Abstract
The growth of legume plants is usually improved by the rhizobacteria inoculation under low phosphorus (P) and alleviation of P nutrition plays important role in plant drought stress response. The aim of this study was to assess the comparative efficacy of two plant growth promoting rhizobacteria namely Burkholderia cepacia B36 and Enterobacter radicincitans D5/23T combined with two sources of phosphates in soybean (Glycine max L.) under low water supply. Plants were grown under P soluble versus insoluble P fertilization for comparing the effects of soybean inoculation on growth, uptake and use efficiency of phosphorus under moderate drought stress. At the beginning of flowering, half of plants was subjected to low water supply (35% water holding capacity, WHC) for 12 days while control plants were well watered - 70% WHC. The plants were harvested at the end of drought and physiological traits and P contents were analyzed. The inoculation treatments showed better plant growth and nutrient uptake when compared to uninoculated control. The application of the Burkholderia cepacia was more efficiently in terms plant growth than E. radicincitans especially under insoluble phosphates. Phosphorus concentrations of shoots and roots increased with both bacterial strains. The bacterial inoculation has much better stimulatory effect on nutrient uptake by soybean fertilized with insoluble phosphates. Study findings indicate that the combined application of PGPR (Burkholderia cepacia B36) and P amendments has the potential to improve P nutrition and growth of soybean cultivated on P-deficient soil under well-watered as well as moderate drought condition.
 
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 In press - Online First. Article has been peer reviewed, accepted for publication and published online without pagination. It will receive pagination when the issue will be ready for publishing as a complete number (Volume 47, Issue 3, 2019). The article is searchable and citable by Digital Object Identifier (DOI). DOI link will become active after the article will be included in the complete issue.
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Highlights
Phosphate deficiency is a major constraint to plant productivity in many soils of the world, in highly weathered soils of the humid tropics and acidic regions (Oberson et al, 2006)
Soybean growth was evaluated by measuring root dry weight, shoot dry weight, and total plant dry weight of inoculated and non-inoculated plants under two P sources and two soil moisture levels at flowering (6 weeks after sowing) stage
The soybean seed inoculation with Plant growth promoting rhizobacteria (PGPR) isolates increased dry weights of shoots of plants grown under low soil water regime, irrespective of P sources compared to uninoculated reference plants
Summary
Phosphate deficiency is a major constraint to plant productivity in many soils of the world, in highly weathered soils of the humid tropics and acidic regions (Oberson et al, 2006). Large amounts of chemical fertilizers are necessary to provide normal growth and yield of crops. A large portion of soluble inorganic phosphate applied to soil as chemical fertilizer is immobilized rapidly after application due to phosphate fixation by aluminum, calcium, iron, magnesium and soil colloids (Rodriguez and Fraga, 1999) and becomes unavailable to plants (Merbach et al, 2010). Phosphorus chemical fertilizer is an expensive input and its use efficiency by crops may range from 10-25% (Bahl and Singh, 1986). The assimilation of fixed soil P as well as hardly soluble phosphates using microorganisms is an option to augment the availability of P available form by the crops (Poonamgautam and Agnihotri, 2003). The interactions between PGPR and phosphorus amendments could be a strategy in improving growth and nutrition of crops in different agroecosystems, in P-deficient soils and water limited condition.
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