Effect of plant growth promoting bacteria (PGPB) on copper toxicity reduction in grape seedlings

Copper (Cu) is one of the heavy metals that are necessary for the growth of plants, but at high concentrations has a toxic effect. Toxicity of copper, that can occur due to the repeated use of fungicides that contain this heavy metal, in many countries is a problem of both agricultural and ecological importance. Due to necessity of multiple use of Cu-containing compounds for combating powdery mildew on perennial plantations, significant amount of Cu is accumulated in the soil and plant organs. There are few ways to reduce the toxicity of copper. One of the most promising is the use of plant growth promoting bacteria and trace elements that contribute to improving plant nutrition. For this scope, suspension of 3 strains of bacteria (Agrobacterium radiobacter, Pseudomonas putida X, Bacillus subtilis L), applied alone and together during the planting of rooted vine cuttings of cv. Victoria, and foliar application of the half of trace elements complex Microcom-VA recommended dose were used. The experiment was performed on the growing platform of IGPPP MECR RM. Vine cuttings were rooted in water, after that planted in plastic pots with 11 kg of soil. Cu was added to the lower layer of soil in pots as CuSO4 · 5 H2O during the planting of cuttings (1200mg Cu per 1kg soil). Different experimental variants were set: Control (without Cu), Cu alone and Cu together with suspensions of bacteria and Microcom-VA. Content of proline and photosynthetic pigments, growth and maturation of the shoots, biomass of roots were determinated. After three months of growth, the content of proline, that is a metabolic compound commonly involved in stress response, notably increased in leaves of plants grown under the surplus of Cu (136 % to the control). In variants with Cu and addition of bacterial suspension in soil and foliar treatment with Microcom-VA a decreasing trend of proline content in leaves was observed. At the beginning of September, when the plant growth process ceased and the shoot maturation began, the proline content in leaves decreased, but compared to control the same trend occurred. Reduction of the proline amount in the leaves in presence of bacteria may be due to bacterial activity in contrasting the oxidative stress, increasing hydration of the leaves, producing biologically active substances, and reducing the amount of a stress-related molecule as ethylene in plant tissues. Moreover, the analysis of total chlorophyll content and chlorophyll fluorescence parameters revealed a higher photosynthetic efficiency of plants grown with the addition of the suspension of bacteria and Microcom-VA with respect to control. Growth and maturation of the shoots under the surplus of Cu in soil have been slightly lower than control, while they significantly increased in all variants with addition of bacterial suspension and Microcom-VA. The weight of root biomass under the Cu surplus in soil was not reduced significantly. At the same time addition of the suspension of bacteria stimulated the root growth in all variants. It is worth highlighting that under these treatments an enhanced growth of small roots, which play a major role in plant nutrition, occurred. The presented data show the possibility of mitigating the toxic effect of excess copper in the soil on the growth and development of grapes seedlings by improving the nutrition conditions of plants supplying Agrobacterium radiobacter, Pseudomonas putida X, Bacillus subtilis L and trace elements.