The phytotoxic actions of two herbicides, glyphosate and amitrole, were compared using corn seedlings. Six days after treatment with amitrole at 1.68ensp;kg/ha, plant heights, leaf lengths, and shoot fresh weights were reduced 20, 20, and 25 respectively, whereas with glyphosate applied at 1.12ensp;kg/ha the inhibition was 62, 60, and 84 respectively. The lowest concentration of glyphosate (0.28ensp;kg/ha) inhibited plant growth more than the inhibition caused by the highest concentration of amitrole (3.36ensp;kg/ha). A purple colour appeared on the whole shoot of plants treated with glyphosate, and with amitrole, only the new growth of shoots was white. Both treatments reduced chlorophyll levels and the amitrole treatment reduced the carotenoid levels more than the chlorophyll levels. The root growth of plants treated with 1.12ensp;kg/ha glyphosate was inhibited more than 80, whereas with amitrole at 1.68ensp;kg/ha, the inhibition was 40. Within 6ensp;h glyphosate severely reduced the rate of root respiration and 12ensp;h after treatment respiration was reduced more than 70. This rapid effect on root respiration was not observed with amitrole, and even 48ensp;h after treatment, the decrease in respiration was less than 30. We conclude that the primary site of action of glyphosate in corn seedlings is in the roots whereas the effect of amitrole is in the shoot.
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