5. Additives

Other materials added to the inoculant formulation include macro- and micronutrients, carbon or mineral sources, hormones, and even fungicides. The aim is to supply microorganisms with protective and/or a nutrient source, to assure better adhesion to seed thus improving the inoculant quality, to make the product more stable, to inactivate the toxins, or to enhance the strain(s) survival during storage and after exposure to environmental stress conditions (high temperature, desiccation). There is a critical interrelation between the strains survival rate and used additives. Some of them (such as glycerol) improve cell viability by protecting cells from desiccation through holding considerable amounts of water. Thus the drying rate is significantly reduced. Each additive should be selected for individual strain in order to provide maximal performance. Moreover, their chemical nature should be complex to prevent them from rapid degradation. Several components have been tested, such as clay and skim milk, xanthan, or sodium alginate with variable results on strain(s) survival during storage and field application. Furthermore, certain signalling molecules added in the growth media and inoculants have been shown to provoke desired physiological activities of used microorganisms. Recently, it was reported that some rhizobial metabolites enhance the performance of Bradyrhizobium spp. and Azospirillum brasilense inoculants when soybean and maize are treated. These metabolites include mainly lipochitooligosaccharides (LCOs also called Nod factors) but also exopolysaccharides and plant hormones. Nod factors were shown to be produced by most rhizobia and are mandatory for the root legume infection and nodule formation. To our knowledge, the use of signalling molecules for improving the crop performance is still limited. However, several legume inoculants containing stimulators of nodulation (flavonoids or Nod factors) are commercially available in North and South America. Stimulators of the mycorrhizal symbiosis have also been identified. Strigolactones are of fundamental and practical interest as they are supposed to play a key role in the establishment of the mycorrhizal symbiosis. It was reported that they act as a hormones in plants, and they may also have a role in the presymbiotic growth of AMF. Application to crops could result in beneficial effects on plant development. However, more investigations are needed to assess the potential of these stimulators for the development of a new generation of mycorrhizal inoculants.