Interaction between boron and aluminum and their effects on phenolic metabolism of Linum usitatissimum L. roots

Aluminum toxicity is the most important limiting factor for plant growth and development in acidic soils (pH < 5.5). Inhibition of root growth has been considered as a sensitive marker of aluminum toxicity and the best indicator of boron deficiency as well. On the other hand cell wall phenolics (lignin and phenolic acids) have important roles in the reduction of extensibility of cell wall under stress conditions. Therefore, the interaction between boron and aluminum on phenolic compounds and the activity of the enzymes involved in their biosynthesis were investigated in flax seedlings. The seedlings were grown in Hoagland's solution and were treated with 3 B levels (4.5, 45 and 450 μM of H3BO3 at deficient, normal, and excess conditions, respectively) and 3 Al levels (0, 50 and 100 μM of AlCl3.6H2O). The results showed that Al treatment did not affect B content of roots (B content of Al-treated and non-treated plants were identical), while B treatment, particularly in higher concentrations, decreased Al content of roots compared with the control plants. In addition, the highest B concentration prevented the inhibitory effect of Al on the root length of plants. High concentrations of B also resulted in the decrease of enzyme activities involved in phenolic compounds (i.e., phenylalanine ammonia-lyase, polyphenol oxidase and peroxidase), decrease of lignin content and wall-bound phenols under Al stress, thereby ameliorating Al toxicity. The results suggest that the requirement of flax plants for B under Al stress conditions is higher than that required for growth in normal conditions without Al.