The metabolic pathways and physiological effects of atrazine in Douglas-fir seedlings Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/xg94hr79p

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  • Herbicides are being used as a tool by forest managers to speed the restocking of sites where tree seedlings face competition from grasses for limited moisture, light and nutrients. Retarded growth or, more commonly, death of the seedlings would occur if a selective herbicide was not used to control competing vegetation on these sites. The chemical used must be selective so that it provides release from competition yet does not damage the crop species. This study investigates the physiological effects of atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) on Douglas-fir [Pseudotuga menziesii (Mirb.) Franco] seedlings and the metabolism of the herbicide by the young trees. Measurements of photosynthesis, respiration and transpiration were made over a 30 day period to elucidate the effects of various levels of the chemical on these processes. The uptake and the metabolism of the herbicide were monitored and this made it possible to correlate the physiological responses noted with the foliar concentration of the herbicide. The rates of photosynthesis were altered drastically by atrazine. Exposure to 1.16 x 10⁻⁶ molar atrazine reduced photosynthesis by 63 percent while treatment with the lowest concentration (1.45 x 1O⁻⁷ molar) resulted in rates which were more than 30 percent greater than the controls. Internal levels of atrazine ranged from 10 to 60 nmoles per gram dry weight needles. There was a close negative correlation (R = 0.873) between the photosynthetic rate and the foliar levels of the herbicide. Respiration was not markedly affected by atrazine except when the photosynthetic rates were greatly altered. This indicated that the substance was not directly regulating respiration, but that the initial drop and subsequent increase in rates were a reflection of the well-being of the plants. Both time after treatment and concentration of atrazine affected the transpirational rates of the seedlings. Transpirational rates paralleled but lagged behind those of photosynthesis when the seedlings were first exposed to the highest concentration of the herbicide. After 30 days exposure to the compound the transpirational rates of the treated seedlings exceeded that of the controls by as much as 80 percent, indicating that the low levels of herbicide were stimulating water uptake. The study of the uptake of atrazine by the tree seedlings indicated that it was a passive process, that is, the rate of herbicide uptake paralleled the rate of water uptake from the growing medium. The investigation of metabolism showed that the partial resistance to atrazine attributed to Douglas-fir is most likely a result of the plant's ability to detoxify a portion of the herbicide taken up. The rate of metabolism was such that at the end of the 30 day experiment only three percent of the herbicide taken up by the plants was in the undegraded form. The plants appear to detoxify atrazine by hydroxylation and peptide conjugation of the parent compound. A proposed mode of action for atrazine was developed from data gathered in this study. It appears that the effects of atrazine on these plants, and probably others, is concentration dependent and that the chemical is affecting two separate metabolic processes. This would account for the often opposite responses of the plant to different levels of the herbicide. At low concentration the chemical appears to act as a growth promoting substance which exhibits cytokinin-like properties. The evidence for this effect was the increased rates of photosynthesis, transpiration and metabolism observed. When internal levels of the herbicide exceeded 30 nmoles per gram dry weight needles the deleterious effects of the compound were observed. Atrazine at these elevated concentrations is a potent inhibitor of photosynthesis. Death of the plant was the ultimate effect of exposure to atrazine concentrations greater than 1.16 x 10⁻⁶ molar.
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