A hydrothermal time model explains the cardinal temperatures for seed germination

Temperature (T) and water potential (y) are two primary environmental regulators of seed germination. Seeds exhibit a base or minimum T for germination (T(b)), an optimum T at which germination is most rapid (T(o)), and a maximum or ceiling T at which germination is prevented (T(c)). Germination at suboptimal T can be characterized on the basis of thermal time, or the T in excess of T(b) multiplied by the time to a given germination percentage (t(g)). Similarly, germination at reduced y can be characterized on a hydrotime basis, or t(g) multiplied by the y in excess of a base or threshold y that just prevents germination (y(b)). Within a seed population, the variation in thermal times to germination among different seed fractions (g) is based on a normal distribution of y(b) values among seeds (y(b) (g)). Germination responses across a range of suboptimal T and y can be described by a general hydrothermal time model that combines the T and y components, but this model does not account for the decrease in germination rates and percentages when T exceeds T(o) . We report here that supra-optimal temperatures shift the psi(b) (g ) distribution of a potato (Solanum tuberosum L.) seed population to more positive values, explaining why both germination rates and percentages are reduced as T increases above T(o) . A modified hydrothermal time model incorporating changes in psi(b) (g ) at T > T(o) describes germination timing and percentage across all T and psi at which germination can occur and provides physiologically relevant indices of seed behaviour.