Ammonium Sulphate Enhancement of Picloram Herbicidal Activity and Absorption in Two Guava Species and Dwarf Beans

Abstract

Studies were conducted to increase picloram herbicidal activity on guava (Psidium guajava L.), strawberry guava (Psidium cattleianum Sabine) and dwarf beans (Phaseolus vulgaris L. cv. Bountiful) using ammonium sulphate as an adjuvant, and to determine the basis for the enhancement effect. Guava and strawberry guava seedlings were grown in pots outdoors for 3 to 4 months and 4 to 5 months respectively, and subsequently in the glasshouse for 2 months, before treatment with picloram alone at 0.25 , 0.50 and 0.75 kg a.i. /ha, or plus ammonium sulfate at 0.10, 0.75, 1.0 and 10% (w/v). Dwarf beans were treated with picloram alone at 0.02 and 0.04 kg a.i. /ha, or plus ammonium sulfate at 0.10, 0.50, 1.0, 5.0 and 10% (w/v). Ammonium sulfate increased picloram herbicidal activity in all three species to varying degrees, depending on picloram rate and ammonium sulfate concentration. Picloram injury was enhanced by ammonium sulfate at all picloram rates shortly after treatment. The higher picloram rates continued to increase injury with time, so that by the end of the experiment on each species, the maximum effect was attained without ammonium sulfate and an- enhancement effect could not be demonstrated. The enhancement effect remained evident at the low picloram rates. Ammonium sulfate concentrations of 0.75% to 1.0% and higher were equally effective at increasing picloram activity at the end of each experiment, but at earlier times, picloram activity increased with increasing ammonium sulfate concentration up to 10%. Paired leaves on the upper stem of strawberry guava and guava seedlings were treated with 14C-picloram alone or plus ammonium sulfate at 0.5% and 10% (w/v) on strawberry guava, and 0.5$ on guava. The 14C-picloram acid was converted to the potassium salt using commercial formulation blank, so that the 14C solutions were equivalent to the picloram solutions used in the activity experiments. After 2 days on strawberry guava, 0.5% ammonium sulfate increased 14C-picloram absorption about five-fold. There was about four times more 14C in the upper stem and attached leaves with ammonium sulfate added than with picloram alone. 14C-picloram absorption from 0.5% and 10% ammonium sulfate treatments was equal. Ammonium sulfate at 0.5% increased picloram absorption by four-fold in guava, but there was no increase in the amount of 14C translocated. All the 14C in both guava species was shown to be 14C-picloram by paper chromatography of extracts. Picloram absorption by dwarf beans, measured by difference between the amounts of picloram applied and the amount recovered, was increased by 0.5% and 10% ammonium sulfate by a similar magnitude as on guava species. Picloram absorption was further studied using detached strawberry guava leaves, and guava leaves in an initial experiment. 14C-picloram absorption was greater through the lower surface than through the upper surface, with or without ammonium sulfate. In strawberry guava leaves, 14C-picloram absorption through the upper and lower leaf surfaces was equal, but the ammonium sulfate enhancement of absorption was much greater through the upper surface. Ammonium sulfate increased the rate of 14C-picloram absorption, but absorption from 14C-picloram alone or plus ammonium sulfate levelled off at 6 to 12 hours. The magnitude of the ammonium sulfate-induced increase in 14C-picloram absorption was not affected by picloram concentration in the range 250 to 2000 ppmw, or by leaf age. 14C-picloram absorption by this detached leaf method was highest at an intermediate ammonium sulfate concentration range, such that absorption with 0.1% and 10% ammonium sulfate was less than with 0.5%; 0.01% had no effect on absorption. Increased 14C-picloram absorption resulted from adding the ammonium salts of sulfate, nitrate, chloride or dibasic phosphate, but not from ammonium carbonate or ammonium molybdenate. Ammonium sulfate was the only one of six sulfate salts tested to increase picloram absorption. pH adjustment of the external solution from 6 to 4 increased picloram absorption, while a pH change from 6 to 8 had no effect. In buffered solution at pH 4 ammonium sulfate did not increase 14C-picloram absorption. Ammonium dibasic phosphate at a solution pH of 7.7 increased picloram absorption as effectively as ammonium monobasic phosphate at pH 4.6. Potassium monobasic phosphate at pH 4.6 increased absorption to about one-half the value with ammonium phosphates added, whereas potassium dibasic phosphate at pH 9.5 had no effect. Stomata are not present on the upper surface of strawberry guava leaves, thus in the measurement of the effect of ammonium sulfate on picloram absorption, cuticular penetration of picloram must have been involved. The evidence suggests that ammonium sulfate has a direct physical effect in the absorption pathway.