Field and greenhouse studies of 2,4-D [(2,4-dichlorophenoxy)acetic acid] and glyphosate [N-(phosphonomethyl) glycine] absorption, translocation, and metabolism were initiated to explain field observations which indicated susceptibility of common milkweed (Asclepias syriaca L.) but not hemp dogbane (Apocynum cannabinum L.) to glyphosate and the reverse response to 2,4-D. Glyphosate was absorbed less than 2,4-D in both species with absorption of both herbicides being greater in common milkweed. Greater herbicide absorption by common milkweed was attributed to less epicuticular wax, less cuticle, lower contact angle of the herbicide spray, and the presence of stomata and trichomes on the adaxial leaf surface. No major translocation differences of the herbicides were noted between species. Translocation of glyphosate was more rapid than that of 2,4-D. More glyphosate than 2,4-D accumulated in areas of high meristematic and metabolic activity. Rapid 2,4-D metabolism occurred in common milkweed leaves above treated leaves. There was no detectable 2,4-D metabolism in hemp dogbane roots after 20 days, while 60% of the 2,4-D in common milkweed roots was metabolized. Limited absorption of glyphosate but not 2,4-D by hemp dogbane and metabolism of 2,4-D but not glyphosate by common milkweed were considered the primary factors involved in explaining observed susceptibility differences.
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