IN-VITRO CULTURED COCKLEBUR (XANTHIUM-STRUMARIUM L) RESPONSES TO DIMERCAPTOPROPANESULFONIC ACID AND MONOSODIUM METHANEARSONATE

Monosodium methanearsonate (MSMA) is an effective herbicide used for t he control of Xanthium strumarium (cocklebur), a serious weed problem in cotton production. Recently, MSMA-resistant cocklebur was reported, which reduced MSMA effectiveness in the cocklebur control strategy. T he mode of action of MSMA may involve the reduction of the pentavalent arsenic in MSMA to a trivalent form, which may then form a complex wi th sulfhydryl-containing enzymes. Dimercaptopropanesulfonic acid (DMPS ) chelates trivalent arsenic, resulting in a nontoxic complex. The obj ective of this study was to determine the effect of DMPS and MSMA on t he growth of in vitro grown cocklebur shoot and callus tissues. Latera l and terminal shoots were grown on basal Murashige and Skoog medium c ontaining 6.25 mg L(-1) (0.0428 mM) MSMA alone and with various concen trations of DMPS. Callus was cultured on basal callus medium also cont aining 6.25 mg L(-1) MSMA alone and an equimolar concentration of DMPS . Shoot injury symptoms, dry root weight, and fresh weight of callus w ere recorded. Susceptible cocklebur shoots showed severe discoloration and death when treated with MSMA. Resistant shoots and cultured shoot tips were not adversely affected. DMPS alone did not inhibit growth o f these tissues. Callus induced from susceptible tissue was inhibited by MSMA and DMPS alone. In all combinations of DMPS and MSMA tested, D MPS did not counteract the MSMA response. Thus, the pentavalent form o f arsenic in MSMA apparently is not reduced to the trivalent form by t he plant and hence is not involved in the mechanism of action