Diversity of Parasitic Cuscuta and their Host Plant Species in a Larrea-Atriplex Ecotone.
Parasitism occurs when an association exists between two species that benefits the parasitic organism at the expense of its host organism. The host constitutes a living microhabitat for the parasite. The monotypic family Cuscutaceae has a cosmopolitan occurrence, represented solely by one genus Cuscuta (dodder), with many species native to North America (Kuijt 1969). This family is often included as a subfamily of Convolvulaceae, but differs in its parasitic life-form (Kuijt 1969). Species of Cuscuta are among the best known of the higher vascular plant parasites because of their extraordinary appearance and behavior (Kuijt 1969).
Multiple species of Cuscuta are known to occur in southern Nevada (Cronquist 1984), with different Cuscuta species parasitizing different xerophytic and halophytic hosts. Cuscuta sauna is known to primarily parasitize halophytes, such as four-winged saltbush (Atriplex canescens), shadscale (A. confertifolia), and iodine bush (Allenrolfea occidentalis) (Kearney and Peebles 1951). Cuscuta californica occurs frequently on white bursage (Ambrosia dumosa) (Beauchamp 1986). Two common host plant species of Cuscuta denticulata are creosote bush (Larrea tridentata) and cheesebush (Hymenoclea salsola) (Beauchamp 1986). Hosts of Cuscuta campestris are mostly herbaceous, including composites and grasses (Munz 1974). In an area with multiple species of Cuscuta, identifications of these parasites are distinguished primarily by their geographical location, floral morphological characteristics, and by their host plant species (Wesley Niles, personal communication).
Seeds of Cuscuta spp. germinate near the soil surface, and may develop a small root system which sustains the plant until the stem reaches a suitable host plant. Many species of Cuscuta lack chlorophyll and do not photosynthesize (Belzer 1984). Cuscuta spp. have only scale-like vestigial leaves, depending solely on exploitation of resources from their hosts for growth, survival, and regulation of their environment (Whitson 1987). The orange-colored, spaghetti-like parasitic threads function as runners, allowing an individual to move from host to host, as well as to densely infest a single host (Kelly et al. 1988). Upon attachment to a host, a Cuscuta plant becomes entirely parasitic, with subsequent degeneration of its terrestrial root system (Kelly 1992), and the resource acquisition is entirely above-ground. A stem of Cuscuta spp. infests its host by coiling around the host stem or leaf and sending haustoria into the host's vascular system (Kelly 1992). Soon after the first haustorial contacts have been ma de, the radicle dies and all contact with the soil is irrevocably lost (Kuijt 1969).
The objective of this study was to examine the distribution of four Cuscuta species on 11 host plant species in the Amargosa Valley of southern Nevada. Specifically, woody and subwoody plant species were identified and assessed for 1) Cuscuta biomass and relative percent cover of host covered by Cuscuta as a measure of the degree of infestation; 2) flowering success between the infested and uninfested host plants; and 3) condition (living or dead) of each infested host individual.
Materials and Methods
Field studies were conducted during Spring 1998 in the Amargosa Valley (roughly 36[degrees]50' N, 116[degrees]05' W) of southern Nevada where multiple species of Cuscuta were present. The woody vegetation zone (elevation 760 m) was dominated by Larrea-Atriplex spp., with numerous individuals of Ambrosia and a few individuals of Hymenoclea, seepweed (Suaeda torreyana), and Allenrolfea. A low abundance of annual and perennial herbaceous species was also present, including milkvetch (Astragalus spp.), desert marigold (Baileya pleniradiata), bugseed (Dicoria canescens), desert globemallow (Sphaeralcea ambigua), and fluff grass (Erioneuron pluclielluin).
The Amargosa Valley was selected on the basis of an abundant infestation of Cuscuta species in a relatively homogeneous habitat with a variety of host plant species. This area thus provides an assessment of host specialization in four Cuscuta species (C. california, C. campestris, C. denticulata, and C. salina) without having the effects of significantly different environmental variables inherent with the selection of separate geographic study sites.
Two hectares northeast of U.S. Highway 95 were selected in the Amargosa Valley. Within these two hectares, all plant species were identified, but only woody species abundance (number of individuals) were determined. The amount of host canopies covered with green leaves and covered with their respective Cuscuta species, along with the flowering success of host plants were visually estimated in 10-% point increments (Lei 1999a, b, and c). Host plants that lacked flower or fruit production and without green leaves on branches were assumed dead. The condition of seven woody and subwoody (suffrutescent) species covered with parasites was recorded as either live (1) or dead (0). All parasitic stems were collected from all woody and suffrutescent host plants with infestations, and were oven-dried at 45[degrees]C for 72 hours in a biology laboratory at the Community College of Southern Nevada (CCSN) to determine biomass accumulation of Cuscuta.
