Cadmium and nickel uptake are differentially modulated by salicylic acid in Matricaria chamomilla plants.

Chamomile (Matricaria chamomilla) is a widely used medicinal plant which also accumulates heavy metals in its above-ground organs. We investigated the effect of the important plant signaling molecule, salicylic acid (SA), on the accumulation of Ni or Cd, by exposing plants over 7 days to 60 microM solutions of individual heavy metals with or without 50 microM SA. Special emphasis was focused on phenolic metabolism-related parameters, not only because of their importance for growth and stress tolerance but also because phenolics are potent antioxidants in human diet. In combined treatments, SA stimulated an increase in soluble proteins of roots and reduced their water content. SA reduced total Cd in the shoot and increased Ni. Total and "intraroot" Ni decreased in Ni + SA treatment, while in the case of Cd, only "intraroot" content decreased in Cd + SA treatment, being correlated with cell wall-bound phenolic acids and lignin. SA was strongly accumulated in roots from the Ni + SA treatment, being correlated with an increase in hydrogen peroxide. In both Cd + SA and Ni + SA treatments, SA enhanced phenylalanine ammonia-lyase activity and accumulation of total soluble phenols, particularly in the roots. Here, we report for the first time that soluble phenols may be involved in Cd shoot-to-root translocation. In the case of Ni, it seems that phenols serve as a root barrier in order to prevent Ni from reaching the above-ground organs. The effects of SA on phenolic metabolism, and the signaling role of ROS in the accumulation of phenols, are discussed.