Jean C. Stutz

Relating foliar dehydration tolerance of mycorrhizal Phaseolus vulgaris to soil and root colonization by hyphae

Mycorrhizal symbiosis can modify plant response to drying soil, but little is known about the relative contribution of soil vs. root hyphal colonization to drought resistance of mycorrhizal plants. Foliar dehydration tolerance, characterized as leaf and soil water potential at the end of a lethal drying episode, was measured in bean plants (Phaseolus vulgaris) colonized by Glomus intraradices or by a mix of arbuscular mycorrhizal fungi collected from a semi-arid grassland. Path analysis modeling was used to evaluate how colonization rates and other variables affected these lethal values. Of several plant and soil characteristics tested, variation in dehydration tolerance was best explained by soil hyphal density. Soil hyphal colonization had larger direct and total effects on both lethal leaf water potential and soil water potential than did root hyphal colonization, root density, soil aggregation, soil glomalin concentration, leaf phosphorus concentration or leaf osmotic potential. Plants colonized by the semi-arid mix of mycorrhizal fungi had lower lethal leaf water potential and soil water potential than plants colonized by G. intraradices. Our findings support the assertion that external, soil hyphae may play an important role in mycorrhizal influence on the water relations of host plants.

Preliminary assessment of arbuscular mycorrhizal fungal diversity and community structure in an urban ecosystem.

Arbuscular mycorrhizal fungal (AMF) species richness, composition, spore density and diversity indices were evaluated in the Phoenix metropolitan area, Arizona, USA at 20 sampling sites selected to represent the four predominant land-use types found in the greater urban area: urban-residential, urban non-residential, agriculture and desert. AMF spores were extracted and identified from soil samples and from trap cultures established using soil collected at each site. Data were analyzed according to land use, land-use history, soil chemistry and vegetation characteristics at each site. Current agricultural sites were associated with decreased spore densities and historically agricultural sites with decreased species richness. Overall species composition was similar to that previously reported for the Sonoran desert, but composition at each sampling site was influenced by the vegetation from which samples were collected. Sites with the highest degrees of similarity in AMF species composition were also similar to each other in native plants or land use. Conversely, sites with the lowest similarity in AMF composition were those from which the majority of samples were collected from non-mycorrhizal plants, predominately ectomycorrhizal plants or bare soil. Spores of Glomus microggregatum were most abundant in urban sites, while those of G. eburneum were most abundant in desert and agricultural sites. Further studies are needed to determine the functional implications of shifts in AMF communities in urban ecosystems, including effects on plant primary productivity.

Interactions between Tamarix ramosissima (Saltcedar), Populus fremontii (Cottonwood), and Mycorrhizal Fungi: Effects on Seedling Growth and Plant Species Coexistence

Little is known about the composition and function of the mycorrhizal fungal community in riparian areas, or its importance in competitive interactions between Populus fremontii, a dominant tree in southwestern United States riparian forests which forms arbuscular and ectomycorrhizas, and Tamarix ramosissima, an introduced tree species that has spread into riparian areas. The objectives of this study were to determine the mycorrhizal status of Tamarixand to evaluate the effect of mycorrhizal fungal inoculation on Tamarix growth and on the coexistence between Tamarix and Populus.Arbuscular mycorrhizal fungal colonization of Tamarix was very low in both field and greenhouse grown roots, but levels of colonization by dark septate endophytes were high. Fungal inoculation had little effect on Tamarix seedling growth in monoculture. When Populus and Tamarix were grown together in a greenhouse pot experiment, fungal inoculation reduced the height and biomass of Tamarix but had no effect on Populus. Fungal inoculation shifted coexistence ratios. When Tamarix and Populuswere grown together, Tamarixplants averaged 20 of pot biomass in the uninoculated control but only 5 of pot biomass in the inoculated treatment. These results indicate that Tamarix is non-mycotrophic and that in this greenhouse experiment inoculation altered patterns of coexistence between Populus and Tamarix.

Geographic isolates of Glomus increase root growth and whole-plant transpiration of Citrus seedlings grown with high phosphorus

Abstract Four Glomus species/isolates from arid, semi-arid and mesic areas were evaluated for their effects on growth and water use characteristics of young Citrus volkameriana (′Volkamer′ lemon) under well-watered conditions, followed by three soil-drying episodes of increasing severity (soil moisture tensions of –0.02, –0.06, and –0.08 MPa) and recovery conditions. Arbuscular mycorrhizal (AM) plants were also compared to non-AM plants given extra phosphorus (P) fertilizer. AM plants and non-AM plants had similar shoot size (dry weight and canopy area), but all AM fungus treatments stimulated root growth (dry weight and length). Leaf P concentrations were 12–56% higher in AM plants than non-AM plants. Enhanced root growth was positively correlated with leaf P concentration. In general, AM plants had greater whole-plant transpiration than non-AM plants under well-watered conditions, under mild water stress and during recovery from moderate and severe soil drying. This suggests a faster recovery from moisture stress by AM plants. AM plants had lower leaf conductance than non-AM plants when exposed to severe soil drying. Although the greatest differences were between AM and non-AM plants, plants treated with Glomus isolates differed in colonization level, leaf P concentration, root length, transpiration flux and leaf conductance.