One-way Analysis of Variance (ANOVA), followed by Tukey's multiple comparison test were used to detect differences in Cuscuta biomass among host species, and to compare means of Cuscuta biomass when a significant infestation effect was detected, respectively (Analytical Software 1994). The ANOVA was also employed to detect survival and flowering success between infested and uninfested host individuals. Mean values were presented with standard errors, and statistical significance was determined at p [less than or equal to] 0.05.
Four species of Cuscuta were found to primarily parasitize seven woody and subwoody species, as well as two annual and two herbaceous perennial species within the study area (Table 1). Hosts of C. campestris were herbaceous plants, including Baileya and Indian ricegrass (Oryziopsis hymenoides), while hosts of C. denticulata were woody plants, such as Hymenoclea and Larrea (Table 1). Herbaceous plants that were rarely infested with parasites included the annuals Baileya and Chaenactis fremontii, as well as the perennials Oryzopsis and Sphaeralcea (Table 2). Conversely, fiddleneck (Amsinckia tessellata), Astragalus spp., grama grass (Bouteloua barbata), Dicoria, and Erioneuron revealed no infestation at all (Table 2).
The percentage of host canopies covered by multiple species of Cuscuta varied significantly among the host types (Table 1; Fig. 1). Although Ambrosia was the most abundant host species, it had a low frequency of C. california infestation. Larrea plants were less abundant than Ambrosia, yet had the highest frequency of C. denticulata infestation (Fig. 1). A significant difference was detected between Larrea and Hymenoclea in percentage of host canopy cover infested by C. denticulata (P [less than or equal to] 0.05; Fig. 1).
Biomass accumulation of Cuscuta species also varied significantly (P [less than or equal to] 0.001) among the infested host plant species (Fig. 2). Larrea plants had the greatest mean percentage covered by C. denticulata, and had the greatest mean C. denticulata biomass because of the large above-ground canopy (Fig. 2). Atriplex spp. also exhibited abundant C. salina biomass on their canopies, whereas other woody taxa contained a relatively low parasitic biomass (Fig. 2). From casual observations, more parasitic biomass was found in the center than around the periphery of shrub canopies. Within the same perennial shrub species, Cuscuta revealed the greatest biomass on individual plants that were more vigorous with a rapid growth rate. Host plants not infested with parasites showed a significantly (P [less than or equal to] 0.01; data not shown) greater percentage of green leaves and active flowering than host plants infested with parasites. Hosts with heavy infestation exhibited a significant (P [less than o r equal to] 0.01; data not shown) reduction in green leaves and flowering success compared to hosts with light or no infestation.
Condition (survival) of woody and subwoody hosts infested with Cuscuta species was observed. Larrea and Atriplex spp. displayed a greater degree of infestation by C. denticulata and C. salina, respectively, based on percent cover and biomass than the more abundant Ambrosia (Figs. 1-2). Among the woody and subwoody plants, Larrea had the highest, while Allenrolfea had the lowest number of dead hosts (Fig. 3). Although most host plants were alive, several dead hosts including Atriplex spp., Larrea, Ambrosia, and Hymenoclea with live parasites were found (Fig. 3). Despite being considered alive, some nearly dead hosts were also found in all seven woody and subwoody species. Within the same host species, significantly greater (P [less than or equal to] 0.05; data not shown) infestation by parasites were observed on individual hosts with greater height and canopy size.
In general, Larrea plants were the most preferred host, followed by Atriplex spp. for the growth of parasites. This study supports the theory that Cuscuta spp. are not a strict generalist in the sense that they do not equally infest all available host types in proportion to host abundance. My data reinforces Kelly et al. (1988) study, suggesting that Cuscuta species are selective consumers with multiple species of Cuscuta parasitizing different species of host plants. Although Ambrosia was the most abundant species, Larrea exhibited the highest mortality from parasitism in this study.
Kelly (1992) found that Cuscuta species made morphological changes gauged towards expected resource (water and nutrient) gain from the host. Species of Cuscuta exhibit behavior in which they grow away from host plants with low nutrient availability and rapidly coiled around host plants with high nutritional value (Kelly 1992). Rejection of a potential host indicates that a parasitic plant may strategically take the risk of forfeiting the initial host for the chance of acquiring better resources from another host later (Kelly 1992).
