INOCULATION WITH N2-FIXING PLANT GROWTH PROMOTING RHIZOBACTERIA TO REDUCE NITROGEN FERTILIZER REQUIREMENT OF LETTUCE

This study was undertaken to determine the effect of three different mixtures of some N2-fixing plant growth-promoting rhizobacteria (PGPR) on growth, yield, element content and nitrate accumulation as well as the effect on the reduction of nitrogen fertilization of lettuce (Lactuca sativa L.). The measurements were made in two separate experiments in 2015 between 6 June – 5 August (Experiment 1) and 2 July – 3 September (Experiment 2) under the field conditions. Butterhead form and heat tolerant summer cultivar ‘Luna’ was used as a plant material. Agrobacterium rubi RK-34, Pantoea agglomerans RK-79 and RK-92, Pseudomonas putida RK-142 and TV-42A, Bacillus megaterium TV-6D, TV-60D and TV-91C, Pseudomonas flourescens TV-11D and Paenibacillus polymyxa TV-12E were used as N2-fixing plant growth promoting rhizobacteria. The treatments were 150 kg N/ha (available dose of AS) as ammonium sulphate (AS) [(NH4)2SO4), (21% N)] and three different mixtures of PGPR. Further, combined uses of decreasing doses of AS (50%, 75 kg ha–1AS and 75%, 112.5 kg ha–1 AS) and PGPR mixtures (M) such as M-1 + 75 AS, M-1 + 112.5 AS, M-2 + 75 AS, M-2 + 112.5 AS, M-3 + 75 AS and M-3 + 112.5 AS were used as additional treatments. All treatments increased the yield and the growth of lettuce according to the control. While inoculation with PGPR mixtures decreased the accumulation of heavy metals such as Cd, Ni, and Pb in lettuce, increased nutrient uptake of lettuce. It was determined that the nitrate accumulation of lettuce (cv. ‘Luna’) in PGPR mixtures were lower than the available dose of AS but higher than control. The yield in M-3 + 112.5 AS (48431 kg ha–1) was similar and in the same statistical group with the available dose of AS (48225 kg ha–1) in both experiments. Furthermore, according to the results of cost analyses, using 25% less of AS (112.5 kg ha–1) with M-3 will supply the same income instead of using AS (150 kg ha–1). It can be clearly said that the mixtures with some N2-fixing plant growth promoting rhizobacteria (PGPR), especially M-3 (P. putida RK-142 + P. flourescens TV-11D + B. megaterium TV-91C), have a great potential to decrease the nitrogen use (25%) for environmentally friendly crop production of lettuce.

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