Effect of mycorrhiza and plant growth promoting rhizobacteria on plant growth rate, flowering time and stevioside accumulation pattern in Stevia rebaudiana Bert.

Stevia rebaudiana Bert. is a medicinal plant، mostly utilized as a suitable sweeter for diabetic patients. In this research، the effect of single or co-inoculation of Arbuscular Mycorrhiza Fungi (AMF) and bacteria (Bacillus polymixa، Pesudomonas putida and Azotobacter chroococcuom) on regenerated plantlets in tissue culture was investigated in intervals of 15، 30، 45، 60 days after planting. Results showed that in comparison to control، inoculation with single and triple treatments significantly increased length، fresh and dry weight of aerial parts and stevioside content until 60 days after planting while the best enhancement was observed in dual inoculations especially in Glomus+Azotobacter treatments. Interestingly، in dual or triple inoculations، maximum relative growth rate was obtained in 15-30 days and reduced in 30-45 days after planting that was in accordance with their flowering time but in single inoculation maximum relative growth rate was available in 30-45 days and with promoting of flowering phase in 45-60 days declined severely. However، maximum stevioside accumulation rate was obtained 30-45 days after planting in all treatments and in 45-60 days after planting reduced severely in single or triple inoculations and more slightly in dual inoculations. Therefore، microorganisms affect relative growth rate، phenology and stevioside accumulation pattern in stevia.