The biochar-based nanocomposites improve seedling emergence and growth of dill by changing phytohormones and sugar signaling under salinity

Biochar-based nanocomposites (BNCs) with a high level of sodium sorption capacity may improve salinity tolerance and seedling establishment of dill. Thus, a pot experiment was conducted to evaluate the effects of solid biochar (30 g solid biochar kg−1 soil) and biochar-based nanocomposites of iron (BNC-FeO) and zinc (BNC-ZnO) in individual (30 g BNC kg−1 soil) and a combined form (15 g BNC-FeO + 15 g BNC-ZnO kg−1 soil) on dill seedling growth in different levels of salt stress (non-saline, 6 and 12 dSm−1). Salinity caused a decrease in emergence percentage and emergence rate of seedlings. Increasing salinity of soil up to 12 dSm−1 decreased the biomass of dill seedlings by about 77%. Application of biochar and particularly BNCs increased the content of potassium, calcium, magnesium, iron, and zinc, reducing and non-reducing sugars, total sugars, invertase and sucrose synthase activities, leaf water content, gibberellic acid, and indole-3-acetic acid in dill plants, leading to an improvement in seedling growth (shoot length, root length, and dry weight) under saline conditions. Sodium content was noticeably decreased by BNC treatments (9–21%), which reduced mean emergence rate and stress phytohormones such as abscisic acid (31–43%), jasmonic acid (21–42%), and salicylic acid (16–23%). Therefore, BNCs especially in combined form can potentially improve emergence and growth of dill seedlings under salt stress, through reducing sodium content and endogenous stress hormones, and enhancing sugars and growth promoting hormones.