Lettuce seed germination and cytokinins: their entry and formation.

Cytokinins overcome the inhibitory effect of abscisic acid on the germination of isolated embryos of Grand Rapids lettuce. We have used this fact to assess the content of cytokinins in embryos removed from intact seeds which have been supplied with these growth substances i.e. to determine hormonal uptake. The method has also been used to examine the hypothesis that phytochrome and gibberellin lead to increases in active cytokinin levels in seeds. Appreciable amounts of cytokinins enter intact seeds yet seem unable to promote germination. On the other hand, little or no cytokinin-like activity seems to result from light or gibberellin treatments. We conclude that there are unresolved difficulties inherent in the "cytokinin hypothesis" of phytochrome and gibberellin action.