Optimum amount of quinoa leaf phosphorus under the inoculation of ‎Trichoderma harzianum‎ and Pseudomonas fluorescens in saline condition

Introduction
Phosphorus is a vital element for plant growth and development, as it participates in various metabolic processes, such as photosynthesis, respiration, energy transfer, and nucleic acid synthesis. Terrestrial plants mainly absorb phosphorus from the soil solution through their roots, mostly in the form of H₃PO₄, H₂PO₄^- and. HPO₄^2-. However, environmental factors, such as salinity, pH, temperature, and other soil properties, influence the availability and uptake of phosphorus. Salinity is a significant abiotic stress that decreases phosphorus availability and uptake. Plants have varying abilities to uptake phosphorus under saline conditions, and this can be improved by the collaboration and sometimes coexistence of microorganisms, such as fungi and bacteria. Quinoa is a halophyte, a salt-tolerant plant, which has been suggested as a potential crop for salt-affected lands, which are increasing due to climate change and human activities. Quinoa has high nutritional and economic value, as well as a broad adaptability to different environmental conditions. However, the information on the management of phosphorus fertilizer requirement for quinoa cultivation, especially in the Iran’s soil conditions, is inadequate and it is necessary to determine leaf optimal phosphorus concentration under saline conditions and the effects of Trichoderma harzianum and Pseudomonas fluorescens inoculation.
 
Materials and methods
The experiment was conducted in a greenhouse environment. Soil with low phosphorus concentration was collected and sieved. Then the soil was sprayed twice with phosphorus fertilizer solution (triple superphosphate) and incubated for four weeks at room condition to obtain nine different phosphorus concentrations. Plastic pots were filled with the soil and leached with saline water (EC = 12 dS.m^-1) until the soil salinity reached EC = 12 dS m^-1. Six pots were assigned for each phosphorus concentration. Each pot contained about 10 kg of soil and 25 quinoa (cultivar TTKK) seeds that were sown in it. After two weeks of growth, the weak seedlings were thinned to four per pot. Then the microorganisms, which were selected and prepared by the Soil Biology Research Department, Soil and Water Institute (T. harzianum and P. fluorescens), were inoculated to the pots. Irrigation was done with saline water (12 dS.m^-1); also, the pots were fertigated with potassium sulfate (0.2 g.pot^-1) and potassium nitrate (0.3 g.pot^-1) three times. The soil was analyzed for ECe and pH. The phosphorus content of mature leaves was examined six weeks before harvesting. The plant yield was also measured after harvesting.‎ Mathematical-statistical model used to compute leaf optimal phosphorus concentration ‎ for quinoa plant under T. harzianum‎ and ‎P. fluorescens inoculation‎ treatment in saline conditions. ‎Identification of significant differences was performed using one-way ‎ANOVA, which p<0.05 is considered significant in differences. ‎Microsoft Excel 2010 and SPSS Version 16 used to perform statistical ‎analyses.‎
 
Results and discussion
In this experiment, the total mean relative yield of quinoa treated with T. harzianum was not significantly different from that of pots treated with P. fluorescens. This implies that microbial treatments had little effect on the final ecological function of the soil and there was still a need for fertilizers to restock the phosphorus, which lost with uptake from the soil. T. harzianum treatment had a greater effect on the relative yield of plants in soils with lower available phosphorus concentrations. However, P. fluorescens treatment achieved the maximum production potential at lower available phosphorus concentrations, but the potential production of quinoa treated with T. harzianum was higher than that of P. fluorescens. The optimal phosphorus concentration of leaves for quinoa in this study was 0.09 mg.kg^-1 for T. harzianum treatment and 0.08 mg.kg^-1 for P. fluorescens treatment, respectively. However, the model-predicted growth potential of quinoa treated with T. harzianum (93%) was higher than that of soils treated with P. fluorescens (81%).
 
Conclusion
This research demonstrated that T. harzianum treatment was more effective than P. fluorescens treatment in saline conditions. Moreover, the quinoa (TTKK cultivar) absorbed a minimal amount of phosphorus in near-optimal conditions, might indicating that this plant required less phosphorus fertilizers in saline soils. Nevertheless, the results of this research needed to be verified and validated in full-scale/field conditions before utilizing them in decision-making and plant policy planning.
 
Acknowledgments
We hereby express our thanks and appreciation to the management, faculty members and staff of the National Salinity Research Center and the Soil and Water Institute of Iran that have helped in the implementation and improvement of this research.