In situ seed germination and mycorrhizal associations of the endangered terrestrial orchid Spiranthes parksii

Background/Question/Methods

The Navasota ladies’-tresses orchid, Spiranthes parksii Correll, is a federally listed endangered species that is restricted to specific habitats in the post oak savanna of central Texas. Patchy distribution of essential mycorrhizal symbionts and scarcity of suitable seed germination sites are common hypotheses explaining the restricted distribution of terrestrial orchids. We examined variation in seed viability and in situ seed germination using buried seed-baits in a variety of habitats including areas in close proximity to S. parksii, areas where S. parksii have not been found but exhibit similar associated plant species and canopy cover as S. parksii habitats, dense post oak-yaupon forest, and open grasslands. We also assessed the distribution of naturally occurring fungi capable of stimulating seed germination in habitats near S. parksii populations and in habitats where S. parksii are absent. At the time of seed dispersal (January), we buried 960 orchid seed-baits, each containing approximately 400 seeds, at 1 cm depth in the four different habitats in west Grimes County, Texas. 

Results/Conclusions

Seed viability, seed germination, developmental stages, and fungal endophyte infection were recorded. The percent of viable (embryo present) seeds remained above 95% after 2 months (spring). Viability rapidly declined below 50% after 6 months (summer) with intact seeds absent in samples after 12 months (winter). Seed germination exceeded 23% after 2 months and remained above 20% after 4 months. Seed development to stage 4 (enlarged embryo, production of rhizoids and leaf primordial) was 1-3% after 6 months.  After 12 months, 2% of the seeds showed signs of development to stage 4 but remaining seeds were not viable. Germination frequencies were significantly higher in close proximity to existing S. parksii plants and in the dense post oak-yaupon forest compared with other habitats. Similarly, higher seed fungal infection was observed close to S. parksii and forest treatments. Data suggest that S. parksii exhibit a transient soil seed bank and last less than a year. Seed germination and protocorm development occur rapidly after dispersal and ungerminated seeds degrade within a single growing season.  Rapid loss of seed embryos after 6 months indicates a small optimal window for germination with the best chance occurring in late winter and early spring months. These findings have implications for conservation of S. parksii and emphasize the need for a combination of in situ and ex situ conservation practices to maintain current S. parksii populations.