Physiological studies on the bolting of lettuce plants. I.

Abstract

1. Wayahead, May King and Great Lakes varieties grown in the field were sampled at intervals of 10 days for the analyses of carbohydrates and nitrogen compounds in the tops and for the estimation of auxin, gibberellin and nucleic acids in the apical buds of lettuce plants.Carbohydrates and RNA contents increased with growth of the plant and reached to their peaks just before the flower bud differentiation, and then decreased with the development of flower organs.Seasonal changes in auxin contents were the same as those in carbohydrates and RNA contents, but auxin contents temporarily decreased just before the flower induction, nearly corresponding with the peak time of RNA contents.Seasonal trend of nitrogen compound contents was in contrast with that of carbohydrates.These tendencies were same in every varieties, though the contents of carbohydrates and RNA at maximum level were higher in Great Lakes (late variety) than in Wayahead and May King (early varieties), and the highest auxin contents were seen in Wayahead followed by May King, and Great Lakes in descending order.2. Seasonal changes of the contents in native gibberellin-like substances and nucleic acids in apical buds of plants treated with high temperature were investigated.Gibberellin-like substances in apical buds of plants grown under the natural conditions were hardly detected until the flower bud differentiation, and increased rapidly after the flower induction, whereas the gibberellin-like substances in the plants treated with high temperature appeared soon after the treatment and increased remarkably after the floral initiation in similar manner as that of control plants grown under the natural conditions. Both the contents of DNA and acid soluble substances were scarcely affected with age of the plant and by the high temperature treatment. The content of RNA increased with growth of the plant grown under the natural conditions, but it decreased soon after the high temperature treatment, and increased to the peak before the flower induction.The content of RNA in the treated plant was one half, as low as that of control plant grown under the natural conditions.3. The flower induction was inhibited by the foliar applications of auxins, but was promoted by the sprays of nucleic acids.The results of gibberellin applications showed that the stem elongation was induced both at high and low temperatures, but the floral initiation was induced by gibberellin applications only at high temerature.4. From the above-mentioned results it may be concluded that the auxin contents increase with age of the plant and they promote the RNA metabolism, and that the flower bud differentiation is induced by the qualitative conversion of RNA which is affected by the temporary reduction of auxin level in the apical bud of lettuce plants.