Hydrotime analysis of the effect of fluctuating temperatures on seed germination in several non-cultivated species

Germination responses to higher doses of fluctuating temperatures in Carduus acanthoides, Cynara cardunculus, Cirsium vulgare and Sysimbrium altissimum seeds were tested. In two separated tests the effect of the increment of thermal amplitude, or the accumulation of a higher number of cycle-doses of fluctuating temperatures, both in water and a range of reduced water (osmotic) potentials, were evaluated. Effects of osmotic potential and temperature were modelled with the hydrotime model. Higher thermal amplitude (Test I) or, the accumulation of a higher number of cycle-doses (Test II), promoted seed germination in water and at reduced osmotic potential, compared to germination scored under non-fluctuating (constant temperature) regimes. The parameters estimated from the hydrotime model, the hydrotime constant (_H), the mean base water potential (ψ_b(50)) and its standard deviation (σ_Ψb) gave a good description of germination time courses in water and at reduced osmotic potential. For all cases incubation under higher doses of fluctuating temperatures, either by a more pronounced thermal amplitude or by a higher number of cycles, shifted ψ_b 50 downwards, explained the promotion of total germination observed under fluctuating temperatures regimes in both tests.