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

Abstract

The longitudinal and lateral transport of 2, 4-D-[1-¹⁴C] has been studied in 6-mm horizontally disposed segments of the hypocotyl of Helianthun annuus. The technique involved the asymmetric application of the auxin in agar donor blocks to either the upper or lower halves of the cut surface of the segments and the measurement of ¹⁴C accumulating with time in split receiver blocks on the upper and lower halves of the cut surfaces at the opposite ends. A highly significant effect of gravity inducing a polar migration of 2, 4-D to the lower side has been established and represents, under optimal conditions, about 10 per cent of the total 2, 4-D in transit. This lateral polar movement shows a tendency to saturate at higher donor concentrations (5 mg/1) and is affected by temperature in precisely the same way as the basipetal longitudinal transport. The optimal flux (intensity of transport) occurs at about 35 °C or slightly above, and above 40 °C both transport systems are seriously impaired. There is some evidence that gravity increases the velocity of 2, 4-D transport in the lowermost tissues of the horizontal hypocotyl but does not affect the transport intensity.