A comparative study of the sorption, persistence, and bioactivity of the enantiopure and racemic forms of abscisic acid in soil under different application regimes

The plant growth regulatory activity of both enantiomers of abscisic acid (ABA) provides a rationale for evaluating the potential benefits of using racemic ABA (RS-ABA) over S-ABA as a plant protection product. For soil application, where phytohormone stability is an important issue, the use of either S-ABA or RS-ABA would require a thorough evaluation of their enantioselective behavior in soil. In this work, a comparative study of the bioactivity of S- and RS-ABA under different regimes of soil application was performed, using Eruca sativa as a target test plant. Under soilless conditions (Petri-dish experiments), S-ABA inhibited the germination of Eruca sativa at lower concentrations (IC50 = 1.1 mg/L) than RS-ABA (IC50 = 1.9 mg/L). Conversely, in soil pot bioassays, RS-ABA inhibited germination at lower concentrations (IC50 = 12 mg/L) than S-ABA (IC50 = 30 mg/L). This was attributed to a more rapid inactivation of the S-enantiomer of ABA by biodegradation in soil in comparison with the R-ABA enantiomer. When applied post-emergence to soil-grown Eruca sativa seedlings, S-ABA exhibited greater activity than RS-ABA in foliar applications, while differences in activity became insignificant upon soil applications. A field experiment confirmed the germination inhibitory activity of both S-ABA and RS-ABA under real environmental conditions, with soil-applied RS-ABA showing slightly greater activity compared to S-ABA. The outcomes of this work demonstrate that the enantiomeric composition can significantly influence the effectiveness of ABA as a plant protection product, and suggest that racemic ABA may offer advantages over S-ABA in certain soil application scenarios due to a slower inactivation of the R-ABA enantiomer.