Habitat-specific seed dormancy-release mechanisms in four legume species

Physical seed dormancy is a common attribute among plants, and a wide range of dormancy-release mechanisms have been described, but their ecological significance is rarely tested through comparative study. This study tests whether dormancy-release responses to wet heat in four legume species with physical dormancy are correlated with habitat: two wetland species (Mimosa pigra and Parkinsonia aculeata, both dispersed primarily by water) and two terrestrial species (Acacia nilotica and Prosopis pallida, both dispersed primarily through vertebrate herbivores). Dormancy release was compared at three moisture levels (80% relative humidity, saturated and submerged) at constant (20-45 degrees C) and diurnally fluctuating (20/40 degrees C) temperatures for 14 d. Seed viability was tested by germinating at 25 degrees C. The functional relationship between temperature and dormancy release after 14 d differed between species: submerged seeds of the two wetland species showed a quadratic response, with low rates of imbibition below 20-25 degrees C and complete imbibition at around 40 degrees C; P. pallida seeds showed a linear positive relationship, whereas there was no temperature response for A. nilotica seeds below 45 degrees C. Surprisingly, dormancy release after 14 d was relatively insensitive to moisture levels, although rate of dormancy release was generally slower under drier conditions. Dormancy release was not influenced by fluctuating temperatures. Seed viability was largely unaffected by temperature or moisture regime, although it did differ with species and was lower for non-dormant seeds. Our results suggest that a functional dormancy-release response to wet heat provides important fitness benefits for wetland species, but not for species dispersed through vertebrate herbivores, for which it may be maladaptive.