Effect of Moisture Stress on the Absorption and Transport of Herbicides in Woody Plants

Abstract

Absorption by foliage and distribution of picloram (4-amino-3,5,6-trichloropicolinic acid) and 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) as affected by moisture stress were studied in mesquite (Prosopis glandulosa var. glandulosa) and winged elm (Ulmus alata) under a controlled environment. Gas chromatographic assay measured the movement of both herbicides into untreated portions of the plant. Mesquite absorbed picloram more rapidly and extensively than 2,4,5-T. After 4 hr the apex contained both herbicides, but only picloram was present in the roots. After 24 hr the apex and roots contained more picloram than 2,4,5-T. The phloem-cortex accumulated greater quantities of picloram than the xylem-pith. After 90 hr herbicide concentrations in most tissues were unchanged or higher than after 24 hr. Mesquite and winged elm transported markedly different amounts of 2,4,5-T. Moisture stress reduced foliar uptake of picloram in mesquite but not in winged elm. Moisture stress did not affect absorption of 2,4,5-T. Stress reduced transport of herbicides differently in the two species, but reduced transport generally paralleled retarded growth. Moisture stress sufficient to slow growth markedly reduced transport of both herbicides into untreated tissues. The largest reductions in transport occurred at moderate stresses or where extensive movement occurred under no stress.