Mode of action of thiocyanates and iodides in aminotriazole formulations

Abstract

Ammonium thiocyanate (I) was shown to inhibit the oxidation of aminotriazole (3‐amino‐1,2,4‐triazole) in two free‐radical generating systems: (a) riboflavin photo‐sensitised oxidation; and (b) oxidation by hydroxyl radicals. Evidence from in‐vitro studies is presented to show that I could enhance aminotriazole performance by being preferentially oxidised within the leaf, thereby preventing aminotriazole free‐radical formation and subsequent conjugation with amino acids and other compounds. This opens up the possibility of a whole new range of additives which could enhance translocation by inhibiting free‐radical reactions. A comparison of possible inhibitors revealed that cyanides, iodides and perhaps bromides may be of use in this respect. In addition to inhibiting free‐radical reactions, I and potassium iodide (II) were shown to have a considerable effect on the uptake of aminotriazole by bean leaves (Phaseolus vulgaris var. Canadian Wonder). In the absence of additives, uptake of aminotriazole was negligible (1.8%); in the presence of a 1:1 molar ratio of I or II, uptake of aminotriazole was increased to 36.6% and 54.2% respectively. At a 3:1 molar ratio of additive:aminotriazole, the respective figures were 63.5% and 81.0%. II appeared to be the more efficient of the two additives at higher humidity. However, the results and observations indicated that the situation may be reversed under low‐humidity conditions.

Mode of action of thiocyanate and iodide in relation to IAA metabolism

“…It is accepted that charge transfers are involved and that the order of reactivity of the anions in question follows the order I' > Br" > CI" (Bellus, 1978), This is consistent with the trends in their oxidative properties and it would follow that the pseudohalide thiocyanate will occupy a position intermediate between iodide and bromide (Adams et al., 1972). In the above work (Cook & Duncan, 1979). the ability of these anions to inhibit the oxidation of aminotriazole in in vitro systems appeared to follow this order.…”

“…Of course, at higher concentrations of thiocyanate, non specific interactions of the typse noted for iodide will undoubtedly occur thereby accounting for the fact that at low levels hormonal effects due to the influence on IAA are easily recognisable (sprout control and leaf abscission) whilst at higher concentrations of chemi-cal more general freezing effects occur. The fact that thiocyanate is an effective free radical scavenger (Adams et ai, 1972;Cook & Duncan. 1979) and also aids foliar uptake of herbicides (Babiker & Duncan, 1975;Cook & Duncan, 1979) are additional worthwhile benefits from this chemical.…”

“…The fact that thiocyanate is an effective free radical scavenger (Adams et ai, 1972;Cook & Duncan. 1979) and also aids foliar uptake of herbicides (Babiker & Duncan, 1975;Cook & Duncan, 1979) are additional worthwhile benefits from this chemical.…”

“…Studies on the use of additives to aid aminotriazole uptake and translocation (Cook & Duncan, 1979) have led to an interest in the use and mode of action of thiocyanate and other related anions, not only as additives in formulations but when applied alone to plants (Stephen, Cook & Duncan, 1979).…”

“…Thiocyanate has been found by chemists to exhibit many of the properties of the halides and so is often classified by them as a pseudohalide, thus, in addition to thiocyanate, the halide anions were investigated as potential additives for aminotriazole formulations (Cook & Duncan, 1979). This study demonstrated that in two in vitro free radical generating systems thiocyanate and iodide would inhibit the oxidation of aminotriazole by being preferentially oxidized and so inhibit the formation of aminotriazole free radicals which is thought to be a first stage in detoxification (Brown & Carter, 1968), Of the other halides tested, bromide showed this ability in only one system, chloride and fluoride gave little or no aminotriazole protection.…”