Recovery of terrestrial plants in vegetative vigor and seedling emergence tests from exposure to atrazine

Abstract

Ten species of terrestrial plants, including 6 dicotyledonous and 4 monocotyledonous species, were exposed to a direct overspray of atrazine according to US Environmental Protection Agency seedling emergence and vegetative vigor study guidelines and subsequently evaluated for potential recovery. For each species, no-observed-effect rate (NOER), 10% effect rate, 25% effect rate, and 50% effect rate values were calculated (where possible) for a variety of guideline-required endpoints (but focusing on growth rate) for both the standard experimental phase and a recovery phase; and the rates subsequently were compared. For the seedling emergence study, the standard experimental (designated test 1) and recovery (designated test 2) phases encompassed days 0 to 14 and days 14 to 28, respectively. Similarly, for the vegetative vigor study, test 1 and test 2 encompassed days 0 to 21 and days 21 to 42, respectively. Plants were exposed to atrazine at nominal application rates ranging from 1.1 g active ingredient (a.i.)/ha (0.0010 lb a.i./A) to 28,000 g a.i./ha (25 lb a.i./A), depending on the species; the 28,000 g a.i./ha rate is greater than 12 times the maximum application rate of 2250 g a.i./ha (2 lb a.i./A) registered on corn. For seedling emergence, only 2 of 10 species tested, cabbage and tomato, provided clear rate responses in the initial 14 d of exposure (test 1). Based on a comparison of x% effect rate (ERx) and NOER values for growth rates of shoot length and shoot dry weight for days 0 to 14 relative to days 14 to 28, recovery was apparent for cabbage shoot length growth rate and tomato shoot length and shoot dry weight growth rates. Test application rates selected for the remaining 8 species showed either a weak response that did not allow a clear assessment of recovery or no response at all. For the vegetative vigor study, 9 of the 10 species tested provided clear rate responses in test 1 (days 0-21); corn did not demonstrate any herbicidal response up to the highest rate tested, 28,000 g a.i./ha. Based on comparison of day 0 to 21 (test 1) relative to day 21 to 42 (test 2) ERx and NOER values for shoot length, average growth rates indicated that 8 of 9 species clearly demonstrated an increase in 2 or more metrics (cabbage did not demonstrate a response based on shoot length). Clear recovery was also indicated by an increase in ERx and/or NOER values from test 1 to test 2 for shoot dry weight average growth rates for 7 of the 9 species (corn did not show a response, and oat and soybean showed variable responses). Thus, in most species, where initial herbicidal effects were observed, the effects are largely ameliorated over time.

Assessment of risks to listed species from the use of atrazine in the USA: a perspective

“…These effects would be mediated via photosynthetic starvation in plants and/or the production of secondary toxic substances (Shimabukuro et al 1971). Dicotyledonous plants are somewhat more sensitive than monocotyledonous plants, (Brain and Hoberg 2016;Dalton and Boutin 2010;White and Boutin 2007) but not to the extent that atrazine could be considered selective. In the laboratory and field, atrazine does not typically cause rapid mortality (e.g., the complete loss of all viability with no potential for recovery) in terrestrial or riparian plants at environmentally realistic exposures and durations (Brain et al 2019;Brain and Hoberg 2016;Dos Santos et al 2020;Wang et al 2015;White and Boutin 2007) and, as discussed above (Section 2.4), plants can recover.…”

“…Dicotyledonous plants are somewhat more sensitive than monocotyledonous plants, (Brain and Hoberg 2016;Dalton and Boutin 2010;White and Boutin 2007) but not to the extent that atrazine could be considered selective. In the laboratory and field, atrazine does not typically cause rapid mortality (e.g., the complete loss of all viability with no potential for recovery) in terrestrial or riparian plants at environmentally realistic exposures and durations (Brain et al 2019;Brain and Hoberg 2016;Dos Santos et al 2020;Wang et al 2015;White and Boutin 2007) and, as discussed above (Section 2.4), plants can recover. Also noted in Section 2.2 above, there is no evidence of bioaccumulation of atrazine in terrestrial plants beyond general uptake as would be expected for an herbicide with its physicochemical properties that result in transport in the xylem with minimal systemic transport when applied to leaves.…”

Adapting population models for application in pesticide risk assessment: A case study with Mead's milkweed

“…In guideline vegetative vigor studies (Organisation for Economic Co‐operation and Development ; US Environmental Protection Agency ), the inhibition in survival, shoot length, and shoot dry weight compared with untreated control plants is measured 3 wk after a single herbicide overhead application to the plants in early growth stages (∼2 wk after germination). To model potential effects of herbicides from a single yearly preplanting application on a hypothetical population of A. meadii growing in a habitat adjacent to a treated area, we used data available from guideline vegetative vigor studies conducted with atrazine (Brain and Hoberg ) and mesotrione (J.R. Porch, K.H. Martin, H.O.…”

Population modeling for pesticide risk assessment of threatened species—A case study of a terrestrial plant, Boltonia decurrens

“…Established plants (including seedlings) experience effects from the herbicide according to dose–response functions for survival and biomass growth. Dose–response functions were estimated using data from standard toxicity tests for atrazine . In the seedling emergence tests, nearly no effects on seedling emergence, survival, or growth were detected.…”