Autecological Studies of Psoralea subacaulis

Psoralea subacaulis T. & G. (Leguminosae) is an acaulescent, herbaceous perennial endemic to the cedar glades of middle Tennessee, northern Georgia, and northern Alabama. It is a spring aspect dominant of the open, well-lighted portions of gravelly cedar glades. In late June or early July, the entire shoot dies back. At the top of the tuberous root, about 5 cm below the surface of the ground, there is a small undifferentiated bud that gives rise to next season's growth. During the hot, dry summer months this bud elongates very little; but with the onset of cooler and wetter weather in the autumn it begins to elongate and the shoot apex shifts from vegetative to floral. Shoot elongation and apex development continue during the winter and by the following spring the shoot, with its apex in an advanced stage of development, emerges from the ground. After meiosis and pollen grain formation in March and early April, the shoot expands rapidly and the reproductive portion of the life cycle is completed before summer. Seeds are shed in late May and lie dormant until at least the following March or April, at which time germination and seedling establishment occur. Plants do not reproduce until they are 5 to 6 years old. Seed dormancy results from two distinct factors: (1) a "hard" seed coat impermeable to water, and (2) one or more water-soluble chemical inhibitors. In order for maximum germination to occur in a minimum time, seeds require both scarification and leaching. Bud expansion and flowering in P. subacaulis are controlled by the annual thermoperiod. In plants which had just become dormant, a 2- to 3-month warm period followed by a 3-month cold treatment initiated normal growth and development. Plants given a 3-month cold treatment not preceded by a warm treatment developed abnormally and did not flower. Adaptations of P. subacaulis to its cedar glade environment include: (1) capacity to complete the most active part of its life cycle during the cool moist season and to remain dormant during the hot, dry summer; (2) delay of germination until spring when moisture and temperature are favorable for seedling establishment; (3) capacity to complete reproductive stages of its life cycle quickly, and (4) capacity to become established in the rocky substrate, where larger plants find establishment difficult.