Respiration and germination rates of tomato seeds at suboptimal temperatures and reduced water potentials

Suboptimal temperature (T) affects germination rates (reciprocal of time to radicle emergence) on a thermal time basis; that is, the Tin excess of a base or minimum temperature multiplied by the time to a given per cent germination (t(g)) is a constant. Respiration rates are also sensitive to T, and proportional relationships are often found between respiration rates and germination rates, Reduced water potential (psi) delays seed germination on a hydrotime basis (i.e. the psi in excess of a base water potential multiplied by t(g) is a constant), It was tested whether respiration rates prior to radicle emergence vary in proportion to T and psi as expected from the thermal and hydrotime models, Respiration rates (CO2 evolution) of cold-tolerant, rapidly germinating (PI 341988) and cold-sensitive, more slowly germinating (T5) tomato (Lycopersicon esculentum Mill.) seeds were evaluated over a range of T and psi conditions, For both genotypes, respiration rates until the beginning of radicle emergence were related to T on a thermal time basis and increased approximately linearly with psi above -2.0 MPa, consistent with the hydrotime model. Respiration rates were uniquely related to germination rates, regardless of whether germination timing was affected by T, psi, or genotype, However, germination timing was unaffected when respiration rates were manipulated by varying O-2 partial pressure. Thus, while both germination and respiration rates vary with T and psi consistent with thermal and hydrotime models of biological time, respiration rates per se were not the limiting factor in germination timing of tomato seeds.