Allelopathic effects of (+)-usnic acid on some phytohormone concentrations in tomato plants

Usnic acid (UA), the main phenolic secondary metabolite in some lichens, affected the phytohormone concentrations in tomato organs. Long-term cultivation of tomato plants in Hoagland media supplemented with 30 μM (+)-UA reduced the phytohormone concentrations than control levels in leaves and roots. The reduction in indole-3-acetic acid (IAA), in leaves and roots was 61% and 66%; 62% and 36% for cytokinins (CKs), and 40% and 71% for abscisic acid (ABA) respectively. This may due to blocking or changing the direction of metabolic transformation of precursor(s) and/or increased activity of enzyme(s) engaged in catabolic pathways. UA also reduced the transport of phytohormones between the main site of their synthesis and other organs. This is evident from the changes in the root/leaf ratios of phytohormone molecular concentrations, which for IAA decreased from 3.0 in the control to 2.6 in plants grown in media with (+)-UA and for ABA from 0.35 to 0.16. The root/leaf ratio of cytokinins molar concentrations increased from 1.5 in control to 2.5 in plants cultivated in media with additional 30 μM (+)-UA supplement.