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Soil infertility is reported to be among the most limiting factors for crop production and yield.
Despite being abundant in most soils, only a small proportion of phosphorus is readily available to
plants due to its high reactivity with soil constituents and slow release from phosphate compounds.
Phosphate solubilising bacteria (PSB) play an important role in phosphorus nutrition. These
microbes can solubilise various insoluble phosphate compounds through different mechanisms
including production of organic and inorganic acids, production of chelating substances and
ammonium assimilation, thus enriching soluble phosphorus into soil solution for plant uptake. Use
of phosphate solubilising bacteria in agriculture has been reported to increase crop yield in different
crops including maize (Zea mays L.). Other than phosphate solubilisation, PSB can also solubilise
micronutrients including zinc (Zn) and iron (Fe). PSB are also known to produce various plant
growth promoting substances such as indole acetic acid (IAA) and siderophore, which are important
for crop growth and development. This study therefore aimed at evaluating the plant growth
promotion potential of phosphate solubilising microorganisms by, in addition to phosphate
solubilisation, looking at their potential for zinc solubilisation, siderophore and IAA production and
plant growth promotion in general. Purified colonies of bacteria from 19 native PSB isolated from
selected field and garden crops grown around Morogoro municipality, Tanzania, were found to be
strong phosphate solubilisers, hence were selected for further studies. Morphologically, these
bacteria were whitish, yellowish to creamy in colour, rod shaped and gram negative. Based on 16s
rRNA gene sequence most of the isolates were found to belong to the bacterial genus Burkholderia
while a few others belonged to the genus Ralstonia. All isolates were positive for IAA and
siderophore production and zinc solubilisation although at varying levels. On phosphate
solubilization, Burkholderia cepacia strain GPY1 isolated from rice was the most promising strain
releasing the highest phosphorus concentration (84.8 mg of soluble P L-1) compared to the lowest
amount (10.85 mg of soluble P L-1) that was released by Burkholderia territorii strain KBB5
isolated from rice. Similarly, Burkholderia cepacia strain ATCC 25416 isolated from rice was the
most promising IAA producer, producing up to 28 mg of L-1, followed by Burkholderia cepacia
strain GYP1 isolate from sweet potato which released 21 mg L-1 of IAA. On the other hand, the
lowest IAA amount (i.e. 1.072 mg L-1) was from Burkholderia territorii strain S2 isolated from rice.
Furthermore, siderophore production as measured in percentage siderophore unit (PSU) was
highest (95 %) by Burkholderia sp. QN m1 isolated from sweet pepper, followed by Burkholderia
territorii strain KBB5 (94.82%) and Burkholderia territorii strain S2 (93.98%) both isolated from
rice, while the lowest percentage siderophore unit was 28.77 % produced by Burkholderia cepacia
strain GYP1 isolated from sweet potato. The highest quantinty of zinc soulubilised was 347.5 mg
of soluble Zn L-1 by Burkholderia territorii strain KBB5 isolated from sweet potato followed by
Burkholderia cepacia strain ATCC (242.1 mg L-1) isolated from sweet potato.
Direct application of bacterial cultures to maize seedlings was observed to significantly (P = 0.05)
increase both plant height and shoot elongation as compared to a water treated control. Bacteria
strains indicated varying abilities in promoting root and shoot elongation. However, strains
belonging to B. cepacia were the most promising plant growth promoters as compared to other
strains. Overall, the finings of this study imply that bacterial isolates can be used as inoculants for
enhancing plant growth and consequently yield. However field trials need to be carried out to
evaluate the performance of the strains under field conditions. |
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