Pyroligneous acid (PA) is a reddish-brown liquid obtained through the condensation of smoke formed during biochar production. PA contains bioactive compounds that can be utilized in agriculture to improve plant productivity and quality of edible parts. In this study, we investigated the biostimulatory effect of varying concentrations of PA (i.e., 0%, 0.25%, 0.5%, 1%, and 2% PA/ddH₂O (v/v)) application on tomato (Solanum lycopersicum ‘Scotia’) plant growth and fruit quality under greenhouse conditions. Plants treated with 0.25% PA exhibited a significantly (p < 0.001) higher sub-stomatal CO₂ concentration and a comparable leaf transpiration rate and stomatal conductance. The total number of fruits was significantly (p < 0.005) increased by approximately 65.6% and 34.4% following the application of 0.5% and 0.25% PA, respectively, compared to the control. The 0.5% PA enhanced the total weight of fruits by approximately 25.5%, while the 0.25% PA increased the elemental composition of the fruits. However, the highest PA concentration of 2% significantly (p > 0.05) reduced plant growth and yield, but significantly (p < 0.001) enhanced tomato fruit juice Brix, electrical conductivity, total dissolved solids, and titratable acidity. Additionally, total phenolic and flavonoid contents were significantly (p < 0.001) increased by the 2% PA. However, the highest carotenoid content was obtained with the 0.5% and 1% PA treatments. Additionally, PA treatment of the tomato plants resulted in a significantly (p < 0.001) high total ascorbate content, but reduced fruit peroxidase activity compared to the control. These indicate that PA can potentially be used as a biostimulant for a higher yield and nutritional quality of tomato.

Hydrochar derived from hydrothermal carbonization (HTC) has been recognized as a potential absorbent and horticultural substrate. However, its practical application has been limited due to its low adsorption capacity and negative effects on plant growth. To address these issues, three pre-treatment methods (water washing, microbial aging, and freezing-thawing aging) were employed to further improve the physical structure and chemical properties of hydrochar. A seed germination test with kale (Brassica oleracea var. acephala D.C) was conducted to evaluate the phytotoxicity of modified hydrochars. The results showed that microbial aging considerably enhanced the physicochemical properties of the hydrochar. Specifically, under microbial aging, the bulk density of microbial-aged hydrochar (MHC) decreased by 8.1%, the porosity increased by 24.8%, and the water-holding capacity increased by 36.54% compared to fresh hydrochar (FHC). Moreover, the surfaces of MHC and freezing-thawing aged hydrochar (FTHC) were observed with rough and cracked surfaces and macro pore structures. Fourier transform infrared (FTIR) spectroscopy revealed that the functional group’s intensities of the four hydrochar materials varied, and that MHC and FTHC had more oxygen-containing groups than the others. Additionally, the surface areas of MHC and FTHC increased by 318.64% and 238.98% compared to FHC, respectively. The seed germination test indicated the strong inhibitory effect of FHC, while MHC significantly (p < 0.05) improved the seed germination rate and root development. These findings suggest that among the different pre-treatment methods, microbial aging demonstrated the greatest potential for practical application in improving the physicochemical properties of hydrochar and promoting seed germination. This study opens up new avenues for further research on improving hydrochar and suggests that future studies should focus on optimizing the aging process.

The cost of producing organic crops is increasing. Agricultural wastes can be used as biostimulants to increase plant growth and productivity and reduce the dependence on chemical fertilizers. A pouch assay and a potted greenhouse experiment were conducted to investigate the effect of pyroligneous acid (PA) and sea lettuce (SL) on kale (Brassica oleracea subsp. acephala (DC.) Metzg) seed germination and growth. Although previous studies have demonstrated that these two biostimulants could promote plant germination and growth, there is little research to compare their effects on seed germination and plant growth. The pouch assay showed that PA liquid affected the seed germination rate under different concentrations; the seed germination rate decreased as the concentration of PA liquid increased. However, the effect of seed germination was less pronounced in SL liquids. Kale seeds treated with 0.01% PA showed the best elongation and seedling growth performance. Moreover, the greenhouse experiment indicates that SL liquids significantly (p < 0.05) affected kale growth production, while PA liquid had less difference on kale growth under various concentrations. The 0.25% PA and 1% SL increased the aboveground fresh weight by ca. 26% and 29%, respectively. Also, the phytochemical contents of kale leaves, including phenolics, flavonoids, ascorbate, and protein, were significantly increased with 0.25% PA and 1% SL application. These results suggest that low concentrations of PA are more suitable for seedling root growth in kale and 1% SL had the most significant growth-promoting effect on kale. Hydrothermal carbonization sea lettuce liquid can be used as a good biostimulant for agricultural production to improve kale germination and growth.