Comparative Assessment of Vegetative and Reproductive Terrestrial Plant Species Endpoints from Exposure to Herbicides and Potential Environmental Implications: A Review

Abstract

To investigate whether vegetative endpoints are protective of reproductive endpoints in terrestrial plant risk assessments (RAs) for authorization of plant protection products (PPPs), we assessed differences in sensitivity to herbicides between these parameters. Published literature and unpublished proprietary data generated for the registration of PPPs were used to compile a database. If reproductive endpoints were systematically more sensitive than the vegetative endpoints on which regulatory decisions are presently based, a concern could be raised about the protectiveness of the current RA process. Vegetative and reproductive endpoints were assessed considering further potentially relevant parameters. Reproductive endpoints were compared with vegetative endpoints of juvenile plants or with those of mature plants. Direct comparison by substance–species combination proved to be most adequate and was used to calculate quotients by effect level. In addition, we assessed the spread between different effect levels, estimating by which factor the conservatism would increase if effect rate (ER)50 endpoints were replaced by ER25 or ER10 endpoints with otherwise unchanged test parameters. Reproductive endpoints were found to be similarly sensitive as vegetative endpoints derived in nontarget terrestrial plant (NTTP) studies conducted following Organisation for Economic Co‐operation and Development (OECD) or US Office of Chemical Safety and Pollution Prevention (OCSPP) guidelines. A switch from vegetative to reproductive endpoints would therefore not significantly change the conservatism (less than a factor of 1.5), whereas the change from ER50 to ER10 would (by a factor of ~5 to 6). However, because ecotoxicological tests on terrestrial plants bear intrinsic high variability that prevents the reliable detection of effects at the 10% effect level, ER10 endpoints are not a reliable basis for RA. No particular family, genus, or species with clusters of distinctly insensitive vegetative and sensitive reproductive endpoints could be identified that would call for regular testing of reproductive endpoints. Also, from the data set available, no specific herbicidal modes of action could be singled out for acting particularly on reproductive endpoints. Integr Environ Assess Manag 2020;16:166–183. © 2019 SETAC

Effects of Low Doses of Herbicides on Different Endpoints in the Life Cycle of Nontarget Terrestrial Plants

“…For plants exposed to herbicide drift at early growth stages, reproductive endpoints were 3 times more sensitive than biomass. However, a re‐analysis of published data by Christl et al (2020) concluded that reproductive endpoints were only slightly more sensitive than biomass.…”

“…However, the observed effects may depend on the herbicide, as shown by Egan et al (2014), who found that for Verbena utricifolia and V. hastata , reproductive measures of inflorescence length were more sensitive than biomass to certain herbicides (i.e., glyphosate), but not to others (i.e., atrazine). Christl et al (2020) did not detect any pattern in specific herbicide groups or plant families showing more sensitivity on reproductive compared with vegetative endpoints.…”

“…For plants exposed to herbicide drift at early growth stages, reproductive endpoints were 3 times more sensitive than biomass. However, a re-analysis of published data by Christl et al (2020) concluded that reproductive endpoints were only slightly more sensitive than biomass. Blackburn and Boutin (2003) demonstrated a negative effect on the germination rate of seeds produced by plants exposed to glyphosate depending on the degree of seed maturity at exposure.…”

“…Although plants are known to be very sensitive to herbicides at early growth stages, it has been demonstrated that reproductive endpoints are more sensitive measures of herbicide effects than vegetative vigor (=biomass) in more than 50% of the examined cases (Boutin et al 2014). However, this continues to be debated: a recent review using comprehensive data available from the literature and unpublished data concluded that reproductive and vegetative endpoints were similarly sensitive (Christl et al 2020). Following exposure at the seedling stage, effects may be observed on growth and/or on flowering and/or on seed set.…”

A Field Study Method as a Potential Higher Tier Option to Refine Herbicide Risk Assessment for Nontarget Terrestrial Plants

“…They are technically challenging to measure (experiences made by the authors during attempts to assess flowering in the pilot studies; data not shown). A recent literature review has revealed that a switch from vegetative to reproductive endpoints would increase conservatism of NTTP testing by only a factor of 1.5 (Christl, Hoen et al 2019) which makes labor‐, time‐, and cost‐intense testing of reproduction questionable.…”