Effect of Plant Growth Promoting Rhizobacteria on the Concentration and Uptake of Macro Nutrients by Corn in a Cd-contaminated Calcareous Soil under Drought Stress

Heavy metals such as cadmium (Cd) are found naturally in soils, but their amount can be changed by human activities. The study of the uptake and accumulation of heavy metals by plants is done in order to prevent their threats on human and animal’s health.Cadmium is a toxic element for living organisms. Cadmium competes with many of nutrients to be absorbed by the plant and interferes with their biological roles. Water stress affects the cell structure and the food is diverted from its normal metabolic pathway. It also reduces the availability and uptake of nutrients by the plant. One reason for the reduction of plant growth under drought stress is the accumulation of ethylene in plants. There are ways to mitigate the negative effects of drought stress that one of which is the use of Plant Growth Promoting Rhizobacteria(PGPRs) to increasing the availability of nutrients. Soil beneficial bacteria play an important role in the biological cycles and have been used to increase plant health and soil fertility over the past few decades.The aim of this study was to investigate theeffect of PGPRson the concentration and uptake of macro nutrients by corn in a Cd-contaminated calcareous soil under drought stress.
A greenhouse factorial experiment was conducted in a completely randomized design with three replications. The treatments were two levels of bacteria (with and without bacteria), four levels of Cd (5, 10, 20, and 40 mg kg-1), and three levels of drought stress (without stress, 80, and 65% of field capacity). The pots were filled with 3 kg of treated soil. Cd was treated as its sulfate salt in amounts of 5, 10, 20, and 40 mg kg-1. The soil was mixed uniformly with 150 mg N kg-1 as urea, 20 mg P kg-1 as Ca (H2PO4)2, 5 mg Fe kg-1 as Fe-EDDHA and 10, 10 and 2.5 mg Zn, Mn and Cu kg-1, respectively as their sulfate salt in order to meet plant needs for these nutrients. Six seeds of Zea mays (var. HIDO) were planted at each pot. Each seed of maize was inoculated with 2 mL (1×108 colony-forming units (cfu) mL-1) of Micrococcus yunnanensis (a gram positive bacterium with the ability of production of sidrophore and phosphate dissolving characteristic). Each pot was irrigated daily with distilled water to near field capacity by weight, until 15 days after corn planting. Then corn was thinned to 3 plants per pot and irrigation was started with different levels of drought stress (without stress (F.C), 80, and 65% of field capacity) by weight. At harvest (8 weeks after planting), the aerial parts of the plants was cut at the soil surface. The harvested plants were washed with distilled water, dried to a constant weight at 65C. Representative samples were dry-ashed and analyzed for macro nutrients.
The results indicated that the inoculation of bacteria increased shoot dry weight (DW) and total uptake of nitrogen (N), phosphorus (P), and potassium (K). Drought stress decreased DW, total uptake of N, P, and K, concentrations of N and K in corn shoots, and concentration of K in the soil. The application of biological fertilizers, such as plant growth promoting rhizobacteria, increase plant growth through increasing microorganism’s activities and population in the soil and so increase macro nutrients uptake by the plant. Phosphate solubilizing rhizobacteria increase plant growth and phosphate availability with production of organic acids and secretion of phosphatase enzymes or protons and conversion of non-soluble phosphates (either organic or inorganic phosphates) to the forms that are more available for the plants and improve their nutrition and increase their growth. Drought stress decreases Dry Matter Weight(DMW) through decreasing relative humidity of the air of plant growth environment and increases evaporation, transpiration, plant temperature and light intensity of the sun. It prevents normal development of roots, water uptake, and plant growth by reducing the moisture content of the soil. It also decreases uptake and availability of Phosphorus in arid soils because plant growth and root activity in arid soils are lower from those of wetlands and as phosphorus is immobile in the soil, its uptake by the plant will decrease. N concentration of plants will increase drought stress conditions through rapid accumulation of amino acids that had not been converted into protein. The combined effects of drought stress and inoculation of bacteria on decomposition of silicates, cause the release of nutrients such as potassium. Increasing levels of cadmium in both cases, with and without bacterial inoculation, decreased DW, N and K uptake by corn because of its toxicity and its competition and interactions with these nutrients.
The inoculation of bacteria mitigated the negative effects of drought stress and cadmium contamination by increasing dry weight of corn and increasing uptake of macronutrients by aerial parts. Drought stress in both cases (with and without bacterial inoculation) reduced shoot dry weight, total uptake of macro nutrients, N and K concentrations in corn shoots and post-harvest potassium concentration in the soil. Cadmium levels decreased shoot dry matter and N and K uptake by the plant. The use of bacteria was more effective at low cadmium and drought stress levels.