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Translocation and Distribution of Picloram in Bean Plants Associated with Nastic Movements

^a Plant Sciences Laboratories, Fort Detrick, Frederick, Maryland 21701

Nastic responses in bean plants (Phaseolus vulgaris L. var. Black Valentine) occur rapidly when very low concentrations of picloram (4-amino-3,5,6-trichloropicolinic acid) are applied to the root systems. The distribution of picloram associated with nastic responses was quantitatively determined in root-treated plants.

Sixty-seven micrograms of ¹⁴C-labeled picloram and 72 µg of unlabeled picloram in 300 ml of nutrient solution were applied to roots of 9-day old bean plants growing in an environmental growth chamber at 24° and 56% relative humidity. Plants were removed from the treatment solution after uptake periods of 3, 6, and 11 hr. Sections were excised from 10 locations on each plant and ¹⁴C content was determined by liquid scintillation techniques. As a qualitative indication of picloram mobility, additional plants were treated for 3 hr and subjected to autoradiographic analysis.

The accumulation of picloram within various parts of the plant increased with treatment time. After 3 hr of picloram uptake, a curvature of 60 to 80° from vertical in the upper 50 to 60 mm of stem corresponded to an accumulation in that part of the plant of 0.300 (second internode sample) to 1.065 (terminal bud sample) ng of picloram per mg fresh weight. The appearance of hyponasty in the trifoliolate leaflets after 6 hr of treatment corresponded to a picloram concentration in the leaf of 0.803 to 0.855 ng per mg fresh weight.

As evidenced by the ¹⁴C count data, the translocation of picloram from the roots to the apical part of the plant was very rapid. Picloram was preferentially accumulated in the terminal bud and first trifoliolate leaflets. Very little picloram was transported to the primary leaves and only at the longer treatment periods. Autoradiographs of plants treated for 3 hr showed a qualitatively similar distribution of ¹⁴C in the plant.