Root morphology, distribution of mycorrhizae, and nutrient status of the terrestrial orchid Spiranthes cernua.

The rhizosphere is a complex of biotic and abiotic factors and their interactions. It includes the soil, the micro- and megafauna, and a variety of autotrophic species. The goal of this study is to understand the belowground dynamics of the terrestrial orchid, Sniranthes cemua. through the characterization of soil nutrients, root system morphology, and mycorrhizal infection. Interrelationships among the soil, the mycorrhizae and S. cemua may explain the role of mycorrhizae in adult chlorophyllous orchids as well as the influence of soil nutrients on mycorrhizae. Understanding mycorrhizal relationships, will also contribute to the conservation and reestablishment of threatened and endangered terrestrial orchids. Spiranthes cemua was studied at Nine-Mile Prairie, near Lincoln, NE, in 1997. The soil is a silt loam to silty clay loam with a pH range of 5.4 - 5.95. The soil contained 4.54 ± 0.61 µg/gds ammonium-N, 1.23 ± 0.11 µg/gds nitrate-N, 5.62 ± 0.13 pg/gds phosphorus (Bray), and 3.38 ± 0.07 µg/gds total carbon. Inorganic nitrogen decreased through the growing season while carbon and phosphorus remained stationary. The size and structure of the root system of S. cemua changed during the period April - July. During this vegetative phase the root system was composed of young and old mature roots and bud roots. Bud roots and old roots were not observed after July. Mycorrhizae infected approximately 12.1% of root cortical cells in S. cemua. however, less than 5% was active infection. Mature roots were significantly less infected than bud roots (10.4 ± 0.01% versus 31.1 ± 0.04%). It is possible that S. cemua. like its congener S. sinensis, has two types of roots, one primarily for mycorrhizal infection and one for storage. In S. cemua bud roots may represent the mycorrhizal roots. During the vegetative phase the plant must manufacture/acquire resources for the growth and maintenance of the current shoot as well as store resources for future growth and reproduction. In S. cemua mycorrhizal activity is greatest when the plant is vegetative and soil inorganic nitrogen levels are highest. This temporal pattern allow the plant to acquire nutrients at the lowest possible cost