Enhanced Response of Bean (Phaseolus vulgaris) and Canada Thistle (Cirsium arvense) to Bentazon or Glyphosate by Gibberellin

Abstract

Gibberellin-enhanced response of bean plants (Phaseolus vulgarisL. ‘Black Valentine’) and Canada thistle [Cirsium arvense(L.) Scop.] to bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] and glyphosate [N-(phosphonomethyl)glycine] was determined on both species under controlled photoperiods in the growth chamber and on Canada thistle under natural conditions in the field. Pretreatment of bean plants in the growth chamber with 10 mg/L gibberellin (GA3or GA4/7) more than doubled the herbicidal effect of 1600 mg/L bentazon treatments. Pretreatment of Canada thistle plants in the growth chamber with 100 mg/L GA4/7increased the herbicidal effect of 50 mg/L bentazon applications more than four-fold, and that of 400 mg/L glyphosate applications by more than two-fold. The phytotoxicity of bentazon to field-grown Canada thistle increased 10% or more in populations pretreated with 25 g/ha gibberellin; and that of 840 g/ha glyphosate was similarly increased. The effectiveness of a higher glyphosate dosage (1680 g/ha) was not significantly increased by GA pretreatment. Plants induced to bolt with gibberellin (GA4/7) were injured more by bentazon than were plants that bolted naturally.