Soil effects on the plant growth inhibitory activity of S-abscisic acid

The use of natural plant growth regulators (PGRs) as ecofriendly agrochemicals is gaining much attention, but the fate of these compounds once they enter the soil environment is poorly understood. In this work, we compared the plant growth inhibitory activity of the phytohormone S-abscisic acid (S-ABA) in the presence of three soils with that observed in soilless (Petri dish) conditions and related the differences in activity to the sorption and dissipation processes of the phytohormone in the soils. In Petri dishes, S-ABA inhibited the germination of Eruca sativa, Allium porrum, Lactuca sativa, and Hordeum vulgare with mean inhibitory concentration values (IC50) in the range of 0.5-8.2 mg/L. Eruca sativa was selected for subsequent studies based on its high sensitivity to S-ABA (IC50 = 0.5 mg/L). The inhibition of germination of E. sativa by S-ABA was fully reversible at a low phytohormone concentration (5 mg/L) and partially reversible at a higher phytohormone concentration (60 mg/L). S-ABA also inhibited the growth of pre-germinated seedlings of E. sativa, albeit at higher concentrations than those at which it inhibited germination. The three soils used in the study weakened the inhibitory activity of S-ABA by soil factors in the range of 0.008-0.380. As S-ABA displayed low or even negative sorption in the soils tested, the decrease in the activity of S-ABA was attributed to its biodegradation in the soils, rather than to a decrease in its bioavailability due to sorption. Despite the reduction in the activity of S-ABA observed in the presence of the soils, the phytohormone still expressed its activity at quite low soil concentrations (0.3-20 mg/kg), showing higher activity in soils where the compound degraded more slowly.