Reduction of Nitrate and Nitrite in Lambsquarters (Chenopodium album) Biotypes Resistant and Susceptible to Atrazine Toxicity

Abstract

The nitrite-reducing activity of the normal susceptible biotype of lambs-quarters (C. album L.) was strongly inhibited by atrazine in the assay medium, both in the case of the in vivo assays of leaf discs in light, and in vitro photoreduction assays of crude extracts. In vitro assays of crude extracts with methylviologen or ferredoxin supplying the reducing potential were not inhibited by atrazine. In the resistant biotype, inhibition of nitrite reduction did not occur with any of the above assays. Atrazine apparently does not inhibit nitrite reductase itself, but rather the availability of photosynthetically supplied electrons for the reduction. Atrazine had no effect when added to the media for either in vivo or in vitro assays of nitrate reduction by either the susceptible or resistant biotype. Young lambsquarters plants were treated with atrazine by spraying the leaves at a rate which was lethal for susceptible plants after 5 or 6 days, but had little effect on the resistant biotype. Nitrite did not accumulate in either biotype, but remained present at the level of about 0.1 .mu.g nitrite N/g fresh wt. The nitrate content of susceptible-type leaves increased to 2 or 3 times the initial level during the first 4 days after spraying. Usually, the only visible effect on the plants during this time was a decreased growth rate. Twenty-four hours after spraying, the following activities had fallen to 25% or less of the activities of solvent-sprayed content plants: in vivo nitrite reductase, in vivo nitrate reducfase, in vitro NADH-nitrate reductase, in vitro reduced FMN-nitrate reductase, and in vitro NADH-diaphorase. In these atrazine-treated plants, in vitro nitrite reductase activity with reducing potential supplied by methylviologen was not affected, nor were any of the above activities in leaves of atrazine-treated resistant plants. The abrupt fall in nitrate reductase represents an effect of atrazine not directly related to inhibition of photosynthesis.