Beneficial microorganisms: a sustainable horticultural solution to improve the quality of saffron in hydroponics

Saffron (Crocus sativus L.) is the most expensive spice in the world. Its organoleptic properties are mainly dictated by the apocarotenoids crocins (dyeing capacity), picrocrocin (pleasant taste), and safranal (scent). Cultivation of saffron in controlled conditions with beneficial microorganisms may increase its profitability. In this study saffron was grown in hydroponics under a greenhouse with the PGPR Bacillus megaterium CB97032 and Paenibacillus durus CB1806 (Pgpr treatment), and the AMF Rhizophagus intraradices (Myc treatment), alone or mixed (Mix treatment). The influence exerted during all crop phases on flowering trend, flower and spice yield, secondary metabolites, ecophysiological traits, and corm production was investigated, with the hypothesis that microbial synergy would have more positively affected these parameters. All the bioinoculants did not positively influence flower and spice yield, but enhanced the content of safranal (up to +96% in all the treated plants). When mixed, the PGPR and AMF improved the total phenolic content (+19%) of the saffron spice. Even if no differences emerged from the ecophysiological analysis, the corm yield was improved for the inoculated plants. The single-type bioinoculants allowed to obtain a higher number of replacement corms (up to +13% for the Myc plants) without reducing their weight, but lowering their size. When mixed, the size of the corms was restored. Together, the PGPR and AMF also increased the corm weight (+24%) of the largest corm fraction (> 1.5 cm). Thus, the bioinoculants may have stimulated the secondary metabolism of the plants by improving quality traits, rather than having acted as biofertilizers by increasing yield, at least during flowering. Overall, AMF and PGPR were proved to be a sustainable horticultural solution in hydroponics to improve the quality of saffron, especially when applied in mixed formulations.