Assessment of mycorrhizal colonization in the genus Dirca

Background/Question/Methods

Species occupying narrow and specialized habitats often have specific physiological, morphological, and genetic traits that allow their persistence in their niches. Dirca L. is a genus of uncommon, deciduous shrubs that are distributed in disparate regions of North America. While the most abundant species, Dirca palustris L., occurs in disjunct populations in eastern Canada and U.S., Dirca occidentalis Gray is endemic to San Francisco Bay, and Dirca mexicana Nesom and Mayfield is known from a single population in Mexico. Most plants of all Dirca spp. prefer north-facing slopes above streams in shaded forests where soil moisture is consistently moderate. While edaphic conditions appear to be important in determining the distributions of Dirca spp., relatively little is known of the chemical and biological characteristics of their root zones. To assess the role of mycorrhizae in the distribution of the genus, we measured mycorrhizal colonization in D. palustris, D. occidentalis, and D. mexicana. Roots from up to 20 individuals of each species were collected during two seasons, stained, and evaluated for the number of vesicles and arbuscules to estimate the among-species and within-species variation of mycorrhizal colonization.

Results/Conclusions

Number of vesicles was the strongest indicator of colonization of roots of Dirca spp. Dirca palustris was the most mycorrhizal species. For both D. palustris (both in Florida and North Dakota) and D. occidentalis, colonization was higher in late-season than in samples collected early in a growing season, suggesting that the importance of mycorrhizal associations in nutrient and water acquisition by the plants varies seasonally. We did not observe significant variation in soil nutrient properties across these sites, however. Mycorrhizal colonization of D. palustris in Florida was greater for plants in riparian niches (119 vesicles/0.25 g roots) than for plants in upland locations. When compared among the sampling locations, D. palustris in Florida exhibited the highest colonization by mycorrhizal fungi (mean = 67 vesicles/0.25 g roots), followed by roots of that species in North Dakota. Dirca mexicana, the species most geographically restricted, was the least colonized regardless of the time of root sampling. Colonization of the other narrowly endemic species, D. occidentalis, also was low. Morphology of vesicles indicates the mycorrhizal fungi of Dirca spp. belonging to the Glomales, but further evaluations, including molecular determinations, will allow a more specific identification of the mycorrhizae associated with Dirca spp.