Physiological Basis for the Different Phloem Mobilities of Chlorsulfuron and Clopyralid

Abstract

Foliar-applied clopyralid was translocated much more readily than chlorsulfuron in the phloem of Tartary buckwheat plants. This result was not due to greater penetration of clopyralid into the treated leaf or to greater retention of chlorsulfuron in the cuticle. Experiments with excised leaf disks indicated that chlorsulfuron was taken up more readily by the leaf tissue and accumulated in the tissue to a higher concentration than clopyralid. Both herbicides effluxed readily from the tissue after transfer to herbicide-free medium, indicating that the accumulation was not due to irreversible binding within the tissue. Chlorsulfuron (2.8 nmol) applied with14C-sucrose reduced14C export from the treated leaf. Chlorsulfuron also reduced export of14C following exposure of the treated leaf to14CO2at 6, 12, or 24 h after herbicide application. This effect of chlorsulfuron could be partially reversed by pretreating the plants with a combination of 1 mM valine, leucine, and isoleucine. In similar experiments clopyralid had no effect on assimilate transport. It is concluded that phloem translocation of chlorsulfuron in sensitive species is limited by a rapid, indirect effect on phloem transport that reduces both its own translocation and that of assimilate.