عنوان مقاله [English]
Introduction Drought stress is one of the important environmental factors that limit distribution and productivity of major crops. Drought stress caused by reducing the availability of external water, which makes reduces the ability of the plant’s roots to take up nutrients and induced cellular and photo-oxidative damages, through the increased accumulations of reactive oxygen species. Plant growth promoting rhizobacteria and arbuscular mycorrhizal fungi by using different mechanisms such as production of siderophores, organic acids, proton, growth regulators, and other chelating agents, and creative of reductive conditions, increase dissolution of minerals and mobility of non-soluble nutrients and thus improve nutrients uptake and yield of plants. They can influence plant root morphology and change the quantity and quality of root exudates. Mycorrhizal symbiosis involves a complex interaction among plant, soil and mycorrhizal fungi. Arbuscular mycorrhizal associations' relationship are rather important in crops because they are believed to increase nutrients uptake, improve plant fitness, and plant water relations and thus increase the drought resistance of host plants. Plant growth promoting rhizobacteria improve water relations of plants in part due to increases of plant growth, nutrient uptake and antioxidant activities. Maize is an effective host of arbuscular mycorrhiza in infertile and drought conditions and its root system consists of different root types. Therefore, the objectives of this study was to evaluate the effects of Glomus intraradices, Pseudomonas fluorescens (as a PGPR bacterium) and drought stress on growth characteristics and micro-nutrients uptake of maize in a calcareous soil under maize cultivation.
Materials and Methods A greenhouse experiment in a factorial completely randomized design was conducted to evaluate the effects of arbuscular mycorrhizal (AM) fungus (Glomus intraradices), Pseudomonas fluorescence, and drought stress on root colonization and absorption of micro-nutrients (Fe, Mn, Zn, Cu) by maize (Zea mays). The factors were consisted of arbuscular mycorrhizal fungus at two levels: G0 (not inoculated with fungus) and G1 ( inoculated with Glomus intraradices), bacteria at two levels: B0 (not inoculated with bacterium) and B1 (inoculated with Pseudomonas fluorescence) and drought stress at four levels: S0 (without stress), S1 (75% FC), S2 (50% FC) and S3 (25% FC). Mycorrhizal inoculum was prepared through the trap culture of forage sorghum (Sorghum biocolor L.) with spore of Glomus intraradices. The potential of inoculum (spore numbers of 12 g-1 substrates and root colonization of 80%) was measured for spore extraction and counting, and evaluation of root colonization. The bacterium used in the present experiment was Pseudomonas fluorescens and provided by soil biology and biotechnology laboratory of College University of Agriculture and Natural Resources of Tehran University, Karaj, Iran. The bacterium had a high ability to dissolve poorly soluble organic and inorganic phosphate compounds, to produce siderophores, indole acetic acid (IAA), and 1-aminocyclopropane-1-carboxylate (ACC)-deaminase enzyme. A non-sterile composite soil sample was collected from depth of 0-30 cm soil surface of Agriculture Research Station of Shiraz University, Shiraz, Iran (fine, mixed, mesic, Calcixerollic Xerochrept). The samples were air-dried and passed through a 2mm sieve. Some physical and chemical properties of studied soil are measured. The seeds were inoculated with 1mL fresh and active suspension of bacterium (population of 1×108 colony-forming units (CFU) per milliliter). After a growth period of 4 months, plant materials harvested and data were subjected to analysis of variance and means were compared by least significant difference.
Results and Discussion In non microbial treatments, wet and dry weights of shoot significantly decreased whereas other measured parameters had not significant changes under drought stress of 25% FC. At each level of drought stress, root colonization significantly higher in mycorrhizal treatments than non mycorrhizal treatments. The highest root colonization percent was observed in treatments of co-inoculation of plant with both inoculants. Co-inoculation of plant with both inoculants significantly increased morphological properties and shoot nutrients uptake except Fe uptake in comparison with non microbial treatments up to drought stress of 50% FC.
Conclusion All measured parameters ( leaf area, wet and dry weights of root, root colonization, shoot micronutrient uptake) except wet and dry weights of shoot significantly decreased with increasing of drought stress up to 25% of FC. Single and co-application of bacterium and fungus decreased the negative effects of drought stress under low levels of water stress. Root colonization significantly increased with single application of fungus and co-inoculation of plant with fungus and bacterium. Co-application of fungus and bactrieum increased shoot nutrients uptake except Fe uptake up to 50% FC in comparison with non inoculated treatments.