Root Growth Inhibition and Lignification Induced by Salt Stress in Soybean

Salt stress was evaluated on root growth, enzyme activities (phenylalanine ammonia‐lyase or PAL and soluble plus cell wall‐bound peroxidase or POD), hydrogen peroxide (H₂O₂) production, total phenolic content and lignin content and composition in soybean (Glycine max L. Merrill) roots. Three‐day‐old seedlings were cultivated in half‐strength Hoagland’s solution (pH 6.0), with or without addition of 50–200 mm of NaCl, into a growth chamber (25 °C, 12/12 h light/dark photoperiod, irradiance of 280 μmol m^?2 s^?1) for 24 h. In general, root length and fresh and dry weights decreased after NaCl treatment. PAL activity decreased, soluble and cell wall‐bound POD activities increased, and H₂O₂ content significantly decreased after NaCl exposure. Consequently, total phenolic and lignin contents and p‐hydroxyphenyl (H) and syringyl (S) monomers of lignin increased in NaCl‐treated roots. Altogether, these results suggest that the effects caused by NaCl may be owing to the enhanced lignin production that solidifies the cell wall and restricts root growth.