Transport of Xenobiotics in Higher Plants I. Structural Prerequisites for Translocation in the Phloem

Abstract

Summary The correlation between the mobility of xenobiotics in the phloem and their chemical structure was investigated using the following substances: phloem-mobile 2,4-D, xylem-mobile 2,4-dichloro-anisole derived from the elimination of the carboxyl group, xylem-mobile defenuron and atrazine, and their ambimobile derivatives N-(p-carboxyphenyl)-N′-methylurea, phenylureidoacetic acid and 2-chloro-4,6-bis-( β -alanino)-l,3,5-triazine. Mobility was characterized quantitatively by the translocation rates and the translocation quotient Q tr . The Q tr was determined by the Sinapis test exhibiting the ratio of mobility in phloem following leaf application and mobility in xylem following root application. Analysis of exudates from detached Yucca inflorescence stalks revealed the presence of phloem- and ambimobile compounds without any metabolic conversions in the phloem sap exclusively. The partition coefficients of substances mobile in the phloem were higher at p H 8 than at p H 5. The corresponding coefficients of xylem-mobile chemicals were of the same value. These results suggest that xenobiotics enter the sieve element-companion cell complex by diffusion. Phloem- and ambimobile compounds are accumulated by an ion trapping mechanism causing their retention in the sieve tube and preventing a complete escape along the path. Consequently, acidic compounds with pK a lower than 7 are expected to be phloem- or ambimobile.