Seed dormancy in red rice. XII: Population-based analysis of dry-afterripening with a hydrotime model

Red rice (Oryza sativa L.) dispersal units (florets) were dry-afterripened for 0-8 weeks and subsequently incubated at 30 degrees C in polyethylene glycol (PEG) solutions with water potentials from 0 to -1.6 MPa. Germination percentages and rates increased with dry-afterripening and water potential of the incubation medium. The seed population exhibited a normal distribution of base water potentials (Psi(b), i.e. minimum water potential allowing germination) among individual seeds, characterized by three parameters: the hydrotime constant (theta(H)), the mean base water potential ((Psi) over bar (b)) and the standard deviation of the base water potential distribution (sigma(psi b)). Changes in germination during afterripening could be described by modifications of such parameters, particularly (Psi) over bar (b), which was employed to derive an index, D(H)(ARx) = (Psi) over bar (b)(AR(X)) - (Psi) over bar (b)(AR(N)), where D(H)(AR(X)) represents a measure of dormancy of the seed population (in MPa) based on the hydrotime model, (Psi) over bar (b)(ARx) is the mean base water potential of the seed population at any afterripening time X, and (Psi) over bar (b)(ARN) is the mean water potential of the non-dormant (fully afterripened) population. The introduction of this index permitted interpretation of afterripening as a measurable reduction in the dormancy status of the seed, with progressive acquisition of both full germinative capacity and maximum germination rate, as anticipated by the hydrotime model. Moreover, secondary dormancy was induced proportionally to the reduction in water potential in the dark. Susceptibility to secondary dormancy induction was defined through D(I)(AR(X)), an index analogous to DH(AR(X)). These indices revealed that, in red rice, both breaking of primary dormancy and the inducibility of secondary dormancy followed decay kinetics with different sensitivities to the duration of dry storage.