Selenium fern application and arbuscular mycorrhizal fungi soil inoculation enhance Se content and antioxidant properties of green asparagus (Asparagus officinalis L.) spears

Asparagus can be considered as target crops for Se biofortification for producing spears with enhanced Se content and antioxidant proprieties. Arbuscular mycorrhizal fungi (AMF) inoculation may affect selenium uptake from soil and the level of antioxidant compounds in vegetable crops. However, for this species the response to selenium fertilization is scarcely known as well as the role of AMF in the biofortification process. Two-year experimental trials on Se fertilization of cultivar Grande were performed on two farms a) to assess the effect of 0 (Se-0), 25 (Se-25), 75 (Se-75) and 125 (Se-125) g ha(-1) of Se as selenate sprayed onto ferns on crop productivity, spear quality, and Se enrichment; b) to consider the potential synergic effect between Se fern fertilization and AMF inoculation (+AMF and -AMF). The cvs. Atlas and 'Italo' were also tested against 'Grande' to measure their genotype-specific responses using a medium rate of Se application (Se-75). Neither fern injury nor changes in spear yield, quality (commercial and organoleptic) or essential/non-essential element contents were observed in Se-treated asparagus plants, whereas an increase in Ca2+ level was detected. Averaged over all the trials, Se content in Se-0 spears was close to 150 mu g kg(-1) DW. It always rose linearly with increasing Se rate, to a much greater extent when Se application was associated with the AMF inoculation, reaching with the highest Se rate values of 1893 and 844 mu g kg(-1) DW with and without AMF inoculation, respectively. However, AMF inoculation was not effective in enhancing the uptake of Se from soil. Variability in Se spear fortification was observed between farms, probably depending on the crop management affecting the growth of the green biomass. The highest Se dose (Se-125) also resulted in an improved content of sucrose in spears and in crowns one year after the Se application. In all trials, total phenols (TP) and carotenoid content, as well as antioxidant capacity (AC) in spears linearly enhanced with the rising of Se dose. A significant correlation between TP and AC in spears of not AMF-inoculated crop was highlighted, while AC in +AMF spears seems to be affected by other antioxidant compounds besides TP. Among the cultivars tested, 'Atlas' seems more prone than 'Grande' and 'Italo' to Se enrichment and, irrespectively to the Se treatment, it had the highest antioxidant capacity. The annual application of Se on asparagus fern is a promising technique for obtaining stable Se enrichment of the asparagus spears over both years and it can be usefully associated with AMF inoculation.