On the flower head formation and development of cauliflower plants. I

Abstract

In order to obtain the physiological data concerning the flower head initiation of cauliflower plants by low temperature, this experiment was carried out with Nozaki-wase grown in the experimental field and with Early Snowball A grown at various temperatures in the phytotron, analysing auxins and gibberellin-like substances in the apical part and chemical constituents in the stem of plants.1. Seedlings planted early in spring received the effect of low temperature immediately after transplanting, while seedlings planted in summer grew for a while under favorable condition for vegetative growth following the transplanting and then were exposed to low temperature. Thus the latter were larger than the former at the stage of flower head initiation.The following analytical results were noted with these materials.(1) Carbohydrates and nitrogen compounds gradually increased with the progress of age and especially soluble sugar was accumulated remarkably in the stem of both plants sown in spring and summer, resulting in the highest level at the stage of flower head initiation followed by a rapid decline.(2) A temporary decline of auxins, especially in the Rf value of IAA, was clearly shown immediately before flower head initiation in both plants sown in spring and summer.(3) The content of gibberellin-like substances was small amount at the vegetative stage and increased considerably after flower head induction.(4) DNA content was fairly constant at the vegetative stage in spite of the different size of seedlings sown in spring and summer, but thereafter gradually increased following flower head induction, while RNA increased rapidly with the progress of age, resulting in a peak immediately before flower head initiation, corresponding with the time of a temporary decline of auxin.From the analyses of nucleotide components of RNA in apical parts of plants at the vegetative stage and reproductive stage, it was found that the ratio of purine base to pyrimidine base was high at the vegetative stage in contrast to the low value at the reproductive stage.2. When Early Snowball A seedlings were exposed to 10, 17 and 25°C respectively, the flower headd ifferentiated after five days at 10°C and after ten days at 17°C, but did not differentiate in the Plants grown at 25°C.Corresponding with those responses to temperature treatment, the following analytical data were obtained.(1) In the stem of plants grown at 25°C, carbohydrates and nitrogen compounds accumulated gradually, whereas the former rapidly increased and the latter decreased with the decline of treated temperature. And the highest vahle of carbohydrales and nitrogen compounds were shown at the stage of flower head induction of plants grown at each temperature.(2) It was observed that the gibberellin content considerably increased following flower head induction, in other words, from the 10th day after the exposure treatment to 10°C and from 15 th day to 17°C respectively.(3) Litlle DNA was found in apical parts of Plants grown at various temperatures during the course of vegetative growth followed by a gradual increase at the reproductive stage.On the other hand, a rapid increase in RNA was found in apical parts of plants grown at 25°C with the progress of age, whereas 10°C and 17°C treatments did significantly decrease the RNA level, the decrease being earlier and greater with the decline of treated temperature.3. From the above-mentioned results it may be suggested that the flower head initiation of cauliflower plants is induced by the qualitative conversion of RNA, that is, relative increase in pyrimidine base and decrease in purine base of RNA component due to a temporary decline of auxins in apical pazts of plants by low temperature.