Submitted to: Chemosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 3, 2010
Publication Date: March 24, 2011
Citation: Ding, W., Reddy, K.N., Zablotowicz, R.M., Bellaloui, N., Bruns, H.A. 2011. Physiological responses of glyphosate-resistant and glyphosate-sensitive soybean to aminomethylphosphonic acid, a metabolite of glyphosate. Chemosphere. 83:593-598. Interpretive Summary: Aminomethylphosphonic acid (AMPA) is formed in glyphosate-treated soybean plants and is known to cause ‘yellow flashes’, but its mode of action is different from that of glyphosate. Greenhouse studies investigated physiological effects of AMPA on glyphosate-resistant (GR) and glyphosate-sensitive (GS) soybean. AMPA was applied to soybean at 0.1 and 1.0 kg/ha, representing a scenario of 10 and 100% degradation of glyphosate (1.0 kg ae/ha use rate) to AMPA, respectively. Visual plant injury was observed on young leaves within 3 days after treatment (DAT) and plants recovered by 28 DAT in both soybean types. AMPA at 1.0 kg/ha decreased significantly chlorophyll content, photosynthesis, and root respiration in both soybean types. AMPA had no effect on nodulation and nitrate reductase activity at 10 DAT, and plant height and shoot dry weight at 28 DAT in GR and GS soybean. The fact that AMPA decreases chlorophyll content prior to reducing photosynthesis rate suggests that AMPA might interfere in chlorophyll biosynthesis in soybean. These data suggest that exposure to relatively high rates of AMPA may impair chlorophyll synthesis and photosynthesis. However, the probability of soybean exposure to greater than 10% of use rate is unlikely. From an environmental toxicity perspective, it is unlikely that inadvertent exposure of soybean to AMPA from either soil carryover or through glyphosate conversion in plant will have a significant effect on soybean.
Technical Abstract: Aminomethylphosphonic acid (AMPA) is formed in glyphosate-treated glyphosate-resistant (GR) and glyphosate-sensitive (GS) soybean [Glycine max (L.) Merr.] plants and is known to cause yellowing in soybean. Although, AMPA is less phytotoxic than glyphosate, its mode of action is different from that of glyphosate and is still unknown. Greenhouse studies were conducted at Stoneville, MS to determine the effects of AMPA on plant growth, chlorophyll content, photosynthesis, nodulation, nitrogenase activity, nitrate reductase activity, and shoot nitrogen content in GR and GS soybean. AMPA was applied to one- to two-trifoliolate leaf stage soybean at 0.1 and 1.0 kg ha-1, representing a scenario of 10 and 100% degradation of glyphosate (1.0 kg ae ha-1 use rate) to AMPA, respectively. Overall, AMPA effects were more pronounced at 1.0 kg ha-1 than at 0.1 kg ha-1 rate. Visual plant injury (18-27%) was observed on young leaves within 3 d after treatment (DAT) with AMPA at the higher rate regardless soybean type. AMPA injury peaked to 46-49% at 14 DAT and decreased to 17-18% by 28 DAT in both soybean types. AMPA reduced chlorophyll content by 37, 48, 66, and 23% in GR soybean, and 17, 48, 57, and 22% in GS soybean at 3, 7, 14, and 28 DAT, respectively. AMPA reduced photosynthesis rate by 65, 85, and 77% in GR soybean and 59, 88, and 69% in GS soybean at 3, 7, and 14 DAT, respectively, compared to non-treated plants. Similarly, AMPA reduced stomatal conductance to water vapor and transpiration rate at 3, 7, and 14 DAT compared to non-treated plants in both soybean types. Photosynthesis rate, stomatal conductance, and transpiration rate recovered to the levels of non-treated plants by 28 DAT. Plant height and shoot dry weight at 28 DAT; nodulation, nitrogenase activity at 10 DAT, and nitrate reductase activity at 3 and 14 DAT were unaffected by AMPA. AMPA reduced root respiration and shoot nitrogen content at 10 DAT. These results suggest that a foliar application of AMPA could indirectly reduce photosynthesis through decreased chlorophyll content in GR and GS soybean up to 14 DAT, but affected plants can recover to normal growth by 28 DAT.