Characteristics of Root‐to‐Shoot Transport of Cytokinin 6‐Benzylaminopurine in Intact Seedlings of Citrus aurantium

Abstract

AbstractTransport of the cytokinin 6‐benzylaminopurine‐8‐14C in the root and shoot of intact Citrus aurantium L. seedlings was studied by “replacing” the 0.5 cm root tip with the uptake solution. The cytokinin was transported basipetally in the root and was distributed in an acropetal direction in the stem and into the leaves. Kinetic analysis of the transport for periods of up to 96 h revealed a characteristic advancing front of the label along the axis of the seedling. The estimated velocity of transport of 6‐benzylamino‐purine‐8‐14C in various regions of the intact root was 2.6 to 5.1 mm/h.The transport of 6‐benzylaminopurine was predominantly in the transpiration stream, in stelar tissues of the root. Conditions of high transpiration favored enhanced transport to the shoot and an overall greater accumulation of the label. The total accumulation of 6‐benzylaminopurine in roots of intact seedlings after 48 h of transport was 354% of that in roots of shoot‐less seedlings. Root girdling and treatment of the root with KCN did not reduce the basipetal transport of the label in the root and into the shoot.Radiochromalogram scanning of root extracts and analysis of the ethanol insoluble‐NaOH soluble fraction revealed considerable metabolic changes in the translocated cytokinin. Only 51% of the radioactivity remained in the original 6‐benzylaminopurine peak after 24 h of incubation. Two other, unidentified, metabolites were detected.It is suggested that all the factors that affect the ascent of sap are involved in the long‐distance transport of cytokinins, and that the rate and mode of transport of cytokinins from the root system to the shoot may be a major factor in the expression of their physiological activity.