Hydrothermal time analysis of tomato seed germination at suboptimal temperature and reduced water potential

Both temperature (T) and water potential (psi) have consistent and quantifiable effects on the rate and extent of seed germination (radicle emergence) Germination at suboptimal T can be characterized on the basis of thermal time, or the T in excess of a base (T-b) multiplied by the time to a given percentage germination (t(g)) Similarly, germination at reduced psi can be characterized on a hydrotime basis, or the psi in excess of a base (psi(b)) multiplied by tg Within a seed population, the variation in thermal times to germination for a specific percentage (g) is based upon the normal distribution of psi(b) values among seeds (psi(b)(g)) Germination responses across a range of suboptimal T and psi might be accounted for by a general hydrothermal time model incorporating both T and psi components We tested this hypothesis for tomato (Lycopersicon esculentum Mill) seeds of two genotypes differing in germination rates and tolerance of suboptimal T and psi For combinations of T (10-25 degrees C) and psi (0 to-0 9MPa), a general hydrothermal time model accounted for approximately 75% of the variation in times to germination within the seed populations of both genotypes, and over 96% of the variation in median germination rates However, psi(b)(g) distributions were sensitive to both the T and psi of imbibition, resulting in a poor fit of the model to specific time course data Analysis of germination timing separately for low and high psi ranges within a given T resulted in specific models accounting for 88-99% of the variation in individual germination times and > 99% of the variation in madian germination rates Thus, for a given T and psi range, the hydrotime model closely matched tomato seed germination time courses Accumulated hydrothermal time accounted well for germination rates at "psi-0 5MPa across suboptimal T if psi(b)(g) was allowed to vary with T Germination did not show a consistent response to T at psi < -0 5 MPa, and estimated Tb values varied over different T ranges Generalization of the hydrothermal time model across the entire range of suboptimal T and psi was limited by physiological adjustments of the seeds to their current environment The hydrothermal time model detected and quantified these adjustment processes that would otherwise not be evident from inspection of germination time courses Temperature and water potential influence the time to germination via physiological mechanisms that reciprocally interact