small compartment with (32)P-labelled soil accessible to AM fungal hyphae and determined expression of orthophosphate (P(i)) transporter genes involved in both AM and direct P uptake. *Gigaspora margarita increased WT competitive effects on rmc. WT and rmc inoculated with Glomus intraradices both showed growth depressions, which were mitigated when G. margarita was present. Orthophosphate transporter gene expression and (32)P transfer showed that the AM pathway operated in single inoculated WT, but not in rmc. *Effects of AM fungi on plant competition depended on the relative contributions of AM and direct pathways of P uptake. Glomus intraradices reduced the efficiency of direct uptake in both WT and rmc. The two-fungus combination showed that interactions between fungi are important in determining outcomes of plant competition.

Impacts of a native parasitic plant on an introduced and a native host species: implications for the control of an invasive weed.

BACKGROUND AND AIMS: While invasive species may escape from natural enemies in the new range, the establishment of novel biotic interactions with species native to the invaded range can determine their success. Biological control of plant populations can be achieved by manipulation of a species' enemies in the invaded range. Interactions were therefore investigated between a native parasitic plant and an invasive legume in Mediterranean-type woodlands of South Australia. METHODS: The effects of the native stem parasite, Cassytha pubescens, on the introduced host, Cytisus scoparius, and a co-occurring native host, Leptospermum myrsinoides, were compared. The hypothesis that the parasitic plant would have a greater impact on the introduced host than the native host was tested. In a field study, photosynthesis, growth and survival of hosts and parasite were examined. KEY RESULTS: As predicted, Cassytha had greater impacts on the introduced host than the native host. Dead Cytisus were associated with dense Cassytha infections but mortality of Leptospermum was not correlated with parasite infection. Cassytha infection reduced the photosynthetic rates of both hosts. Infected Cytisus showed slower recovery of photosystem II efficiency, lower transpiration rates and reduced photosynthetic biomass in comparison with uninfected plants. Parasite photosynthetic rates and growth rates were higher when growing on the introduced host Cytisus, than on Leptospermum. CONCLUSIONS: Infection by a native parasitic plant had strong negative effects on the physiology and above-ground biomass allocation of an introduced species and was correlated with increased plant mortality. The greater impact of the parasite on the introduced host may be due to either the greater resources that this host provides or increased resistance to infection by the native host. This disparity of effects between introduced host and native host indicates the potential for Cassytha to be exploited as a control tool.

Priority effects produced by plant litter result in non-additive competitive effects.

Litter may indirectly affect competitive interactions. It is not clear whether these changes are additive or non-additive indirect effects. Non-additivity could result from: (1) changes in biomass allocation patterns by competitors towards organs not directly involved in resource acquisition (e.g., longer hypocotyls); (2) changes in the proportion of different functional groups (e.g., grasses and forbs) that possess different competitive abilities; or (3) through priority effects caused by subtle changes in timing of emergence. We used a combination of field and glasshouse experiments in which Eucalyptus obliqua seedlings were grown either with or without leaf litter (grass litter/eucalypt litter), and with or without competitors. Eucalypt species growing in the field and in pots attained more biomass with litter than without when competitors were absent. Competition substantially decreased the biomass of eucalypt seedlings. Competitive intensity was heavily influenced by litter type and was most intense in the presence of grass litter. Litter produced a small change in patterns of biomass allocation in the competing herbaceous vegetation, and there was a slight (marginally non-significant) indication of a change in the proportion of grasses relative to forbs when litter was present. However, when the integral of competitor biomass over time was used to calculate competitive intensity, the combined effects of the experimental factors (litter and competition) became additive, suggesting that the effect of leaf litter on the timing of germination and establishment in the grasses and forbs, relative to that of Eucalyptus seedlings, was the principal mechanism by which leaf litter altered the interaction strength of the species studied.

Soil phosphorus heterogeneity and mycorrhizal symbiosis regulate plant intra-specific competition and size distribution.

We investigated the interactive effects of soil phosphorus (P) heterogeneity, plant density and mycorrhizal symbiosis on plant growth and size variability of Trifolium subterraneum. We set up mesocosms (trays 49×49 cm and 12 cm deep) with the same amount of available P, but distributed either homogeneously or heterogeneously, in randomly arranged cells (7×7 cm each) with high or low available P. The trays were planted with either 1 or 4 seedlings of T. subterraneum per cell. Half of the trays were inoculated with spores of the mycorrhizal fungus Gigaspora margarita. We harvested the plants when leaves just started to overlap, 8 weeks after planting. Plants growing in high P cells had the lowest percentage infection, but the highest mean shoot and root biomass and root length. The mean size of the plants in each cell was determined mainly by local P concentration. However, in plants growing in high density, low P cells, ca. 20% of the variability in plant biomass was explained by the number of adjacent cells with high P. Patchy trays had the highest total shoot biomass, independently of mycorrhizal infection or plant density. Inoculated trays (M) had higher total shoot biomass and relative competition intensity (measured as reduction in plant biomass due to increased density) than non-inoculated trays (NM). Plant density reduced the plant response to mycorrhizal infection, and its effect was independent of P distribution. All populations growing in patchy trays, and low density mycorrhizal ones, had the highest plant-size inequality, presumably because patchy distribution of P and mycorrhizal infection increased competitive asymmetry. We conclude that mycorrhizal symbiosis has the potential to strongly influence plant population structure when soil nutrient distribution is heterogeneous because it promotes pre-emption of limiting resources.

Germination requirements and responses to leaf litter of four species of eucalypt.

We studied how the responses of four species of eucalypt to leaf litter related to their germination responses to light and water availability. Two of the species (Eucalyptus obliqua and E. baxteri) have a mesic distribution, while the other two (E. oleosa, and E. incrassata) are more xerophytic. We studied the effect of litter on emergence of the four species in a glasshouse experiment. Litter did not affect the emergence of E. incrassata and E. oleosa, but enhanced the emergence of E. obliqua and E. baxteri. Litter increased the seedling mortality of all four species. Germination responses to light and water availability were studied in growth cabinets under controlled conditions. The germination of E. obliqua and E. baxteri was substantially lower under fluorescent light than in darkness, but that of E. oleosa and E. incrassata was not affected by the light environment. The germination of E. obliqua and E. baxteri was significantly reduced by reduced water potential (ψa). Substantial germination of E. oleosa and E. incrassata occurred even at very low ψa (less than -1.05 MPa). We conclude that both the shade and the humid micro-environment provided by litter may have contributed to the emergence responses of the four species to litter, and these responses may correspond to ecological adaptations to the different environments in which they live.

Experimental evaluation of the foliar flag hypothesis using fruits of Rhus glabra (L.).

I tested experimentally whether the presence of colorful plastic ovals (simulating foliar "flags") attached to infructescences of Rhus glabra increase fruit removal by birds in a forest-oldfield border. I used a factorial experimental design testing for the effect of size (small or large) and color (yellow or red) of the flags. There was also a control, without flags. Large red flags increased the percent of fruits removed from the panicles, but yellow and small flags had no effect. My results give partial support to Stiles (1982) hypothesis that early color change of leaves close to the fruits in some plants may serve as visual signals that attract frugivorous birds and enhances seed dispersal.