ADAMS, CHRISTOPHER A.1*, JERRY M. BASKIN1, and CAROL C. BASKIN1,2. 1School of Biological Sciences, University of Kentucky, Lexington, KY, 40506; 2Department of Agronomy, University of Kentucky, Lexington, KY, 40506. - Comparative seed dormancy in three Aristolochia species: A test of two hypotheses on changes in ecological requirements in plant lineages.
The overall purpose of this research is to examine two opposing views
on changes in physiology of closely-related plant taxa that evolved in
response to changes in climatic and other environmental factors
following geographical breakup of the "Arcto-Tertiary"
forest. The first view, advocated by Daniel I. Axelrod, suggests that
although modern taxa may have slight to moderate morphological
differences compared to Tertiary ancestors, their ecological
tolerances and physiological requirements essentially have remained
unchanged. The contrasting view, put forth by Jack A. Wolfe, is that
taxa diverged in physiological and ecological requirements as lineages
were exposed to climatic changes through geologic time. These two
hypotheses are being evaluated via a comparative study of seed
dormancy and germination characteristics of three closely-related
Aristolochia species: the California endemic A.
californica, the Appalachian endemic A. macrophylla, and
the southeastern U.S. species A. tomentosa. The three species
are the North American members of a monophyletic group (subg.
Siphisia) that became geographically separated and diverged,
presumably in the late Tertiary. Seeds of all three species have
linear underdeveloped- and physiologically dormant embryos, thus
morphophysiological dormancy (MPD). Seeds of A. californica
require warm (e.g., 30/15C) stratification for dormancy-break and a
low (e.g., 15/6C) temperature regime for nondormant seeds to
germinate. In contrast, seeds of A. tomentosa and A.
macrophylla require cold (5C) stratification for dormancy-break,
but nondormant seeds will germinate over a wide range of temperature
regimes (i.e., 15/6-35/20C). These preliminary results seem to
indicate that seeds of A. tomentosa and A. macrophylla
have nondeep simple MPD and those of A. californica nondeep
complex MPD. In any case, seed dormancy-breaking and germination
requirements of the Mediterranean-climate species, A.
californica, differ considerably from those of the temperate
rainy-climate species, A. tomentosa and A. macrophylla,
thus supporting Wolfe's view on physiological changes in a lineage.
Key words: Aristolochia subg. Siphisia, monophyletic group, morphophysiological dormancy, physiological changes in lineages, seed germination, Tertiary Period