Growth and yield response of upland rice (Oryza sativa L.) to actinomycete inoculation

There is an urgent need to increase rice production in the upland areas. This study explored the potential of the actinomycete, Streptomyces mutabilis, as microbial inoculants to improve the growth and yield of upland rice. At present, there is no commercially available actinomycete inoculant in the Philippines. The existing inoculants are not actinomycetes. The actinomycete in this study solubilizes, phosphate and produces both 1-aminocyclopropane-carboxylic acid (ACC) deaminase and indole 3-acetic acid (IAA). Streptomyces mutabilis increased shoot length of rice by 28% relative to the uninoculated treatment 5 days after inoculation under laboratory conditions. The highest shoot length, 68 mm plant-1, was recorded in treatment with S. mutabilis isolate over uninoculated control (53 mm plant-1). In a pot experiment, S. mutabilis plus full rate of fertilization (RRF) significantly increased grain yield by 128% relative to full fertilizer treatment alone. Streptomyces mutabilis in combination with half rate of fertilization increased grain yield by 73% relative to S. mutabilis alone. Response to inoculation in the field was observed in terms of significant increases in plant height, root depth, tiller number, biomass, and grain yield. Under field conditions, S. mutabilis + 1/2 RRF significantly increased grain yield of NSIC Rc192 and PSB Rc23 by 101% and by 35%, respectively, relative to 1/2 RRF alone but not significantly different from RRF, an indication of its potential to reduce fertilizer input by half. Highest benefit cost ratio and return on investment of 2.41 and 140.99%, respectively, was obtained with 1/2 RRF + inoculant. The enhanced growth and the significant increase in grain yield due to S. mutabilis inoculation demonstrate the potential of actinomycete as plant growth-promoting inoculant for upland rice. However, further field assessment is needed to provide more information on the performance of actinomycete inoculant under limited moisture conditions in upland rice production.