Time course of exudation from excised corn root segments of different stages of development.

Xylem exudates were collected at hourly intervals from short segments which had been excised from two portions of the primary root of corn (Zea mays L.) seedlings and partially immersed in experimental salt solution containing (86)Rb. All segments showed variation in rates of output of both volume and ions for several hours, after which a steady state was attained which persisted for many hours. Apical segments produced little or no exudate for several hours and did not reach a steady state until 18 or more hours after excision. Basal segments produced exudate containing detectable quantities of isotope within an hour and they reached a steady state about 12 hours after excision. During their respective steady states, apical segments produced three times the volume per hour and translocated eight times as much Rb per hour as did basal segments.These differences in the time course of exudation and the large differences in output are interpreted as indicating that two independent systems of ion transport operate simultaneously in intact roots. If this interpretation is correct, then it would appear that one of these systems has a greater potential capacity for ion transport than the other.