Effect of plant growth promoting rhizobacteria, salicylic acid and brassinosteroid on Physiological properties of Black Mustard in cadmium stress condition

Cadmium contamination has decreased crops quality and quantity due to its uptake by plants and inhibition of its proper growth. Plants of Brassicacea family like black mustard have a high growth capacity in heavy metals polluted conditions. Some rhizobacteria can help to the movements of heavy metals in the soil. Plant growth regulators also have an important role in the induction of plant reactions to most of abiotic stresses such as heavy metals. The aim of this experiment was the evaluation of effect of growth promoting rhizobacteria, salicylic acid and brassinosteroid on reduction of cadmium stress effects in black mustard.

In order to study the effects of growth promoting rhizobacteria and plant growth regulators on physiological properties of black mustard under cadmium stress conditions, a factorial experiment was conducted based on randomized complete block design with four replications at research greenhouse of Mohaghegh Ardabili university. Experimental treatments included two cadmium levels (0 and 100 milligrams per kilograms of soil), three levels of rhizobacteria (control, Azospirillum, Pseudomonas) and three levels of growth regulators (control, Salicylic acid, Brasinosteroid). Before planting the seeds were inoculated with rhizobacteria and also growth regulators were sprayed in two stages during growth period on plants. After harvest some traits including MDA, proline, soluble sugars, photosynthetic pigments, membrane stability index and secondary metabolites were measured.

Results of means comparisons indicated that cadmium application by some effects including oxidative stress induction, increased MDA, proline, soluble sugars, phenol and flavonoid contents of leaves and decreased chlorophyll a and b, carotenoids and anthocyanin contents, membrane stability index and plant yield. Malondialdehyde and proline contents were reduced significantly by rhizobacteria inoculation and growth regulators application and phenol and flavonoid contents as well as plant yield were enhanced in both cadmium levels by rhizobacteria. Whereas growth regulators spray increased chlorophylls a and b contents, soluble sugars, membrane stability index, phenol, flavonoid, anthocyanin and plant yield and also the rhizobacteria enhanced soluble sugars, anthocyanin content and membrane stability index significantly. In addition, there was no significant difference between salicylic acid and brassinosteroid in all traits and pseudomonas inoculation compared to azospirillum indicated a significant superiority regarding some traits including membrane stability index and plant yield.

It can be concluded that cadmium contamination via the oxidative stress induction leaded to cell membrane destruction and increasing malondialdehyde content and also it resulted decreasing chlorophyll content of plant that is associated with increasing proline synthesis under stress condition. As a result of cadmium contamination stress, the activity of carbohydrates synthesis enzymes was increased and resulted increasing soluble sugars content and consequently increasing secondary metabolites content. By the application of rhizobacteria and plant growth regulators, the effects of oxidative stress were decreased and it leaded to enhancing membrane stability and reducing malondialdehyde content. Additionally, these treatments increased chlorophyll production and decreased proline content. By increasing chlorophyll content affected by these treatments, soluble sugars production in plants was increased and it resulted increasing secondary metabolites production in plant. Considering these results, the application of growth regulators and growth promoting rhizobacteria in order to decrease cadmium stress effects on black mustard grown in contaminated soils is recommended.