On the abscission of debladed petioles in Coleus rhenaltianus, especially in relation to the effect of gravity

By means of experiments with Coleus rhenaltianus it could be demonstrated that the effect of proximally as well as of distally applied auxin (IAA) on the abscission of debladed petioles, is an indirect one. By growth processes that are induced by the application of auxin at the proximal side of the abscission zone, e.g. by the development of new adventitious roots and by the elongation of roots that are already present, a substance that retards the abscission is drained off from the petioles. This substance is produced in the petioles, and probably also in the leafblades, when the latter are exposed to the light, and it was found to migrate through the parenchyma. A low concentration of auxin that is distally applied, accelerates the abscission of the petioles, provided that there is growth activity in the part at the proximal side of the abscission zone. The accelerated abscission in horizontally placed plants is due to growth processes that are induced at the lower side of the stem by the gravitational force. The difference in abscission time between the petioles of plants that are kept in the vertical position, and those that are placed horizontal, is independent of distally applied sugar and of the concentration of distally applied auxin. The abscission proves to be retarded when the petioles are debladed a week after the plants had been brought in the horizontal position or when there is no source of auxin proximal of the abscission zone (young leaves). In the horizontal position the gravitational force affects a lateral shift of auxin from the tip to the lower side of the stem, and it hampers moreover the basipetal transport of substances that are produced in the leaf blades, and by which the growth of roots, the growth at the lower side of the stem and the development of new adventitious roots are promoted. By means of paper chromatography it was found that the substance from the petioles which retards the abscission, is most probably identical with an auxin-like substance that was extracted from full-grown leaves, and of which it could be shown that it is not an indole derivative. In plants in the normal position the abscission is accelerated by the longitudinal component of gravity. Elimination of the longitudinal component hampers the transport of the substance which retards the abscission, a transport that is directed to centres of growth, and so it can exercise its influence on the petioles themselves, and retard their abscission. The retardation of the abscission in plants that are rotating parallel to the horizontal clinostat axis, is influenced by the revolving speed of this axis. This retardation is greatest at a revolving speed of 1-3 min. In slowly rotating plants the retardation of the abscission is partly compensated by the effect of the transversal component of gravity which promotes the abscission. The effect of the elimination of the longitudinal component is independent of the presence of a proximal source of auxin, e.g. of the tip and of young leaves.