Origin and basipetal transport of the IAA responsible for rooting of carnation cuttings.

Abstract

The origin and transport of the IAA responsible for rooting was studied in carnation (Dianthus caryophyllus L.) cuttings obtained from secondary shoots of the mother plants. The presence of mature leaves in the cuttings was essential for rooting. Removal of the apex and/or the youngest leaves did not reduce the rooting percentage as long as mature leaves remained attached. Removal of mature leaves inhibited rooting for a 24-day period during which the basal leaves grew and reached maturity. After this period rooting progressed as in intact cuttings. Auxin (NAA + IBA) applied to the stem base of defoliated cuttings was about 60% as effective as mature leaves in stimulating rooting. Application of NPA to the basal internode resulted in full inhibition of rooting. The view, deduced from these results, that auxin from mature leaves is the main factor controlling the rooting process was reinforced by the fact that mature leaves contained IAA and exported labelled IAA to the stem. The distribution of radioactivity after application of (5-3H)-IAA to mature leaves showed that auxin movement in the stem was basipetal and sensitive to NPA inhibition. The features of this transport were studied by applying 3H-IAA to the apical cut surface of stem sections excised from cuttings. The intensity of the transport was lower in the oldest node than in the basal internode, probably due to the presence of vascular traces of leaves. Irrespective of the localization of the sections and the carnation cultivar used, basipetal IAA transport was severely reduced when the temperature was lowered from 25 to 4 degrees C. The polar nature of the IAA transport in the sections was confirmed by the inhibition produced by NPA. Local application of IAA to different tissues of the sections revealed that polar auxin transport was associated with the vascular cylinder, the transport in the pith and cortex being low and apolar. The present results strongly support the conclusion that IAA originating from the leaves and transported in the stem through the polar auxin transport pathway was decisive in controlling adventitious rooting.