Studies on the Geotropism of Roots: I.GROWTH-RATE DISTRIBUTION DURING RESPONSE AND THE EFFECTS OF APPLIED AUXINSI

Abstract

Experiments are described in which the normal geotropic responses of the roots of Pisum sativum seedlings have been compared with those obtained in the presence of auxins (indole-3-acetic acid and 2:4-dichlorophenoxyacetic acid) in the external medium. The courses of positive curvature resulting from short exposures (40 minutes) and also subsequent recovery phenomena on a horizontal klinostat have been followed. A photographic recording technique allowed the determination of absolute growth-rates of both upper and lower sides of the root during the course of each experiment. Positive curvature started at its maximum rate (0.30–0.32 deg./min.) after a reaction time of 11.5 minutes and continued constant at that rate for about 60 minutes after stimulation ceased. Recovery took place at a similar rate of curvature and was complete after a further 150–200 minutes. During the phase of positive curvature overall root growth-rates were considerably reduced and were slowly restored to normal during recovery. Low concentrations (1 part in 10 ¹¹ ) of both auxins increased the rate of positive curvature by 30–40 per cent. and shortened the reaction time roughly in proportion. The growth-rates of both sides of the root were increased to the same extent during both curvature and recovery. High concentrations (10 ⁻⁸ IAA and 3.10 ⁻⁸ 2: 4-D) reduced the rates of curvature by 50 per cent., lengthened the reaction time, and inhibited the growth of both sides of the root during both curvature and recovery. Neither concentration of either auxin otherwise affected the time course of response and recovery. It is suggested that geotropic response is due to the de novo production of an endogenous inhibitor in the extending cells of the lower side of the root whence it may later spread to the upper side. The complete independence of the growth actions of this inhibitor and of the applied auxins suggests that it is not indole-3-acetic acid or any similar compound. Recovery may be very largely independent of both inhibitor and auxins and due to the action of another growth factor limiting cell length. The implications of these findings and of the attendant theories are fully discussed.