The Selectivity and Activity of A-triazine Herbicides in Banana Plants

Barba, Ramon C.
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Field screening trials in Hawaii showed ametryne (2-methylmercapto-4-ethylamino-6-isopropylamino-s-triazine) to be a potentially important herbicide for bananas. Ametryne exhibited more effective weed control than atrazine (2-chloro-4ethylamino-6-isopropylamino-s-triazine). Furthermore, atrazine caused injury to bananas in 2 of 3 locations used. Investigations were conducted to explain the differential selectivity of the 2 closely related s-triazine herbicides to bananas. Sand culture experiments showed that both herbicides at concentrations of 0.4, 4, and 16 ppm in 2 liters of nutrient solution injured the plants. Plants grown in pots containing soils from the experimental fields showed increasing phytotoxicity to the herbicides in the following order: Poamoho soil, Waimanalo soil, and Kauai soil. Ametryne was less injurious than atrazine but differences were not as pronounced as that under field conditions where ametryne did not exhibit phytotoxicity. Phytotoxicity was directly related to leachability of the herbicides and negatively related to adsorption capacity of each soil for the herbicides. Organic matter content seemed to partly explain the response observed. Selectivity appeared to be largely attributed to differential position of roots of banana plants in relation to herbicide placement rather than to inherent selectivity. Apparently, atrazine being less absorbed by soil and more readily leached, moved to a greater depth upon receipt of a given amount of water resulting in crop injury. Laboratory studies confirmed results of greenhouse experiments that absorption, translocation and degradation of the herbicides by the plants did not account for the selectivity observed. Both of the herbicides were partly metabolized by the plants to their common hydroxy derivative and 2 other unidentified metabolites after 3 and 7 days of exposure to nutrient solutions containing C14-labelled ametryne and atrazine. When applied to the leaves with a surfactant the herbicides were absorbed and translocated in an acropetal direction to a very limited extent. Higher leaf content of free ammonia, free nitrate, and non-protein nitrogen, and lower chlorophyll content, were associated with plants receiving phytotoxic concentrations of ametryne and atrazine after 1 and 4 weeks from treatment. It appears that phytotoxic concentrations of these herbicides resulted in disturbance of the incorporation of nitrogen into more complex nitrogen containing plant constituents as proteins. In these experiments atrazine at 2 and 10 1b/A and ametryne at 10 1b/A applied to soil were phytotoxic to banana plants grown in pots. Ametryne at 2 1b/A did not injure the plants.
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