Glyphosate in the rhizosphere-role of waiting times and different glyphosate binding forms in soils for phytotoxicity to non-target plants

Glyphosate is the most widely used non-selective, systemic herbicide. It is easily translocated from shoot to roots and released into the rhizosphere, where it is immobilized at the soil matrix or microbially degraded. However, contradictory results are reported in the literature concerning the bio-availability of glyphosate residues in soils and the potential risks for intoxication of non-target organisms. This study addresses the question whether plant residues of glyphosate-treated weeds (model plant perennial rye grass. Lolium perenne L.) or direct soil application of glyphosate bears an intoxication risk for subsequently cultivated sunflower (Helianthus annuus L.) seedlings. The experiments were conducted as greenhouse studies on two soils with contrasting properties (acidic, sandy Arenosol, calcareous loess subsoil). Also the potential role of different waiting times between glyphosate application and sunflower cultivation was considered. On both soils, sunflower seedling growth and biomass production was strongly impaired by glyphosate pre-sowing treatments in the variants with 0d waiting time and recovered within a waiting time of 7-21 d. Generally, the detrimental effects were more pronounced after glyphosate weed application (90% biomass reduction) compared with direct soil application (55-70% biomass reduction) at waiting time 0 d. The inhibitory effects on seedling growth were associated with a corresponding increase in shikimate accumulation in the root tissue as physiological indicator for glyphosate toxicity. Glyphosate intoxication of sunflower seedlings was also associated with an impairment of the manganese-nutritional status, which was still detectable after a waiting time of up to 21 cl, particularly on the Arenosol in the variants with glyphosate weed application. These findings indicate an important and yet uninvestigated role of glyphosate in plant residues in determining the risk of non-target plant intoxication