Growth and Physiological Responses of Greenhouse-Grown Blue Grama to Atrazine
- J. A. Morgan and
- W. G. Knight
Blue grama [Bouteloua gracilis (H.B.K.) Lag. ex Steud.] is an important native forage and the dominant C4 species on the shortgrass prairie. Sublethal concentrations of the herbicide atrazine (6-chloro-N-ethyl-N1-|l-methylethyl]-l,3,5-triazine-2,4-diamine) have increased range production, tissue N, and drought survival of blue grama. The mechanisms(s) for these responses are not well understood. This experiment was conducted to evaluate C exchange rates,water use, and productivity of blue grama to variable concentrations of atrazine. Plants were grown from seed (Lovington cv.) in soilpacked, column-lysimeters. Treatment concentrations of 0.0, 0.5,1.0, and 2.0 mg of atrazine per column were achieved by irrigating columns containing 8-wk old plants with solution atrazine and/or tap water. Nine days following the initial atrazine application, CO2 exchange rates of plants treated with intermediate and high rates of atrazine averaged 28 μmol plant−1 s−1, 43% lower than rates observed for control and low atrazine treatments. This depression, which persisted for 30 d, was due to inhibition of leaf photosynthetic capacity. As a result, aboveground productivity of the intermediate and high atrazine treatments was inhibited 28% compared to the control. Similar atrazine-induced reductions in evapotranspiration were noted. By the end of this period, chlorophyll concentrations of all atrazinetreated leaves had increased to an average 0.41 g m−2 compared to 0.33 g m−2 in control leaves. Shoot N concentrations were also affected by atrazine, averaging 23.3 g kg−1 in intermediate and high atrazine leaves and 14.9 g kg−1 in control leaves. Thirty-seven days after the initial atrazine application, all remaining plants were clipped to 5-cm height to begin a regrowth cycle. Twenty days after clipping, regrowth of low atrazine-treated plants was 22% greater than controls. During the initial (2 wk) regrowth period, both increased plant photosynthesis and increased evapotranspiration rates were observed, but only for the intermediate treatment plants. Over the final period studied (2-4 wk regrowth), no differences, compared to controls, were found for any parameters measured. No evidence for a direct stimulatory effect or hormonal-type mode of action was found for atrazine.
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