Temperature Relations for Bud Growth in the Root Geophyte Pediomelum subacaule and Ecological Implications

Abstract

Pediomelum subacaule (Leguminosae) is a tuberous root geophyte endemic to cedar glades of the southeastern United States. The annual shoot of P. subacaule emerges in early spring and dies with the onset of drought in late spring or early summer, by which time a small (< 1.0 mm in length) perennating bud has formed at the top of the root, about 50 mm below the soil surface. In a phenology study conducted in a non-temperature-controlled greenhouse, little bud growth occurred in July and August (ca. 32/21 C), but by mid-November the buds had reached a height of about 20 mm. The buds grew very little from mid-November to mid-February. From mid-February to late March, however, they grew an additional 30 mm and emerged at the soil surface in March. New absorbing roots were produced in March. After 36 wk, buds on roots incubated at 15/6, 20/10, and the simulated summer-autumn-winter-spring temperature sequence elongated 50 mm, while those incubated at 5, 25/15, 30/15, 30/20, and 35/20 C elongated to only 11-31 mm. New absorbing roots were produced only by roots incubated at 15/6, 20/10, and the sequence of simulated habitat temperatures. The temperature relationships of bud growth in P. subacaule are a phenological adaptation of this drought-intolerant species to its summer-dry habitat.