RETENTION OF XENOBIOTICS ALONG THE PHLOEM PATH

Detached leaves of Cyclamen persicum Mill. can be used as a simple sou rce-sink system. Phloem transport in the excised material was monitore d by the noninvasive C-11-technique. Assimilate movement stopped immed iately when the petiole was cut off. However, within 20 min a recovery of transport was observed. The translocation rate in the detached lea f was only 13% of that in the intact plant. C-14-Xenobiotics and [H-3] sucrose were injected into the upper petiole parenchyma (source). They moved downstream by a symplastic route. The stump of the petiole was inserted into a buffer solution containing ethylenediaminetetraacetic acid (sink). After 3 h, the distribution of sucrose and xenobiotics wa s determined in five subsequent segments of the petiole (path). The re tention coefficient (r) was calculated from the ratio of radioactivity in the vascular bundle to that in the petiole parenchyma. The distrib ution along the vascular path was given by a geometric progression, wh ereas its constant was the transport coefficient (q). Values of r and q corresponded with the degree of phloem mobility and ambimobility. Fo ur groups of compounds were classified: (i) acidic substances with log K-ow = -2 to -2.4 (K-ow is the partition coefficient octanol/water) a t pH 8 (pH of sieve tube sap), retained by ion trapping and exhibiting small lateral efflux (q greater than or equal to 0.7; maleic hydrazid e, dalapon); (ii) acidic substances with log K-ow = -0.7 to -0.8 at pH 8, retained by ion trapping and subjected to a moderate lateral efflu x (0.7 > q > 0.5; 2,4-dichlorophenoxyacetic acid, 2-methyl-4-chlorophe noxyacetic acid, bromoxynil); (iii) nonionised substances retained by optimum permeability, exhibiting a considerable lateral leakage (q les s than or equal to 0.5; glyphosate, amitrole); (iv) substances without basipetal transport in the phloem (atrazine, diuron). Retention of su crose corresponded quantitatively with that shown in group (i). This c lassification was also supported by results of uptake and efflux exper iments using the isolated conducting tissue. Theoretical translocation profiles were calculated from the determined transport coefficients ( q).