THE INDUCTION OF LATERAL ROOTS BY INDOLEACETIC ACID AND ROOT DECAPITATION

Abstract

THE ANATOMY of lateral root formation has been known for a considerable time from the classic histological work of van Tieghem and Duliot (1888) which showed the endogenous origin of laterals within the primary root axis. Yet relatively little is known even now concerning the physiological processes involved. That artificially applied auxin stimulates lateral root formation in many plants was early recognized. Bouillenne and Went (1933) attributed increased root branching in the roots of Acalypha cuttings to the applied hormone not utilized in the initiation of the adventitious roots induced on the stem itself. Zimmerman and Hitchcock (1935) reported that lateral root formation on aerial roots of Cissus was induced byapplied artificial auxin or by root decapitation. They postulated the formation of a factor by the primary root tip which prevented lateral branching but which became inoperative when the tip was interfered with or no longer actively growing. The fact that auxin is definitely one factor controlling root branching was shown by Thimann (1936) in experiments treating the roots of Avena and Pisum seedlings with indoleacetic acid. He also found that removal of 1 mm. root tips from these seedlings resulted in increased lateral root formation in both species. In Avena, which has large amounts of natural auxin, Thimann reported that the principal influence on root branching is exerted by the tip. Thus it was indicated in these experiments that auxin is not the only factor controlling lateral root formation. In his study of auxin production in isolated pea roots grown in nutrient solution, van Overbeek (1939) found that 10 mm. root decapitation after 2 weeks' subculture resulted in a marked increase in lateral root formation on the basal portion of such roots. Delarge (1941) reported that, although indoleacetic acid caused branching in initially excised root tips of Zea and Triticum grown in sterile culture, subsequent retreatment with the auxin caused no further lateral root formation. Delarge suggested that there was a depletion of some factor necessary for lateral root formation. Recently, Nutman (1948) has proposed that root meristems, both those of the root and those of effective root nodules, produce a substance which is inhibitory to the initiation of lateral root primordia. With a view to further elucidating the physiological process of lateral root formation and the