Introduction: Phosphate is a structural component of phospholipids, nucleic acids, nucleotides, coenzymes, and phosphoproteins. Phospholipids are important in membrane structure. Nucleic acids of genes and chromosomes carry genetic material from cell to cell. As a monoester, phosphorus provides an essential ligand in enzymatic catalysis. Phytic acid, the hexaphosphate ester of myo-inositol phosphate, is the most common phosphorus reserve in seeds. Inorganic and organic phosphates in plants also serve as buffers in the maintenance of cellular pH. Total phosphorus in plant tissue ranges from about 0.1 to 1%. Phosphorus deficiency suppresses or delays growth and maturity. Although phosphorus- deficient plants are generally stunted in appearance, they seldom exhibit the conspicuous foliar symptoms Phosphorus 55 characteristic of some of the other nutrient deficiencies. Furthermore, appreciable overlap often occurs with the symptoms of other nutrient deficiencies. Plant stems or leaves are sometimes dark green, often developing red and purple colors
Soil salinity is a major problem in arid and semi-arid regions, causing land degradation, desertification, and resulting food insecurity. Salt-affected soils and phosphorus deficiency are common problems in arid and semi-arid regions of the country. Soil salinity limits plant growth by limiting water availability, creating nutritional imbalances, and creating osmotic stress in plants. Quinoa is a plant that is tolerant to environmental stresses and a functional grain with high nutritional value. With the introduction of new quinoa varieties and the production of certified seeds since 2021, this plant has officially entered the cultivation pattern. The economic expansion of the area under cultivation of crops should be based on a correct fertilization program based on soil testing. The aim of this experiment is to determine the critical phosphorus limit for quinoa at different salinities based on soil testing and leaf analysis.

Materials and Methods: An experiment was conducted in a completely randomized design with three factors of soil phosphorus including 10 levels (S1-S10), phosphorus application at two levels (application and non-application) and two levels of irrigation water salinity (6 and 12 dS/m) with three replications in pots. In this experiment, the values calculated manually using the phosphorus fertilizer equivalent formula according to the soil test, phosphorus was added from triple superphosphate at the rates of 25.25, 50.5, 67.32, 92.57, 109.46; 134.71, 151.54, 176.79 and 218.87 mg Kg of soil with 15% moisture content and was incubated for one year due to the process of phosphorus fixation by gypsum and lime in the soil. Then, to apply phosphorus levels (0 and 9 mg phosphorus/Kg) soil was added from a triple superphosphate source. To determine the critical phosphorus level in leaves at the flowering stage, samples were taken from leaves close to the flowers. Biomass, seed weight, and harvest index were measured. The phosphorus content in shoots and seeds was also measured by the dry digestion method.

Results and Discussion: The results showed that severe salinity stress reduced grain yield and biomass by 17 and 24 percent, respectively, but the harvest index increased. Phosphorus fertilizer application increased grain yield and harvest index. Salinity stress increased phosphorus concentration in leaves and shoots and ultimately phosphorus uptake, but grain phosphorus decreased. The greatest positive effect of phosphorus was observed in soils with moderate salinity. The interaction results showed that soil P also played an important role in determining the effect of salinity and phosphorus. Soils with 8 mg/Kg and 13 mg/Kg of available phosphorus showed better performance under moderate stress conditions and phosphorus application, while soil with 16 mg/Kg of available phosphorus had the lowest values under severe salinity conditions. The critical phosphorus limit for soils with moderate and high salinity was 8.9 and 13.8 mg/Kg, respectively, and based on leaf phosphorus, 1.9 and 2.25 g/Kg.

Conclusion: In general, these results showed that optimal management of phosphorus fertilization and reduction of the effects of salinity stress improved growth and yield of quinoa. This is especially important in areas with moderate salinity for sustainable use of soil and water resources. Careful management of phosphorus nutrition under saline conditions is essential to improve phosphorus allocation to seeds and maintain crop yield and quality.