[Experiments in the analysis of the geotropic perception : V. The influence of the gravitational field on the auxin-sensitivity of Helianthus-hypokotyls]

Abstract

Experiments with hypocotyls of decapitated Helianthus seedlings have been carried out to determine whether a change in their sensitivity to growth substances participates in the geotropic reaction chain. As a test for such an influence the elongation of pre-stimulated and of non-stimulated hypocotyls in identical IAA-solutions was compared. 1. First the elongation of the samples in 10(-4) m IAA buffer solution after horizontal exposure of varying duration was examined and compared with the behaviour of vertical controls. In all cases the pre-induction increased the total elongation of the samples. After a 15 min-stimulation the effect was already distinct, and after a 30 min-induction it attained its maximum (+42%); a further prolongation of the exposure time led to a gradual decrease of the effect. 2. In a second series of experiments the influence of the IAA-concentration on the elongation of the samples in the incubation bath was examined after a 1 hour-period of horizontal exposure. The absolute elongation of the stimulated hypocotyls as well as of the vertical controls reached its maximum value in the 10(-4) m-step of the series. A further increase in the IAA-concentration already reduced the elongation in both cases. However, the relative furtherance of the elongation by the gravitational stimulus continued slightly even in the 10(-3) m-step. From this fact it can be concluded that the observed effect of the geoinduction can only be due to an increase of the auxin-sensitivity of the organ. Another conceivable possibility, namely, an increase of the active auxin uptake from the external solution, must be excluded because from the 10(-4) m-step on such an increase would already lead to an accumulation of supraoptimal IAA-concentrations within the tissue. 3. If, after their horizontal exposure, the hypocotyls are split into their upper and lower halves, it can be shown that the geoinduction promotes the elongation of both parts in the auxin solution. However, this effect is considerably more pronounced in the lower halves. This asymmetric change explains the formation of after-curvatures which appear in auxin-depleted hypocotyls symmetrically supplied with IAA after their horizontal exposure. 4. Vertical inverse position also produces in auxin-depleted hypocotyls an increase of their IAA-sensitivity, approximately corresponding to the effect of a horizontal exposure of the same duration. Here again the reaction proved to be an optimum function of the induction time. The maximum, +24,5%, was reached after a 1 hour-inverse position. 5. The gravity-induced increase of the auxin-sensitivity develops only in the presence of oxygen. When the plants were set in an inverse position in an atmosphere of pure nitrogen and were afterwards incubated under aerobic conditions, the effect completely failed to appear. 6. Rotation of the plants on the clinostat parallel to its horizontal axis also causes a rise in the auxin-sensitivity. The strongest effect was observed after a 3 hours' stay on the clinostat: +13,5%. 7. The increase of the auxin-effect brought about by geoinduction can probably be attributed to a spatial shifting in the gravitational field of a cofactor of the growth substance which is available even in the decapitated organ.