SELECTIVE INDUCTION OF ETHYLENE AND CYANIDE BIOSYNTHESIS APPEARS TO BE INVOLVED IN THE SELECTIVITY OF THE HERBICIDE QUINCLORAC BETWEEN RICEAND BARNYARDGRASS

The quinolinecarboxylic acid quinclorac, which is structurally related to natural auxins, is a rice herbicide with selective activity agains t dicot and monocot weeds. As demonstrated for monocot and dicot speci es, stimulation of ethylene production by the herbicide was positively correlated with its phytotoxic effect. Treatment of susceptible barny ardgrass plants (Echinocloa crus-galli) at the second leaf stage with 20 mug/plant (approx. 3 x 10(-5) mol . L-1) quinclorac via the root ap peared to specifically induce ethylene biosynthesis in the root tissue . However, the 1-aminocyclopropane-1-carboxylic acid (ACC) produced an d its derivatives predominantly accumulated in the shoot tissue, where levels of ACC, N-malonyl-ACC (MACC), ethylene formation, and HCN incr eased up to 3-, 60-, 5-, and 3-fold, respectively. Concomitantly, inta ct root tissue was 50-fold less susceptible to damage by quinclorac th an intact shoot tissue. The observed HCN level in the latter tissue we nt up from 12 to 38 mumol . L-1 after treatment with quinclorac. Among the other phytohormones analyzed, only the immunoreactive 3-indoleace tic acid content increased in shoot tissue. Additional treatment with the ACC synthase inhibitor aminoethoxyvinylglycine decreased quinclora c-induced ethylene production and the phytotoxic effects on shoots of barnyardgrass. Quinclorac-treated rice plants (Oryza sativa) showed no notable changes in ethylene production and endogenous ACC, MACC, HCN and immunoreactive phytohormone contents and, shoots of rice plants we re 200-fold less susceptible to damage by quinclorac than those of bar nyardgrass. In comparison with quinclorac, 2,4-dichlorophenoxyacetic a cid (2,4-D) caused considerably lower increases in ethylene production and endogenous levels of ACC, MACC and HCN in barnyardgrass shoots. T his coincided with a 30-fold lower phytotoxic effect of 2,4-D on this plant part. Furthermore, exogenously applied KCN caused phytotoxic sym ptoms on barnyardgrass very similar to those exerted by quinclorac. We suggest a possible relationship between phytotoxic susceptibility and accumulation of endogenous toxic HCN formed as a co-product during qu inclorac-stimulated ethylene biosynthesis.