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

Experiments are described in which the normal geotropic responsesof the roots of Pisum sativum seedlings have been compared withthose obtained in the presence of auxins (indole-3-acetic acidand 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 horizontalklinostat have been followed. A photographic recording techniqueallowed the determination of absolute growth-rates of both upperand lower sides of the root during the course of each experiment. Positive curvature started at its maximum rate (0.30–0.32deg./min.) after a reaction time of 11.5 minutes and continuedconstant at that rate for about 60 minutes after stimulationceased. Recovery took place at a similar rate of curvature andwas complete after a further 150–200 minutes. During thephase of positive curvature overall root growth-rates were considerablyreduced and were slowly restored to normal during recovery. Low concentrations (1 part in 10¹¹) of both auxins increasedthe rate of positive curvature by 30–40 per cent. andshortened the reaction time roughly in proportion. The growth-ratesof both sides of the root were increased to the same extentduring both curvature and recovery. High concentrations (10^–8IAA and 3.10^–8 2: 4-D)reduced the rates of curvature by 50 per cent., lengthened thereaction time, and inhibited the growth of both sides of theroot during both curvature and recovery. Neither concentration of either auxin otherwise affected thetime course of response and recovery. It is suggested that geotropic response is due to the de novoproduction of an endogenous inhibitor in the extending cellsof the lower side of the root whence it may later spread tothe upper side. The complete independence of the growth actionsof this inhibitor and of the applied auxins suggests that itis not indole-3-acetic acid or any similar compound. Recoverymay be very largely independent of both inhibitor and auxinsand due to the action of another growth factor limiting celllength. The implications of these findings and of the attendant theoriesare fully discussed.