Effects of Combined Abiotic Stresses on Growth, Trace Element Accumulation, and Phytohormone Regulation in Two Halophytic Species

Trace element contamination of lands is a serious environmental problem that limits yield and threatens human health. To study the combined effect of high salinity and toxic levels of trace elements on halophytes, the performance of two marsh species, Atriplex halimus and Suaeda fruticosa, grown for 1 month with an irrigation solution supplemented with 200 mM NaCl and 400 mu M Cd2+ or 400 mu M Cu2+ was evaluated. The effect of the combined stress conditions on hormone signaling was also assessed. Biomass production and chlorophyll content decreased under Cd2+ stress in both species, whereas Cu2+ had a lower impact on plant performance. The different plant sensibilities to the two trace elements assayed indicate that each metal has a different effect on plants. Furthermore, the deleterious effect of metal toxicity was alleviated when NaCl was added to the irrigation solution, demonstrating that NaCl improves plant performance and tolerance of halophytic species to cope with trace element intoxication. Results show that both species accumulated important quantities of Cd2+ and Cu2+ in roots (Cd2+: 2,690-3,130 mu g g(-1) DW and Cu2+: 2,070-2,770 mu g g(-1) DW); this finding allows us to classify these species among the hyperaccumulator plants. Cd2+ and Cu2+ differently affected endogenous phytohormone contents in both species. Data suggest an essential involvement of roots on the regulation of tolerance to trace elements. Therefore, indole-3-acetic acid levels increased in roots of both species irrigated with high levels of Cd2+, which suggests that the auxin may stimulate root promotion and growth under these stress conditions. Other compounds, classically considered as "stress hormones" showed very different patterns of accumulation. Whereas, salicylic acid (SA) levels in roots and leaves increased in response to Cd2+, root contents of jasmonic acid (JA), and abscisic acid (ABA) decreased. In leaves, the rambling pattern of accumulation observed for JA and ABA suggested the lack of a specific role in regulation against trace element toxicity. Together, data suggest that SA could act as a specific signal that detects trace element toxicity, whereas JA and ABA promote general responses against abiotic stress.