The role of growth regulators in the differentiation of flowers and inflorescences

Abstract

Growth regulators participate in the differentiation of floral parts, determining the developmental path of the respective type of inflorescence. The effect depends on the expression of the peculiarities of floral part differentiation, the recognition of the character of endogenous substances in certain stages and the choice of the suitable regulator for application. In the primitive flower ofPapaver petals and stamens are formed from the peripheral meristem with a lower content of auxins and a higher level of gibberellic substances. The pistil arises later from central tissues with a higher level of auxins and inhibitory substances. The stamens are more sensitive to the higher level of auxin substances, and by a suitable application of GA₃ and BAP they can be transformed into petals; in this way double flower forms arise. In the differentiation of floral parts ofCampanula, Rosa andMelandrium similar regularities assert themselves in time successions, but in another spatial arrangement. Sex differentiation of diclinous flowers ofMelandrium is based on differences in heterochromosomes XY and XX. The rise of the zygomorphic flower ofVeronica is accompanied by a different distribution of endogenous substances which affect the development of petals, stamens and the pistil.

The differentiation of flowers in the racemose inflorescence occurs in the acropetal succession, and lateral primordia inCampanula develop into actinomorphic regular flowers, whereas inDigitalis they are zygomorphic and only the terminal flower is peloric. In the initial phases the staminate tassel and the pistillate ear in maize are identical. Earlier differentiation of the terminal pistillate tassel is connected with a higher level of gibberellins and the later development of the lateral pistillate ear is accompanied by the increase in auxin-like substances and inhibitions. Similar correlations were found in the development of staminate catkins and the differentiation of pistillate flowers in terminal buds ofJuglans regia. By the application of auxin-like substances it is possible to achieve the transformation of primordia of the staminate tassel into the pistillate ear in maize or to regulate the number of staminate catkins and pistillate flowers on twigs of the walnut tree. In the capitulum of the sunflower differences arise between peripheral pistillate ray flowers and hermaphrodite tubular ones. By applying GA₃ and BAP the number of ray flowers is increased. If the normal course of inflorescence differentiation is affected with a suitable type of regulator, a range of floral abnormalities appears which permit to assess the intervention in different developmental stages and the reaction of the primordium to the applied type of regulator. Abnormalities also suggest some phylogenetic correlations.