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 (H(2)O(2)) 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 degrees C, 12/12 h light/dark photoperiod, irradiance of 280 mu mol 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 H(2)O(2) 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.