Variation in thermal and hydrotime requirements for seed germination of Chaco seasonally dry forest species in relation to population environmental conditions and seed mass

Thermal and hydrotime requirements for seed germination can vary with the species, environment in which the seeds developed and/or seed mass. We investigated seed germination responses to temperature and water potential of Aspidosperma quebracho-blanco, Bulnesia retama, Parasenegalia visco, Parkinsonia praecox and Tecoma stans, all of which are native tree species of Chaco seasonally dry forests in northwest Argentina. We determined inter-and intraspecific variability in seed germination through thermal and hydrotime models and correlated model parameters with environmental variables of seed provenance and seed mass. Seeds of all species germinated to high percentage over a range of temperatures. Seeds of A. quebracho-blanco, P. praecox and T. stans were more tolerant to water stress than those of B. retama and P. visco. Thermal and hydrotime parameters for germination varied amongst species and populations. There was a negative correlation between thermal (theta(T(50))) and hydrotime (theta(H)) constants and base temperature (T-b) and water potential (psi(b(50))). Seed mass correlated negatively with T-b and positively with theta(T(50)). Species and populations were grouped according to thermal and hydrotime parameters and seed mass but not according to seed provenance. These results suggest that seed germination traits related to temperature, water potential and seed mass are species specific, and the variations within a species are probably due to a high phenotypic plasticity. These species showed a complex set of germination strategies that give them a competitive advantage in different environments, depending on dispersal time, seed mass and local environmental conditions during the germination period.