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Picloram residues in potatoes and carrots and picloram photodecomposition. Soniassy, Ranjit Nunderdass


Residues of picloram in four varieties of potatoes, given pre-emergence treatments with picloram at 2 oz per acre and picloram at 2 oz + linuron at 24 oz per acre were determined by electron capture gas chromatography. Average residue levels of 3.9 and 2.7 ppb (fresh weight) were obtained for the picloram and picloram + linuron treatments. This difference was significant at the 1% level. Tuber injury, ranging from formation of corky tissues on the surface to splitting, was observed with the picloram treatments. No such injury was observed with the picloram + linuron treatments. Yields were noticeably higher in the picloram + linuron treatments. The reduced picloram residues could thus be associated with the diluting effect of higher yields which resulted from the addition of linuron. A similar experiment using four varieties of carrots given either pre-emergence or post-emergence treatments with picloram at ½ and 2 oz per acre respectively gave no detectable picloram residues. Using ¹⁴C carboxyl labeled picloram this finding was further investigated and it was shown that picloram was absorbed by the foliage and roots and translocated throughout the whole plant. The leaves accumulated four times more radioactivity than the taproots. The radioactivity in the leaves and taproots was in the form of the parent picloram molecule. The picloram present in the taproot was located mainly in the xylem. A study of the stability of picloram, its potassium salt and its methyl ester under short wave ultraviolet light (253.7 nm) revealed that all three compounds were degraded into several photoproducts. The methyl ester was the least stable, being 85% degraded after one hour exposure. Picloram and its potassium salt were more stable, each being 50% decomposed after one hour exposure. Partial polymerization of all three compounds may also have taken place.

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