The effect of foliar application of glycine betaine on biochemical, physiological, and agronomic traits of quinoa plant (Chenopodium quinoa Wild.) under different irrigation regimes

 Quinoa (Chenopodium quinoa Wild.) is a pseudocereal that is one of the oldest crops in the Americas and a native plant in the Andes region. Compared to other grains, quinoa has more protein and a more balanced amino acid composition with 8-5% lysine and 1.5-2.4% methionine.
Drought, heat, salinity, etc. are types of abiotic stresses that reduce plant growth and cause a sharp drop in crop yield due to various changes at the physiological, morphological, and molecular levels. In addition, drought stress may cause the production of reactive oxygen species in plants, which damage lipid and protein structures and cause the cell membrane to lose permeability and selectivity. Leakage of intracellular ions leads to disturbance in metabolism, chloroplast decomposition, and reduction of chlorophyll content.
Glycine betaine not only acts as an osmotic regulator but also stabilizes the structure and activity of enzymes and protein complexes and maintains the integrity of membranes against the damaging effects of drought. Glycine betaine treatment increases the growth, survival, and tolerance of plants to different stress conditions by regulating different metabolic processes, improving the rate of absorption of pure CO2, maintaining proteins, enzymes, and lipids of the photosynthetic apparatus, and maintaining the flow of electrons through thylakoid membranes.
This research was conducted to investigate the effect of glycine betaine foliar application on agronomic, biochemical, and physiological traits of quinoa under water stress conditions.

 The experiment of split plots based on randomized complete blocks design with three replications was performed at the station of Research in the 2020-2021 and 2021-2022 crop years, Agriculture and Natural Resources Center in Kerman province of Iran. The main factor included three levels of irrigation treatment (irrigation to the full maturity stage (control), irrigation to the beginning of the flowering stage, and irrigation to the beginning of the development stage) and the secondary factor included two levels of glycine betaine (0 and 3 mM). Biological and seed yields and harvest index, biochemical traits including proline and total phenol and flavonoid contents, and physiological traits including the activity of antioxidant enzymes such as catalase, ascorbate peroxidase, peroxidase, and polyphenol oxidase were measured. Variance analysis of all traits and LSD mean comparison test at five percent level was conducted with SAS software version 9.2.

 The effect of irrigation factor and glycine betaine and their interaction effect on the most measured traits were significant. The highest biological and seed yields were observed in control condition and the application of glycine betaine. The lowest of them were in plants grown under irrigation condition to the beginning of the flowering stage and non-application of glycine betaine. Water stress was increased the content of proline and total phenol and flavonoid and the activity of antioxidant enzymes in both level of glycine betaine. The foliar application of glycine betaine caused an increase in the biological and seed yields and biochemical and physiological traits at all three irrigation conditions.

 Under water stress conditions at both glycine betaine levels, the biological and seed yields decreased while the biochemical and physiological traits increased. These results show that the quinoa plant responds to water stress with enzymatic and non-enzymatic defense systems. The application of glycine betaine led to the improvement of biological and seed yields and biochemical and physiological traits in all three irrigation treatments. So, glycine betaine can be used to compensate for the harmful effects of water stress in quinoa.