ACTION OF KINETIN ON PHOTOSENSITIVE GERMINATION OF LETTUCE SEED AS COMPARED WITH THAT OF GIBBERELLIC ACID

1. Experiments with the seeds of Grand Rapids lettuce showed that the germination induced by gibberellic acid or by red light is strongly accelerated by kinetin, although the latter itself can promote the germination in the dark only slightly. The reversible light reactions of the phytochrome system interact with kinetin treatment just as effectively as with water-imbibed controls. The site of primary action of red light is not altered by kinetin treatment. Kinetin does not modify the water uptake of the seeds for at least 8 hours. 2. Kinetin was found to inhibit the growth of the hypocotyl and root of the seed, but to promote very markedly the expansion of the cotyledons. This effect was observed not only with cotyledons in intact seeds but also with isolated cotyledons. The expansion of kinetin-treated cotyledons is further promoted by red light, but not by far-red, as is also the case with germination itself. 3. A number of purine derivatives which have been reported to promote germination also cause expansion of isolated cotyledons. 4. Gibberellic acid promotes both hypocotyl elongation and cotyledon expansion in the dark, but this effect does not interact with the phytochrome system. The site of action of gibberellic acid probably lies in the axis. 5. It is concluded, therefore, that the site of kinetin action is in the cotyledons, whose expansion helps to break the seed-coat when light or gibberellin has contributed the primary stimulus.