Abstract
The physiological effects of the herbicide quinclorac (3,7-dichloro-8-quinolinecarboxylic acid) in corn and the involvement of light on the herbicide-induced chlorosis were investigated. Quinclorac was highly inhibitory to corn growth. However, different from the tested auxins, quinclorac showed no promotion in an auxinic activity assay (corn coleoptile elongation). Quinclorac induced significantly lower amounts of ethylene in corn coleoptile sections compared with IAA, NAA, and 2,4-D under darkness. Furthermore, quinclorac did not bind with a putative corn membrane-bound auxin receptor. Quinclorac caused chlorotic discoloration in the youngest leaf under light. When the quinclorac-treated corn leaf disks were continuously exposed to light, a significant decrease in total chlorophyll content was observed. No decrease in total chlorophyll content of the herbicide-treated leaf disks under dark conditions was detected. There was a significant correlation between light intensity and the reduction in total chlorophyll content of the leaf disks induced by quinclorac. Moreover, quinclorac considerably increased ethylene production in leaf disks under light conditions, whereas no enhancement of ethylene production in the dark was observed. 2,4-D slightly stimulated ethylene production under both the light and dark conditions. There was a high negative correlation between quinclorac-induced ethylene production and total chlorophyll content in quinclorac-treated leaf disks. These results suggest that the physiological actions of quinclorac applied to corn are not necessarily the same as 2,4-D and other auxins. Ethylene biosynthetic reaction may be closely related with the major herbicidal action of quinclorac, similar to that of 2,4-D. However, our data indicate that the quinclorac-enhanced ethylene biosynthetic reaction and the herbicide-induced chlorosis in corn leaves are light dependent.
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