Ecology of rare North American plant species Bartonia paniculata subsp. texana and Platanthera cooperi

Date

2023-12

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Abstract

Bartonia paniculata subsp. texana is an endemic, imperiled plant occurring within the baygalls of the West Gulf Coastal Plain Forested seep (Southern Type; WGCP-FS-ST) habitats of Southeast Texas, which are also considered rare plant communities. Number of individuals and populations of the taxon have both declined by 50% over the last 57 years. Yet, niche preferences within the WGCP-FS-ST of B. paniculata subsp. texana are not characterized, which could enable targeted surveys to identify additional populations within its dwindling habitat. We investigated whether plant communities and environmental conditions within baygalls that support B. paniculata subsp. texana were distinguishable from locations that do not support the taxon. Our results indicate that B. paniculata subsp. texana prefers narrow niches with its baygall habitat. These areas are characterized by identifiable understory vegetation composition (Shannon Index = 1.4, Simpson Index = 0.62), canopy openness (16%), ground cover (live vegetation= 68%, debris= 11%, bare soil= 30%), and soil moisture (23%) as compared to baygalls that do not consistently support the taxon. However, overstory vegetation and soil physicochemical properties between occupied and unoccupied sites were similar. Abstract Chapter 3: Distribution of orchid populations and their fitness is shown to be correlated to the identity, distribution, and abundance of the associated orchid mycorrhizal fungi (OMF), which in turn may be influenced by soil physicochemical characteristics. Roots of plants from larger populations of Platanthera cooperi were dominated by fungi belonging to the Tulasnellaceae (Tul) and hosted Ceratobasidiaceae (Cerato) in low abundances. The soil in these populations had higher concentrations of soil phosphorus (P) and lower concentrations of soil zinc (Zn) in comparison to the smaller populations. We hypothesized that the dominant fungus (LT) in large populations has a competitive advantage at the natural levels of P (295 mg/l) supporting its mycorrhizal association with the orchid. Conversely, we expected suppressed growth of LT under high Zn (41 mg /l) conditions. We cultured isolates representing Tul and Cerato, either individually or in co-culture, on media supplemented with P and/or Zn found in the soils of large and small populations of the orchid. Surface area comparisons after nine days showed that any media in which Zn was manipulated suppresses both fungi yielding smallest surface areas whether cultured individually or together. Highest surface area was recorded for both fungi on media supplemented only with P when they were cultured individually. The two fungi only physically encountered each other when co-cultured on P-supplemented media, where Cerato was dominant over Tul even though its growth was reduced. These results confirm the ability of both Tul and Cerato to occupy similar P niches while suggesting that Cerato may outcompete Tul if they co-occur in the same space. Germination and protocorm formation % after 40 days showed that the Tul leads to high germination and protocorm rates in the natural levels of P and Zn at large populations. Cerato resulted in higher germination %, protocorm formation %, and protocorm to germination raito as compared to Tul. No germination occurred in high Zn treatments. However, fungal DNA sequencing results results revealed the presence of Tul in 62.5 % and Cerato in 37.5 % of the orchid seeds that turned into protocorms in the presence of co-culture of Tul and Cerato. Altogether, our data suggest that although the orchid-OMF-soil interactions are important, the host orchid is likely the more direct determinant of which fungi get recruited into the roots.


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Keywords

Orchid mycorrhizal fungi (OMF), Platanthera cooperi, symbiotic seed germination, soil nutrients, abiotic characteristics, rare plant, Bartonia paniculata subsp. texana, habitat characteristics

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