Seed quality standards enable seed users to achieve their objectives in the establishment of uniform seedlings to a high and reliable level for a range of agricultural and horticultural crops, growing systems and market outlets. Quality standards of commercial seed lots are determined by their positions on the seed survival curves and the shape of their germination progress curves. Although comparative descriptions of germination curves can be achieved by the calculation of the mean germination time (MGT; delay to radicle emergence), single early counts of radicle emergence provide a convenient means of predicting MGT and differences between seed lots. Evidence is presented for an ageing and metabolic repair hypothesis as the overall physiological basis to explain the principles behind the standard germination and vigour tests (ageing, electrolyte leakage, cold test, germination rate and seedling size). The work of the International Seed Testing Association (ISTA) in developing convenient, inexpensive and internationally repeatable tests is illustrated.

Seeds of the Brussels sprouts cultivar Asmer Aries and the cauliflower cultivar Hipop were subjected to ageing at 20% moisture content and 45°C for 24 or 30 h, respectively; all seeds retained high germination after ageing. Aerated hydration of unaged and aged seeds of both cultivars for 4–8 h at a range of temperatures (10–30°C), followed by drying, resulted in improved performance, except that germination percentage and rate of cauliflower were lower at 10°C. Thus, all treated seeds showed greater germination rate and seedling root length than the control, which may have resulted from the advancement of the process of germination. The deleterious effect of aerated hydration at 10°C on cauliflower could be explained by damage due to rapid imbibition; seeds that had imbibed slowly to close to full imbibition (41% moisture content) before aerated hydration showed no decrease in germination. The improvement of aged seeds after aerated hydration was also revealed by higher germination after the controlled-deterioration test, which indicated less deterioration in treated seeds. Furthermore, the optimum improvements for all seeds were observed at 25°C and were greater when the water was aerated than non-aerated. These observations indicate the activation of metabolic repair processes during aerated hydration, leading to a reversal of the deterioration sustained during ageing.