Water Absorption by Needles of Ponderosa Pine Seedlings and Its Internal Redistribution

The maximum amount of 2,4-D one could reasonably expect in the receiver block would not give a concentration of more than 1 to 10 mg/l since in table II only 1 % of the auxin was transported. These concentrations of 2,4-D would give a synergistic response with the IAA rather than an inhibitory one (table I). It is concluded, therefore, that smaller amounts of IAA are transported when the sections have been pretreated with 2,4-D. PRE-TREATMENT OF INTACT SEEDLINGS WITH 2,4-D OR TIBA: The applicability of these results to whole plants is of interest in connection with herbicide work. The transport of IAA was, therefore, measured in sections of stems taken from bean seedlings which had previously received a foliage application of 2,4-D or TIBA. Fifty micrograms were applied to one of the primary leaves of the seedlings in the light, and at 24 hours 5-mm sections were removed from the epicotyl and petiole. The sections were set on moist filter paper for two hours to allow any auxin already present to be transported out. After this time, their ability to transport IAA was tested as before. Analysis made in other studies (3) showed that 2,4-D is transported from the leaf to the epicotyl and that these 5-mm sections would contain about 0.5 ,ugm of 2,4-D under the above conditions. If all of this 2,4-D moved into the receiver blocks, which is extremely unlikely, the final concentration would be 15 mg/l. This concentration would tend to increase the response given by IAA rather than to depress it. The Avena curvatures produced by the receiver blocks after a two-hour translocation period are recorded in table V. The donor blocks contained 0 and 10 mg/l. Quite substantial amounts were transported through both epicotyl and petiole sections of untreated plants, but much smaller amounts moved through sections taken from seedlings which had been previously treated with 2,4-D or TIBA. To this extent the data for TIBA confirm Kuse's conclusion previously mentioned. It seems clear, therefore, that the mechanism which is responsible for the transport of IAA through excised sections of bean stems is disrupted when 2,4-D or TIBA is applied to the foliage. If this transport mechanism operates in intact plants, it seems certain that when these compounds are applied to the leaves or stems they will interfere very greatly with auxin transport.