Other Herbicides and Mechanisms

Abstract

The total herbicide sodium chlorate may be reduced by the enzyme nitrate reductase to yield the highly reactive and acutely phytotoxic chlorite ion (CIO ^-₂ ):

This has been shown to be true for Chlorella [1–3], Chlamydomonas [3], and tomato [4]. The same obviously applies to other higher plants which have been used as starting material for the selection of resistant lines, as discussed below. However, CIO 3 does not act as an inducer of nitrate reductase. The alternative reduction of nitrate or clorate by nitrate reductase is a competitive process. The k_m for chlorate has been estimated as 2.3 mM for Chlamydomonas reinhardii, 1.2 mM for Chlorella vulgaris, 3.9 mM for Chlorella pyrenoidosa, and 4 mM for tomato. The reduction product chlorite leads to the rapid inactivation of nitrate reductase and possibly other enzymes in the vicinity and may thereby display its phytotoxic action.