Effect of low doses of metamitron and glyphosate on growth and chlorophyll content of common lambsquarters (Chenopodium album)

Common lambsquarters was treated with metamitron (M) and glyphosate (G) to investigate chlorophyll content as a single, biological parameter for indicating sublethal-treated weeds. In three different greenhouse experiments the herbicides were applied at dose rates of 1/8, 1/4 and 1/2 times the M-recommended dose rate (= 0.7 kg ai ha-1), and 1/16, 1/8, and 1/4 times the G-recommended dose rate (= 1.44 kg ai ha-1), respectively. Three weeks after herbicide application the chlorophyll content in leaves of plants treated with 1/2 M was lower than the control. At the same time the chlorophyll content in leaves of plants treated with 1/8 and 1/4 M was higher than the control. The chlorophyll content in the leaves of plants treated with 1/16 and 1/8 G was also higher than the control, but the chlorophyll content in the leaves of plants treated with 1/4 G was equal to the control. A dose of 1/2 M killed 30% of available plants within 29 d, but most of the plants were still alive after 29 d when 1/4 M, 1/8 M, 1/4 G, 1/8 G and 1/16 G had been used. The fresh weight of the biomass reflected the fitness of the plants observed with time and was related to chlorophyll content. The dry weight of the biomass, however, was probably related to the typical action of metamitron and glyphosate. Measurements with a remote sensing technique supported the measurement of chlorophyll with a spectrophotometer confirming a significant discrepancy with time between a herbicide's affect on chlorophyll content and on plant growth. Because differences in chlorophyll content may have caused differences in the progress of plant degradation in sub-lethal- and lethal-treated plants, it is concluded that the chlorophyll content can be used as a single, biological parameter for indicating sub-lethal dose rates of metamitron and glyphosate.