ECOTYPIC DIFFERENTIATION FOR FLOOD-TOLERANCE AND ITS MORPHOLOGICAL CORRELATES IN CHAMAECRISTA-FASCICULATA

Chamaecrista fasciculata is widespread in eastern North America and co mmonly found on dry sandy soils. In contrast, large seeded C. fascicul ata var. macrosperma is restricted to fresh-water tidal marshes of the central coastal plain of Virginia, USA. Greenhouse experiments were c onducted to determine if var. macrosperma represents a flood-tolerant ecotype and if flood tolerance is associated with decreased tolerance to dry conditions. Morphological measurements on root and shoot growth were made to determine correlates and possible mechanisms of toleranc e to flood and dry conditions. Flood tolerance in var. macrosperma was confirmed. This ecotype had greater seedling survivorship and greater accumulation of biomass under flooded conditions than the upland form . Vegetative performance of the two types was comparable under well-wa tered but drained conditions. Flood tolerance is associated with incre ased seed mass, a two-fold greater rate of shoot elongation, and incre ased shallow lateral root growth. The evolution of flood tolerance doe s not result in a decrease in vegetative tolerance to dry conditions e ven under competitive conditions. The large seeded, flood-tolerant eco type may be absent in drier habitats because of a trade-off between in dividual seed mass and seed number.