Rooting and growth of pre-germinated sugarcane seedlings inoculated with diazotrophic bacteria

Abstract

Renovation of Brazilian sugarcane plantations using the pre-sprouted seedling technique (PSS) was undertaken to increase sanitation and productivity while reducing the cost of sugarcane planting. The objective of this study was to evaluate growth, nutrient acquisition, and contribution of biological nitrogen fixation (BNF) in two sugarcane cultivars RB867515 and IACSP95-5000, inoculated with five strains/species of diazotrophic bacteria, applied together and individually, for 60 days during growth. The cultivars were grown in a greenhouse in boxes filled with a sterile substrate followed by growth in tubes filled with a commercial substrate in the second phase; finally the plants were transferred to pots containing soil enriched with 15N and grown outdoors. The treatments used were control (no inoculation); mixed inoculation with the five strains, Gluconacetobacter diazotrophicus (PAL5T), Herbaspirillum rubrisubalbicans (HCC103), Herbaspirillum seropedicae (HRC54), Nitrospirillum amazonense (CBAmC) and Paraburkholderia tropica (PPe4T); and individual inoculation with each strain. Increases of up to 50% in the root dry mass was observed following mixed inoculation of the seedlings in the germination phase, with a significant increase in root initiation, volume, and area, especially of the fine roots. Inoculation of seedlings of both cultivars led to increased biomass in tubes of non-sterile substrate. In soil, a higher nitrogen accumulation was also observed in the cvar. IACSP95-5000 in all treatments except plants inoculated with N. amazonense. For the cvar. RB867515 shoots, the opposite was observed were the highest N accumulation was observed for N. amazonense, H. seropedicae followed by the mixture. In this experiment using soil labelled with 15N there was a response on N uptake caused by all bacteria used in one cvar but only by 2 of the 5 strains in other cvar. Using the 15N enrichment of the plants the results showed that the control and inoculated plants all derived 60–70% of N from the atmosphere, and inoculation had no effect on BNF contribution. For P and K levels, RB867515 showed no effect of inoculation, but the other cvar. did respond to inoculation.