Bioavailability and bioactivity of the phytohormone S-ABA in soils

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Phytohormones are receiving increasing attention in modern farming as environmentally friendly substances to enhance crop yield and quality in the face of harsh environmental conditions [1,2]. Little attention has been given, however, to what happens to these substances once they penetrate the underground soil. S-abscisic acid (S-ABA) is a plant growth inhibitory phytohormone currently authorized for several uses in agriculture at a maximum application rate of 2 kg/ha, but there is very little information about the sorption and dissipation processes it may undergo in different soil types and on how its activity in soils may differ from that observed under soilless conditions. In this work, we characterized the sorption and dissipation of S-ABA in two soils, one alkaline and one acid, and evaluated how the soils altered the germination inhibitory activity exhibited by S-ABA under Petri dish conditions using Eruca sativa as a sensitive test plant. S-ABA displayed negative sorption in the alkaline soil and low sorption in the acid soil, with its degradation occurring faster in the alkaline than in the acid soil. In the presence of the soils, the inhibitory concentrations (IC50) of S-ABA for the germination of Eruca sativa became 25 to 125 times greater than those measured under soilless (Petri dish) conditions, where the IC50 value was recorded as 0.5 mg/l. The rapid biodegradation of S-ABA in the alkaline soil led to a greater reduction in bioactivity compared to the acid soil. Despite the reduction in bioactivity observed in the presence of the soils, S-ABA expressed its germination inhibitory activity at quite low soil concentrations compared to previously assayed plant growth inhibitory compounds. This suggests that that its activity under certain soil scenarios may be significant. [1] El-Sabagh A., Mbarki S., Hossain A., Iqbal M.A., Islam M.S., Raza A. et al. 2021. Potential role of plant growth regulators in admininstering crucial processes against abiotic stresses. Frontiers in Agronomy, 3, 648694. [2] Zahid G., Iftikhar S., Shimira F., Ahmad H.M., Kaçar Y.A. 2023. An overview and recent progress of plant growth regulators (PGRs) in the mitigation of abiotic stresses in fruits: a review. Scientia Horticulturae, 309, 111621.

Funded Project PID2020-112563RB-I00 and pre-doctoral contract PRE2021-100664 financed by the Spanish Ministry of Science and Innovation, with EU FSE+ funds