Growth physics and water relations of red-light-induced germination in lettuce seeds

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Abstract
Red light (R) and gibberellins (GA) each induce a water potential decrease in the axes of lettuce (Lactuca sativa L.) embryos resulting in germination of intact “seeds” (achenes) or an increase in growth of the axes of isolated embryos. The fruit coat and endosperm are a substantial barrier to the penetration of exogeneous GA. Isolated embryos take up 35 times as much [3H]GA1 as the embryos of intact seeds and respond to less than 1·10-10 M GA3 or GA4+7. We calculated that only 1·10-8 M of either GA3 or GA4+7 would result in 50% germination if the GA were able freely to penetrate the fruit coat. Exogenous GA3 or GA4+7, at concentrations insufficient to cause germination, result in an apparent synergistic promotion of germination when suboptimal R is applied. Yet suboptimal concentrations of exogenous GA3 or GA4+7 and suboptimal R result in only additive increases in the growth response in axes of isolated embryos. Dose-response curves demonstrate quantitative increases in the growth response of the isolated axes after R or GA treatments insufficient to induce germination in intact seeds, indicating that a threshold potential must be achieved by the embryonic axes before germination can occur.