Mechanisms of canola imbibition kinetics and vigour dynamics under sub-optimal hydration: implications for dry sowing

Early and dry sowing is adopted in canola grown in Mediterranean climates to manage uncertainty in the timing of the germinating rainfall and maximise seasonal opportunities. Under these conditions, seeds remain ungerminated in the seedbed while imbibing water and enter partial hydration states that may promote physiological ageing and reduce germination and establishment success. Canola seed water content approached a near-equilibrium plateau within about 14 h across water potentials and declined from 0.7 g H2O g-1 dry mass at 0 MPa to 0.12 g H2O g-1 dry mass at -20 MPa. Germination required a critical internal water content of 0.6 g H2O g-1 dry mass, corresponding to a median base water potential of -1.31 MPa. There was increasing loss in vigour with greater temperature and exposure duration, and the water potential response was non-monotonic, with comparatively higher vigour at -20 MPa and a pronounced decline near -5 MPa under high temperature conditions. These results indicate that seed vigour is regulated by a hydration-dependent metabolic state, with damage potentially greatest at intermediate water potentials. This highlights that successful early and dry sowing depends not only on rapid water uptake, but also on the capacity of seeds to maintain quality during prolonged imbibition in soil. Together, the critical water content for germination initiation, water imbibition process model and hydrothermal ageing responses provide a framework to determine establishment risk and inform management for early and dry sowing.