There are several potential explanations for the dispersal of Cuscuta seeds. First, the seeds of Cuscuta species appear to be dispersed haphazardly; animals as dispersal agents are probably rare and unimportant (Kuijt 1969). Second, some Cuscuta seeds may fall to the ground and germinate there. Third, seeds of Cuscuta may be transported by wind or water to new hosts (Kuijt 1969). Fourth, the reason why some Cuscuta species infested more than others is not completely known. It may be a maternal influence when C. denticulata seedlings from plants grown on Larrea select Larrea more often than Cuscuta spp. seedlings from other host species. Consequently, the emerging C. denticulata seedlings would grow directly and lead to a more vigorous growth on the Larrea plants. Although a number of annual and perennial herbaceous species were not infested by parasites, these species were potential hosts because they were present in the community.
Biomass accumulation of Cuscuta species differed significantly among host species. Woody perennials showed the greatest Cuscuta biomass, whereas annuals, biennials, and herbaceous perennials exhibited minimal or no infestations in this study. A limited Cuscuta infection success on biennials and herbaceous perennial hosts is related to host morphology (lifeform) since the presence of scattered vascular bundles in grasses may not be amenable to resource exploitation by Cuscuta species (Kelly and others 1988). Additionally, host phenology (developmental stage) is also likely to determine the biomass and abundance of Cuscuta species. Mature or long-lived plants generally have a greater canopy size compared with juvenile or short-lived plants. Within the same host species, a relatively large host canopy size would indicate a greater surface area available on secondary branches for successful colonization and establishment of Cuscuta species. In this study, Cuscuta accumulated more tissue or biomass on those hosts that grew better and faster, presumably due to higher moisture content and nutritional value. Not all xerophytic plants functioned equally as hosts for multiple species of Cuscuta in this study. Although water and nutrient availability were not quantitatively examined, infection of parasite on the host, as demonstrated by the higher probability of infestation on Larrea and Atriplex spp. in comparison to Ambrosia, may be attributed to proportionally different water and nutrient uptake by these host plant species, or to different characteristics of the hosts and/or parasitic species.
Host morphology and phenology were two main factors that partially determined which species of Cuscuta parasitizing which species of host plants. Within the same host species in this study, species of Cuscuta tended to select individual hosts with greater height and canopy size. Seedlings appear to grow toward a source of high nutrient and, to a lesser extent, moisture (Kuijt 1969). Host morphology and phenology may be strongly associated with the availability of water and nutrients. Host preference in multiple species of Cuscuta are still not fully understood. Further research, including water and nutrient status of existing and potential host plants, at various geographical locations, are required to improve our understanding regarding the host preference in parasitic Cuscuta plants.
Valuable field assistance provided by Steven Lei, David Valenzuela, and Shevaun Valenzuela was gratefully acknowledged. Helpful comments by David Charlet greatly improved the manuscript. The Department of Biology at the Community College of Southern Nevada (CCSN) provided logistical support.
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Four species of Cuscuta parasitizing 11 host species in a Larrea-Atriplex ecotone in the Amargosa Valley of southern Nevada. Lifeforms (S = Shrub, Ss = subshrub, A = Annual, B = Biennial, and HP = Herbaceous Perennial) of host plants are shown. Names of host and parasitic species are arranged alphabetically.
Parastic species Host species Lifeform Cuscuta california Ambrosia dumosa S C. campestris Baileya pleniradiata HP Chaenactis fremontii A Oryzopsis hymenoides HP C. denticulata Hymenoclea salsola S Larrea tridentata S Sphaeralcea ambigua HP C. salina Allenrolfea occidentalis S Atriplex canescens S Atriplex confertifolia S Suaeda torreyana Ss
Percentage of host canopies (mean [+ or -] S.E.) (annuals, biennials, and herbaceous perennials) covered by multiple species of Cuscuta in a Larrea-Atriplex ecotone in the Amargosa Valley of southern Nevada. Species of Cuscuta generally exhibited little or no infestation on herbaceous host plants. Host species names are arranged alphabetically and their lifeforms are shown below. Symbols of host plant lifeforms are explained in Table 1.
Host species Lifeform % cover of Cuscuta spp. Amsinckia tessellata A Not infested Aristida glauca HP Not infested Astragalus spp. HP Not infested Baileya pleniradiata A 9.9 [+ or -] 1.3 Bouteloua barbata A Not infested Bromus rubens A Not infested Chaenactis fremontil A 10.2 [+ or -] 2.4 Cirsium neomexicanum B Not infested Cryptantha spp. A Not infested Dicoria canescens A Not infested Erioneuron pluchellum HP Not infested Malacothrix glabrata A Not infested Muhlenbergia porteri HP Not infested Oryzopsis hymenoides HP 8.3 [+ or -] 2.2 Sphaeralcea amnbigua HP 6.7 [+ or -] 2.1 Sporobolus cryprandrus HP Not infested
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|Author:||Lei, Simon A.|
|Publication:||Bulletin (Southern California Academy of Sciences)|
|Date:||Apr 1, 2001|
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