Acidification, growth promoter, and red light effects on germination of skotodormant lettuce seeds (Lactuca sativa)

Abstract

Increasing the period of dark storage (DS) within a uniform seed lot of lettuce (Lactuca sativa L. cv. Grand Rapids) increased the degree of secondary dormancy (skotodormancy) induced, as evidenced by a loss of sensitivity to red light (R) and growth promoters such as gibberellin A3 (GA3), kinetin, thiourea, and ethylene. Differential degrees of skotodormancy were induced in lettuce seeds from three different sources (lot I < lot II < lot III). Either R or GA3 significantly increased the germination of skotodormant seeds if seeds were first immersed in strongly acidic solutions (pH ≤ 3.0) and then rinsed with water. The critical factor was found to be pH, not ionic strength. Promotion of germination by R or GA3 was positively related to increasing acidity, and also to the duration of DS. Sensitivity of the germination response to acid immersion varied with the seed lots, as little as 1 s being effective with some seeds. Acid treatment thus enables skotodormant seeds to recover much of their responsiveness to R and GA3. Extended DS (21 days) produced such pronounced skotodormancy (seed lot III) that acid immersion followed by usual R and GA3 produced only 10% germination. Continuous R or a seed-piercing treatment produced complete germination in otherwise skotodormant seeds. Responsiveness of germination to thiourea, kinetin, and ethylene, however, was not regained by treatment with acid. The action of these three chemicals on skotodormant seeds thus seems to be dependent on an active phytochrome and GA3-sensitive system. Acid-immersion treatments appear primarily to weaken membrane barriers of the endosperm cells, with resulting increased sensitivity of seeds to R treatment and GA3 penetration.