Studies on Foliar Penetration: VI. FACTORS CONTROLLING THE PENETRATION OF 4-AMINO-3,5,6-TRI-CHLOROPICOLINIC ACID (PICLORAM) INTO THE LEAVES OF PHASEOLUS VULGARIS

The evaluation of the factors controlling the penetration of4-amino-3,5,6-trichloropicounic acid (picloram), containing¹⁴C in the carboxyl group, into leaves of Phaseolus vulgarisis complicated by the ability of the leaf tissue to decarboxylatethe compound rapidly. The degradation system becomes fully activatedonly after some hours following treatment with picloram whilethe capacity for decarboxylation declines with the age of theleaf. When allowance for decarboxylation is made, the pattern of penetrationfor picloram resembles in particular that of 2,2-dichloropropionicacid (dalapon). At the abaxial surface penetration continuesover 24 h at a fairly constant rate but in light, which enhancespenetration, a ‘surge’occurs between the secondand sixth hour. Raising the external concentration promotespenetration and in light entry is directly proportional to concentrationup to at least 2.5 x 10^–l M. The response to varying lightintensity resembles that of dalapon rather than 2,4-D. Evenlow intensities promote penetration; the effect is maximal atabout 10 000 lx at the abaxial surface but continues at leastup to 20 000 lx at the adaxial surface. Penetration is retarded at 4°C but the decarboxylation systemremains functional. In light, but not in darkness, penetrationis strikingly reduced by 3-(3,4-dichlorophenyl)-l,l-dimethylurea(DCMU). Raising the external pH from 4.2 to 7.2 retards entry. It is concluded that the factors which determine the penetrationof picloram into Phaseolus leaves also control the entry ofa wide range of organic compounds and inorganic ions.