Studies on the mode of action of asulam in bracken (Pteridium aquilinum L. Kuhn) I. Absorption and translocation of [14C]asulam

Studies of the absorption and translocation of foliage-applied ring-labelled ¹⁴C]asulam methyl (4-aminobenzenesulphonyl) carbamate] were carried out using glasshouse and field-grown bracken plants. Translocation of ¹⁴C from the treated frond was primarily according to a 'source to sink’pattern with intense accumulation of radioactivity in the metabolically active sinks viz. rhizome apices, frond buds, root tips and young frond tissue. In the case of field bracken, translocation and distribution of ¹⁴C was extensive in the rhizome system, accumulation occurring in the active as well as dormant buds situated on the non-frond-bearing and storage rhizome branches. Treatment of fully expanded fronds with 100μl of ¹⁴C]asulam (1 mg, 1.0–1.5 μCi) as 2 μl droplets resulted in a rapid initial uptake during the first week, followed by progressive entry and distribution with time. Basipetal translocation to the rhizome system was positively correlated with total uptake. High humidity (95%) and high temperature (30°C) stimulated uptake and subsequent basipetal translocation to a considerable degree. Uptake was greater through the stomatal-bearing abaxial than through the adaxial cuticle. Incorporation of a surfactant (Tergitol-7, 0.1%) increased penetration by up to 30%. Uptake declined markedly as the frond aged, while translocation was predominantly acropetal in young treated fronds, becoming exclusively basipetal when the fronds matured. Optimum uptake and maximum distribution of ¹⁴C]asulam in the rhizome and its associated buds was achieved when treatments were applied to almost fully expanded fronds. The translocated ¹⁴C (asulam and possibly some of its metabolites) showed a considerable degree of persistence in the rhizome system, 8% of the applied activity still remaining in the rhizome 40 weeks after treatment.