The Lateral Transport of 2, 4-Dichlorophenoxyacetic Acid in Horizontal Hypocotyl Segments of Helianthus annuus

The longitudinal and lateral transport of 2, 4-D-1-¹⁴C] hasbeen studied in 6-mm horizontally disposed segments of the hypocotylof Helianthun annuus. The technique involved the asymmetricapplication of the auxin in agar donor blocks to either theupper or lower halves of the cut surface of the segments andthe measurement of ¹⁴C accumulating with time in split receiverblocks on the upper and lower halves of the cut surfaces atthe opposite ends. A highly significant effect of gravity inducinga polar migration of 2, 4-D to the lower side has been establishedand represents, under optimal conditions, about 10 per centof the total 2, 4-D in transit. This lateral polar movementshows a tendency to saturate at higher donor concentrations(5 mg/1) and is affected by temperature in precisely the sameway as the basipetal longitudinal transport. The optimal flux(intensity of transport) occurs at about 35 °C or slightlyabove, and above 40 °C both transport systems are seriouslyimpaired. There is some evidence that gravity increases thevelocity of 2, 4-D transport in the lowermost tissues of thehorizontal hypocotyl but does not affect the transport intensity.