Survival of endophytic bacteria in polymer-based inoculants and efficiency of their application to sugarcane

Studies have been conducted to evaluate maintenance of cell viability and stability, as well as to select cheap carriers to extend the shelf life of plant beneficial bacterial inoculants for agricultural crops. The purpose of this study was to evaluate the shelf life and the colonization efficiency of novel liquid and gel-based inoculant formulations for sugarcane. The different inoculant formulations were all composed of a mixture of five strains of diazotrophic bacteria (Gluconacetobacter diazotrophicus, Herbaspirillum seropedicae, H. rubrisubalbicans, Azospirillum amazonense and Burkholderia tropica), which are recognized as sugarcane growth promoters. Different inoculant formulations containing as carrier the polymers carboxymethylcellulose (CMC) and corn starch (60/40 ratio) at five different concentrations (named PIC, for Polymeric Inoculant Carrier) were supplemented, or not, with 2 % MgO, an interfacial stabilizing agent. Bacterial survival in the different formulations during storage was evaluated under controlled conditions, and two experiments with mini-cuttings of sugarcane variety RB72454 were carried out under greenhouse conditions. Laboratory tests showed that in the formulation composed of 0.8 g of the polymeric mixture per 100 g of the final product (PIC 0.8), survival of G. diazotrophicus and A. amazonense was around 10(9) CFUa (TM) mL(-1) after 120 days of storage, regardless of the supplementation with MgO. The other formulation (2.2 g of polymeric mixture, PIC 2.2) presented survival levels of 10(8) CFUa (TM) mL(-1) for up to 60 days of storage for all the individual strains. In the greenhouse, sugarcane seedlings showed a positive growth response 50 days after inoculation when inoculated with the mixture of five bacteria, with and without PIC 2.2. The polymer carriers described here allowed for the long-term survival of the five different bacterial strains tested. In addition, short-term experiments in the greenhouse showed that their application as part of an inoculant on sugarcane cuttings was at least as effective in terms of bacterial colonization and the promotion of plant growth as that of the bacterial mixture without carriers.