Metalliferous habitats and seed microbes affect the seed morphology and reproductive strategy of Arabidopsis halleri.

Purpose: Plant reproduction in metalliferous habitats is challenged by elevated concentrations of metal trace elements in soil. As part of their survival strategy, metal-tolerant plants have adjusted reproductive traits, including seed morphology, dormancy, and germination rate. These traits are particularly relevant, yet poorly understood, in metal hyperaccumulators that are promising candidates for phytoremediation. Methods: We assessed seed shape characteristics, dormancy, and germination rate in the hyperaccumulating model species Arabidopsis halleri. Seed morphological parameters were evaluated using seeds collected from two metalliferous and two non-metalliferous sites (~ 1000 seeds per location). We also addressed the potential influence of seed surface-associated microbes and endophytic fungi on germination success. Results: Seeds from non-metallicolous populations were on average 18% bigger than those from metal-contaminated post-mining sites, which contrasts the general expectation about reproductive parts in metallicolous plants. Irrespective of their origin, surface-sterilized seeds had up to ~ 20% higher germination rates and germinated earlier than non-sterilized seeds, hinting at a negative effect of seed-associated microbial communities. Surface sterilization also facilitated the emergence of an endophytic fungus (Aspergillus niger) that is a known seed-borne pathogen. Interestingly, A. niger actually promoted germination in surface-sterilized seeds from some locations. Conclusion: Despite species-wide metal tolerance in A. halleri, metalliferous conditions seem to differently affect reproductive traits compared to non-metalliferous environments (e.g., smaller seeds). Yet, higher germination rates in these populations hint at the potential of A. halleri to successfully colonize post-mining habitats. This process is modulated by site-specific interactions with seed microbiota.

Phytoextraction efficiency of Arabidopsis halleri is driven by the plant and not by soil metal concentration.

The hyperaccumulation trait allows some plant species to allocate remarkable amounts of trace metal elements (TME) to their foliage without suffering from toxicity. Utilizing hyperaccumulating plants to remediate TME contaminated sites could provide a sustainable alternative to industrial approaches. A major hurdle that currently hampers this approach is the complexity of the plant-soil relationship. To better anticipate the outcome of future phytoremediation efforts, we evaluated the potential for soil metal-bioavailability to predict TME accumulation in two non-metallicolous and two metallicolous populations of the Zn/Cd hyperaccumulator Arabidopsis halleri. We also examined the relationship between a population's habitat and its phytoextraction efficiency. Total Zn and Cd concentrations were quantified in soil and plant material, and bioavailable fractions in soil were quantified via Diffusive Gradients in Thin-films (DGT). We found that shoot TME accumulation varied independent from both total and bioavailable soil TME concentrations in metallicolous individuals. In fact, hyperaccumulation patterns appear more plant- and less soil-driven: one non-metallicolous population proved to be as efficient in accumulating Zn on non-polluted soil as the metallicolous populations in their highly contaminated environment. Our findings demonstrate that in-situ information on plant phytoextraction efficiency is indispensable to optimize site-specific phytoremediation measures. If successful, hyperaccumulating plant biomass may provide valuable source material for application in the emerging field of green chemistry.

Endosperm prevents toxic amounts of Zn from accumulating in the seed embryo - an adaptation to metalliferous sites in metal-tolerant Biscutella laevigata.

Seed germination represents the first crucial stage in the life cycle of a plant, and the seed must contain all necessary transition elements for the development and successful establishment of the seedling. Problematically, seed development and germination are often hampered by elevated metal(loid) concentrations in industrially polluted soils, making their revegetation a challenging task. Biscutella laevigata L. (Brassicaceae) is a rare perennial pseudometallophyte that can tolerate high concentrations of trace metal elements. Yet, the strategies of this and other plant species to ensure reproductive success at metalliferous sites are poorly understood. Here we characterized several parameters of germination and used synchrotron X-ray fluorescence microscopy to investigate the spatial distribution and concentration of elements within B. laevigata seeds from two metallicolous and two non-metallicolous populations. We find that average germination time was shorter and the seed weight was lower in the metallicolous compared to the non-metallicolous populations. By allowing for at least two generations within one growth season, relatively fast germination at metalliferous sites accelerates microevolutionary processes and likely enhances the potential of metallicolous accessions to adapt to environmental stress. We also identified different strategies of elemental accumulation within seed tissues between populations. Particularly interesting patterns were observed for zinc, which was found in 6-fold higher concentrations in the endosperm of metallicolous compared to non-metallicolous populations. This indicates that the endosperm protects the seed embryo from accumulating toxic concentrations of metal(loid)s, which likely improves reproductive success. Hence, we conclude that elemental uptake regulation by the seed endosperm is associated with enhanced metal tolerance and adaptation to metalliferous environments in B. laevigata.

Coming Late for Dinner: Localized Digestate Depot Fertilization for Extensive Cultivation of Marginal Soil With Sida hermaphrodita.

Improving fertility of marginal soils for the sustainable production of biomass is a strategy for reducing land use conflicts between food and energy crops. Digestates can be used as fertilizer and for soil amelioration. In order to promote plant growth and reduce potential adverse effects on roots because of broadcast digestate fertilization, we propose to apply local digestate depots placed into the rhizosphere. We grew Sida hermaphrodita in large mesocosms outdoors for three growing seasons and in rhizotrons in the greenhouse for 3 months both filled with marginal substrate, including multiple sampling dates. We compared digestate broadcast application with digestate depot fertilization and a mineral fertilizer control. We show that depot fertilization promotes a deep reaching root system of S. hermaphrodita seedlings followed by the formation of a dense root cluster around the depot-fertilized zone, resulting in a fivefold increased biomass yield. Temporal adverse effects on root growth were linked to high initial concentrations of ammonium and nitrite in the rhizosphere in either fertilizer application, followed by a high biomass increase after its microbial conversion to nitrate. We conclude that digestate depot fertilization can contribute to an improved cultivation of perennial energy-crops on marginal soils.

Intraspecific variation in response to magnitude and frequency of freeze-thaw cycles in a temperate grass.

Winter warming and its accompanying predicted decrease in snow pack for northern temperate regions may increase frost damage to plants induced by an increase in freeze-thaw cycles (FTCs) due to reduced insulation. FTC frequency, minimum temperature during freezing and pre-existing local adaptations potentially all influence site-specific plant responses to future climatic changes. Within a chamber experiment, frost sensitivity towards recurrent FTCs was determined in 12 Dactylis glomerata populations from various European sampling sites differing in temperature and precipitation. After winter hardening, plants were frozen at -4 and -8 °C at frequencies of one, three and seven FTCs within a 1-week treatment phase. The control was kept at 4.5 °C. Plant survival, leaf elongation, chlorophyll content and above-ground net primary productivity (ANPP) decreased with lower minimum temperatures and higher FTC frequencies, while lower freezing temperatures generally proved more influential than increased freezing frequencies. Plant survival rates correlated with the amount of annual precipitation at seed origin, as individuals from comparably drier sites exhibited higher survival rates. This response, however, was limited in its effect to low freezing temperatures (-8 °C) and low and medium freezing frequencies (1 and 3 FTCs). In the set of surviving plants, water availability at seed origin best explained the plants' growth responses to FTC treatment. The observed intraspecific variation emphasizes the ecological importance of potential local adaptations within a more variable future winter climate.