The effect of lead on seed imbibition and germination in different plant species

A study was conducted to determine the extent to which seed coats are a barrier to lead and to what degree germination is affected by this heavy metal. The study was carried out on 25 species of plants from 12 families, represented by different varieties and populations. In all,:34 types of seeds were tested. Comparative observations were conducted on the effect of barium nitrate on seed imbibition and germination. Seeds were treated with solutions of the following salts during imbibition: 100, 1000, and 10 000 mg dm(-3) Pb2+ from Pb(NO3)(2) and 1000 mg dm(-3) Ba2+ from Ba(NO3)(2). The histochemical rhodizonate method was used to determine the distribution of lead and barium in the seeds. Water uptake and germination dynamics were also studied. It was found that 28% of the studied plant species had seed coats that were permeable to lead ions, 39% were permeable to barium ions. In both cases these species belonged to three families. Papilionaceae, Crucifereae and Graminae. Lead delayed germination and lowered the ability of seeds to germinate in a dose-dependent manner in the species with highly lead-permeable seed coats. In some other species, germination was delayed only a few hours. In all, a significant effect of lead on germination was found in over half of the studied species. It was shown that lead did not act by inhibiting water uptake during imbibition. It was also shown that seed coat permeability varied during imbibition of seeds with coats highly permeable to lead. Seed coats were impermeable to lead in the first period of imbibition when water uptake is intense. In the final stages of imbibition, when water uptake is reduced. seed coats became more permeable to lead. The lead that penetrated into the embryos in the final stage of imbibition delayed germination. This shows that seed coats are selectively permeable to lead ions. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.