Assessing diversity of arbuscular mycorrhizal fungi in a local community: role of sampling effort and spatial heterogeneity

Diversity of arbuscular mycorrhizal fungi (AMF) was assessed in two 9.2 × 9.2-m plots planted with landscape trees and shrubs at an experimental site in Phoenix, AZ, USA. Twenty-five soil samples were collected in a regular grid pattern from each plot, and AMF species were identified using trap cultures. A total of 12 species were detected, with 7 species detected in one plot and 11 in the other. We found that sampling effort had a major impact on assessing species richness and composition in this local community. Fifteen samples would be necessary to detect 70–80% of species present in each plot. A limited number of additional undetected species are likely to be present in both plots, based on the sampling effort curves and jackknife estimates. Only two species, Glomus eburneum and Glomus microaggregatum, were detected in over 50% of the samples from both plots, and rank–frequency plots revealed a lognormal species distribution. Despite the patchiness of plants in the plots, the number of species detected per point exhibited spatial structuring only at the smallest sampling scale in a single plot, and only a single species in each plot was not randomly distributed. These results indicate that sampling effort and strategy can affect perceptions of AMF community structure.

Glomus eburneum and G. luteum, two new species of arbuscular mycorrhizal fungi, with emendation of G. spurcum

Two species of arbuscular mycorrhizal fun- gi, Glomus eburneum and G. luteum, are described and the description of G. spurcum is emended. All species produce spores singly in soil. Glomus ebur- neum spores are hyaline or white to cream, usually irregular (40 X 60 to 140 X 160 jim diam) and more rarely globose (40-140 ,um diam). The spore wall consists of two adherent permanent hyaline layers, neither of which reacts in Melzers reagent. The out- er layer is thin (<1.2 ,lm), and the inner layer is finely laminate. Spores of G. luteum are globose to subglobose, 60-180 jLm diam and pale yellow to dark yellow with a brownish tint. The spore wall consists of four layers, the two outer layers often degrading at maturity. The outer layer is mucilaginous and stains pinkish-red in Melzers reagent. The second layer is hyaline and semirigid. Rigid, pale yellow to brownish yellow laminae comprise the third layer. Mature spores exhibit a fourth layer that is thin, flex- ible and may separate from the laminate layer under pressure. Both species were identified from pot cul- tures established with soil and root fragments from a semiarid giant sacaton (Sporobolus wrightii) grassland and subsequently cultured on sudangrass, Sorghum sudanense. Both species formed arbuscular mycorrhi-

AMF Associated with Indigenous and Non-indigenous Plants at Urban and Desert Sites in Arizona

Arbuscular mycorrhizal fungal (AMF) biodiversity from 30 sites throughout the Phoenix, USA, metropolitan area was compared to determine the impact of urbanization on AMF communities. Spores from pot cultures started with soil collected from non-indigenous and indigenous plants at urban sites and from indigenous plants at desert sites were identified. The total number of species detected, number of species per plant and species richness (no. of AMF species/sampling site) were fewer at the urban sites in comparison to desert sites, but were similar between urban sites with indigenous plants and those with non-indigenous plants. There was a significant overlap in the species composition between desert and urban sites with about 70% of the species detected at both urban and desert locations. The relative frequency of AMF species varied between desert and urban sites with several frequently detected species from desert areas (detected in 蠅40% of collected samples) detected in fewer than 10% of samples from urban areas. Although it appears that urbanization has an impact on AMF communities, this effect does not appear to be linked to the presence of non-indigenous plant species in these areas. Future studies will be needed to determine the drivers of alterations in AMF community structure in urban areas.

Micropropagation of Cowania subintegra and Cowania stansburiana

Both Cowania subintegra Kearney and C. stansburiana Torr. were successfully propagated in vitro. Shoot proliferation occurred from shoot tips of green-house grown C. subintegra using a modified Murashige and Skoog medium supplemented with 4.4 μM 6-benzyladenine and 0.5 μM indole butyric acid. Excised microshoots (1.5–3.0 cm long) of both species were rooted using a two-step process in which they were cultured for 3 days in a root initiation medium with 2.7 μM naphthaleneacetic acid and then transferred to a low nitrogen root elongation medium without auxin. Plantlets were successfully transferred to soilless potting mix.