Effects of heavy metals on seed germination and early seedling growth of Arabidopsis thaliana

Seed is a developmental stage that is highly protective against external stresses in the plant life cycle. In this study, we analyzed toxicity of essential (Cu2+ and Zn2+) and non-essential heavy metals (Hg2+, Pb2+ and Cd2+) on seed germination and seedling growth in the model species Arabidopsis. Our results show that seedling growth is more sensitive to heavy metals (Hg2+, Pb2+, Cu2+ and Zn2+) in comparison to seed germination, while Cd2+ is the exception that inhibited both of these processes at similar concentrations. To examine if toxicity of heavy metals is altered developmentally during germination, we incubated seeds with Hg2+ or Cd2+ only for a restricted period during germination. Hg2+ displayed relatively strong toxicity at period II (12 - 24 h after imbibition), while Cd2+ was more effective to inhibit germination at period I (0 - 12 h after imbibition) rather than at period II. The observed differences are likely to be due in part to selective uptake of different ions by the intact seed, because isolated embryos (without seed coat and endosperm) are more sensitive to both Hg2+ and Cd2+ at period I. We assessed interactive toxicity between heavy metals and non-toxic cations, and found that Ca2+ was able to partially restore the inhibition of seedling growth by Pb2+ and Zn2+.