Studies on Foliar Penetration

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Abstract
The evaluation of the factors controlling the penetration of 4-amino-3,5,6-trichloropicounic acid (picloram), containing 14C in the carboxyl group, into leaves of Phaseolus vulgaris is complicated by the ability of the leaf tissue to decarboxylate the compound rapidly. The degradation system becomes fully activated only after some hours following treatment with picloram while the capacity for decarboxylation declines with the age of the leaf. When allowance for decarboxylation is made, the pattern of penetration for picloram resembles in particular that of 2,2-dichloropropionic acid (dalapon). At the abaxial surface penetration continues over 24 h at a fairly constant rate but in light, which enhances penetration, a ‘surge’occurs between the second and sixth hour. Raising the external concentration promotes penetration and in light entry is directly proportional to concentration up to at least 2.5 × 10−l M. The response to varying light intensity resembles that of dalapon rather than 2,4-D. Even low intensities promote penetration; the effect is maximal at about 10 000 lx at the abaxial surface but continues at least up to 20 000 lx at the adaxial surface. Penetration is retarded at 4°C but the decarboxylation system remains functional. In light, but not in darkness, penetration is 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 penetration of picloram into Phaseolus leaves also control the entry of a wide range of organic compounds and inorganic ions.

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