Effects of Inoculation of Rhizomicrobial Strains on Plant Growth at the Early Germination Stage

Abstract

Abstract Plant-growth-promoting rhizobacteria can affect plantgrowth by various direct and indirect mechanisms. This study wasconducted to determine the ability of some rhizobacterial strains toenhance the seed germination of Lactuca sativa (lettuce) andRaphanus sativus (radish). Seeds were inoculated using a sporesuspension (1×10 7 cfu mL −1 ) and incubated in a growth chamberat 28 o C under dark conditions and 65% RH. Azotobacterchroococcum and LAP mix inoculation increased the plumulelength of L. sativa by 1.3, 0.8, and 0.7 cm, respectively, incomparison to the uninoculated control. Pseudomonas putidashowed an increase of only 0.6 cm in plumule length whencompared to the control. Inoculation of A. chroococcum, P. putida,and LAP mix enhanced the seed germination rate of R. sativus, by10, 5, and 30%, respectively, in comparison with the uninoculatedseeds. The results demonstrated that the inoculation of seeds byselect rhizobacterial strains showed remarkable enhancement tothe radicle length of lettuce and radish seedlings.Keywords PGPR · Plumule length · Radicle length ·RhizobacteriaIntroductionThe application of massive amounts of chemical fertilizers andfungicides has obstructed the development of sustainableagriculture. Free-living soil bacteria that are beneficial to plantgrowth, usually referred to as plant-growth-promoting rhizobacteria(PGPR), are capable of promoting plant growth by colonizing theroots. Wu et al. (2005) were among many researchers who haveclearly demonstrated the effects of PGPR strains on differentcrops. An improvement in seed germination in a variety of plantsby treatment with rhizobacteria has been reported. These plantsinclude sorghum (Raju et al., 1999), pearl millet (Niranjan-Raj etal., 2004), as well as wheat and sunflower (Shaukat et al., 2006).Rhizobacteria that possess some direct mechanism or capacityto promote plant growth are promising alternatives to chemicalusage and helpful in maintaining a cleaner ecosystem. Most of theinterest has been focused on non-symbiotic rhizobacterial strains,particularly Pseudomonas and Bacillus species. However, free-living bacteria that have unique growth-enhancing effects on hostplants need to be studied further (Ping and Boland, 2004; Bais etal., 2006). Bacterial inoculants enhance plant growth, germination,and seedling emergence (Lugtenberg et al., 2002).Increasing the synthesis of plant hormones such as indole aceticacid (IAA) and gibberellins is an important factor that triggers theactivity of specific enzymes that promote seed germination andgrowth (Bharathi et al., 2004). The free-living, heterotropicAzotobacter helps in nitrogen fixation and thus enhances theseedling’s progress. Genetic improvement of PGPR strains, inorder to enhance their colonization and efficacy, may result in oneor more factors related to plant growth promotion (Glick 1995;Lubeck et al., 2000; Bloemberg and Lugtenberg, 2001).Biofertilizers are composed of effective microorganisms thatcolonize the rhizosphere and promote growth by increasing theavailability of primary nutrients to the host plant when applied toseed, soil, or even plant surface (Vessey, 2003). Furthermore, theuse of co-inoculation of PGPR, with a greater understanding oftheir functions, is of interest as these formulations may increaseuniformity in the field (Jetiyanon and Kloepper, 2002; Siddiquiand Shauka, 2002). Rhizosphere microorganisms tend to producelarge quantities of growth-promoting substances that affect, eitherdirectly or indirectly, the plant morphology and yield. Growthregulators control growth by influencing the physiological andmorphological processes at trace concentrations (Arshad andFrankenberger, 1998).