Growth Regulators in Populus tremula

The seasonal variation in capacity to form suckers and in auxin level in bark and wood was determined in root segments of aspen (Populus tremula L.). Auxin occurred in the roots from May to October but not in November. The highest auxin level was found during the period of shoot growth. During this period the capacity of root segments to form suckers was low. Auxin level decreased in isolated root segments during the first 24 hours after excision and was low during the period of sucker induction. The relation of endogenous auxin level to control of sucker formation is discussed. The experiments do not exclude the possibility that the auxin effect is mediated through inhibitors.