Growth Hormones As Related To The Setting And Development Of Fruit In Nicotiana Tabacum

STUDIES OF growth hormones in plants have revealed many interesting aspects of the problem of fruit development. That growth hormones have a causal relationship to fruit development was first demonstrated when Gustafson (1936) induced parthenocarpic fruit development in tomato, Petunia, Salpiglossus and pepper. Mature fruits without seeds were produced through the use of the growth promoting chemicals indoleacetic, indolepropionic, indolebutyric and phenylacetic acids applied to the cut style. Gustafson (1937) found that extracts of pollen can induce fruit development in many plants and that this effect is not specific. The results of investigations by Laibach (1932) established the close relationship of the effective substances in pollen extracts to the growth hormones. Extracts of pollen and also of animal tissues caused the gynostemium to swell in certain tropical orchids and promoted the growth in length of the oat coleoptile. These facts were the basis for an hypothesis advanced by Gustafson (1939) that growth hormones from the pollen grains and pollen tubes initiate the early stages of fruit development, and after fertilization the developing embryo provides additional growth hormones. This paper deals with direct measurements of the growth hormones in the style, ovary and pedicel of pollinated flowers. EXPERIMENTAL METHODS.-The plant used in the experiments has been Nicotiana tabacum L., Lizard's Tail variety. It was found that flowers and fruit setting could be obtained continually by cutting back the flowering shoots on plants brought in from the field in October and kept in the greenhouse. The concentration of growth hormones was determined by placing the plant tissue on a block of agar and allowing the diffusion of the growth hormones into the agar to continue for several hours (Went, 1928), and the subsequent application of this block unilaterally to a deseeded Avena coleoptile using Skoog's (1937) modification of the standard Avena test method (Went and Thimann, 1937). Pollinations were made at different times previous to the diffusion of the hormones. At the time that some flowers were pollinated, others had the tip of the style covered with small lead foil caps to prevent pollination, and these were used as controls. Only flowers which had the stigma receptive and the anthers in the process of dehiscing were used. The flowers were removed from the plants just before the experiment, and each group of pollinations was 1 Received for publication June 4, 1942. The investigations reported here were carried out by the writer as an F. C. and Susan Eastman Newcombe Fellow in plant physiology and a Horace H. Rackham Predoctoral Fellow. The writer expresses his gratitude to Professor F. G. Gustafson for his suggestions and interest in the course of this study. Papers from the Department of Botany of the University of Michigan, No. 815. treated as a unit in the diffusion of the hormones. Each pistil was divided into three parts: the ovary, and two equal lengths (1.8 cm.) of the style. The three portions of a single pistil were kept together throughout the experiment so that results for each individual pistil were obtained. Immediately upon excision, the basal cut surfaces of the portions were placed for five to ten minutes on moist filter paper to prevent their drying out until set upon the agar blocks which were 2.7 mm. X 2.9 mm. X 1.6 mm. Small supports of copper wire were used to hold the segments of the style in a vertical position, and care was exercised to ensure the existence of a film of water between the cut tissue surface and the surface of the agar. One and one-half per cent agar was used in experiments 1 and 2. The weight of the ovaries forty-five to sixty hours after pollination was great enough to crush the agar blocks to some extent, therefore, 3 per cent agar was used in experiment 3. All diffusions were for a period of three hours in an Avena test darkroom at a temperature of 25?C. and in the saturated atmosphere of a large petri dish. At the end of the diffusion period the plant tissues were removed and the agar blocks were applied unilaterally to the deseeded Avena coleoptiles. The curvatures of the coleoptiles, after five hours, are recorded in table 1. Both the styles that were used for the determination of growth hormones and others pollinated at the time of the experiment were dissected under a binocular dissection microscope; the cortex was removed from the style and stigma. The strands of conducting tissue and the pollen tubes were stained with aqueous solutions of magenta red and light green S.F. Yellowish, according to the procedure of Buchholz (1931) and the growth of the pollen tubes was observed. RESULTS AND DISCUSSION.-Growth hormones as related to pollination and fertilization.-Table 1 shows that in only a few instances were appreciable quantities of diffusible growth hormones found in the styles or ovaries of unpollinated flowers. The small amounts and their infrequent occurrence are in striking contrast with the results for pollinated flowers. In the styles from flowers pollinated fourteen hours previous to the diffusion of the growth hormones in the first experiment, the pollen tubes had grown through the apical half of the style and a few millimeters into the basal half. Considerable quantities of growth hormones were obtained from both the apical and basal portions of the styles with those from the basal portions being somewhat larger. The quantities obtained from the ovaries were in most cases less than those from the styles at this time. In the second experiment the growth of the pollen tubes was slower than in the first, the tubes having passed through only the apical half of the style twenty hours after pollination. Smaller amounts of