Seed priming with pyroligneous acid mitigates aluminum stress, and promotes tomato seed germination and seedling growth

Aluminum (Al) toxicity in acidic soils is a major constraint for seed germination and crop growth. Pyroligneous acid (PA) is rich in bioactive compounds that can enhance crop growth and tolerance to environmental stresses including toxic trace elements, but under studied. In this study, we investigated the effect of tomato (Solanum lycopersicum L. 'Scotia') seed priming with different rates of PA (i.e., 0, 0.5:100, 1:100, 1:300, 1:600, 1:900, 1:1200, and 2:100 PA/ddH2O (v/v)) on germination and seedling growth under different growing medium Al concentrations (i.e., 0, 0.5, and 1.25 mM aluminum chloride termed Al). The results showed that priming tomato seed with 2:100 PA for 24 hr significantly (p < 0.01) enhanced seed germination indices and seedling growth. PA significantly (p < 0.05) improved seed germination index and seedling vigor irrespective of the imposed Al stress compared to the control, but not seed germination percentage. Priming with PA also increased the total lengths and surface areas of seedling hypocotyls and roots, root volume, and seedling fresh weight. In most cases, seedling growth of both the control and the PA primed groups were not affected by the 0.5 mM Al. Additionally, hydrogen peroxide and malonaldehyde contents of seedlings were reduced while proline and soluble protein contents were significantly (p < 0.001) increased in PA primed seedlings compared to the control. Furthermore, PA-primed seedlings exhibited enhanced peroxidase (POD) activities, and relatively high expression of auxin response factor and antioxidant genes (i.e., glutathione reductase, POD, superoxide dismutase, catalase, and ascorbate peroxidase 1). These findings suggest that seed priming with PA can mitigates Al stress, and improve tomato seed germination and seedling growth via improving antioxidant defense system against Al-induced oxidative stress. Future studies will be required to investigate molecular mechanisms.