ATRAZINE DETOXIFICATION IN PANICUM-DICHOTOMIFLORUM AND TARGET SITE POLYGONUM-LAPATHIFOLIUM

A resistant (R) biotype of the dicotyledonous weed Poligonum lapathifolium found in Spain was 35.5 times more resistant to atrazine than a susceptible (S) biotype. The ED(50) values for the inhibition of shoot growth in Panicum dichotomiflorum, a species never adequately controlled by atrazine, harvested from untreated cornfields and from cornfields which had been repeatedly treated with atrazine were 3.2 and 2.3 kg a.i. ha(-1), respectively. Chlorophyll fluorescence intensity and fast fluorescence induction curves showed that atrazine inhibited whole-leaf photosynthesis in the S Polygonum biotype and in both Panicum biotypes. The Panicum biotypes later recovered fluorescence activity, at a slightly faster rate in plants from the atrazine-treated fields. Electron transport in chloroplast thylakoids isolated from leaves of R Polygonum biotypes was 781 times less sensitive to atrazine than in S biotypes, while thylakoids of both Panicum types were equally sensitive to atrazine. The R and S biotypes of Polygonum accumulated greater amounts of radio-labeled atrazine than both Panicum types after treatment via roots. Absorption and translocation were not different for any of the Polygonum or Panicum types. Both species partly metabolized atrazine via conjugation in leaf tissue within 24 hr. However, the Panicum types did so to a greater extent and more rapidly than Polygonum. The extent of conjugation was slightly but significantly higher in leaves of Panicum plants from atrazine-treated fields (91%) than in those from untreated fields (80%), and two different conjugates (glutathione- and cysteine-atrazine) were found. Thus, the atrazine-resistance mechanism in the Polygonum biotype is due to reduced affinity for atrazine at its target site, while the natural tolerance of Panicum is due to atrazine metabolism via glutathione and cysteine conjugation, which is slightly higher in plants from atrazine-treated fields. (C) 1995 Academic Press, Inc.