Summary• Selenium (Se) hyperaccumulation has a profound effect on plant-arthropod interactions. Here, we investigated floral Se distribution and speciation in flowers and the effects of floral Se on pollen quality and plant-pollinator interactions.• Floral Se distribution and speciation were compared in Stanleya pinnata, an Se hyperaccumulator, and Brassica juncea, a comparable nonhyperaccumulator. Pollen germination was measured from plants grown with varying concentrations of Se and floral visitation was compared between plants with high and low Se.• Stanleya pinnata preferentially allocated Se to flowers, as nontoxic methylselenocysteine (MeSeCys). Brassica juncea had higher Se concentrations in leaves than flowers, and a lower fraction of MeSeCys. For B. juncea, high floral Se concentration impaired pollen germination; in S. pinnata Se had no effect on pollen germination. Floral visitors collected from Se-rich S. pinnata contained up to 270 lg g , concentrations toxic to many herbivores. Indeed, floral visitors showed no visitation preference between high-and low-Se plants. Honey from seleniferous areas contained 0.4-1 lg Se g)1 , concentrations that could provide human health benefits.• This study is the first to shed light on the possible evolutionary cost, through decreased pollen germination in B. juncea, of Se accumulation and has implications for the management of seleniferous areas.

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Selenium (Se) is an essential micronutrient for many organisms, but is also a toxin and environmental pollutant at elevated levels. Due to its chemical similarity to sulphur, most plants readily take up and assimilate Se. Se accumulators such as Brassica juncea can accumulate Se between 0.01% and 0.1% of dry weight (DW), and Se hyperaccumulators such as Stanleya pinnata (Brassicaeae) contain between 0.1% and 1.5% DW of Se. While Se accumulation offers the plant a variety of ecological benefits, particularly protection from herbivory, its potential costs are still unexplored. This study examines the effects of plant Se levels on reproductive functions. In B. juncea, Se concentrations >0.05–0.1% caused decreases in biomass, pollen germination, individual seed and total seed weight, number of seeds produced, and seed germination. In S. pinnata there was no negative effect of increased Se concentration on pollen germination. In cross-pollination of B. juncea plants with different Se levels, both the maternal and paternal Se level affected reproduction, but the maternal Se concentration had the most pronounced effect. Interestingly, high-Se maternal plants were most efficiently pollinated by Se-treated paternal plants. These data provide novel insights into the potential reproductive costs of Se accumulation, interactive effects of Se in pollen grains and in the pistil, and the apparent evolution of physiological tolerance mechanisms in hyperaccumulators to avoid reproductive repercussions.

Elemental hyperaccumulation protects plants from many aboveground herbivores. Little is known about effects of hyperaccumulation on belowground herbivores or their ecological interactions. To examine effects of plant selenium (Se) hyperaccumulation on nematode root herbivory, we investigated spatial distribution and speciation of Se in hyperaccumulator roots using X-ray microprobe analysis, and effects of root Se concentration on root-associated nematode communities. Perennial hyperaccumulators Stanleya pinnata and Astragalus bisulcatus, collected from a natural seleniferous grassland contained 100–1500 mg Se kg−1 root dry weight (DW). Selenium was concentrated in the cortex and epidermis of hyperaccumulator roots, with lower levels in the stele. The accumulated Se consisted of organic (C-Se-C) compounds, indistinguishable from methyl-selenocysteine. The field-collected roots yielded 5–400 nematodes g−1 DW in Baermann funnel extraction, with no correlation between root Se concentration and nematode densities. Even roots containing > 1000 mg Se kg−1 DW yielded herbivorous nematodes. However, greenhouse-grown S. pinnata plants treated with Se had fewer total nematodes than those without Se. Thus, while root Se hyperaccumulation may protect plants from non-specialist herbivorous nematodes, Se-resistant nematode taxa appear to associate with hyperaccumulators in seleniferous habitats, and may utilize high-Se hyperaccumulator roots as food source. These findings give new insight into the ecological implications of plant Se (hyper)accumulation.

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