Soil selenium (Se) application for Se biofortification and for protecting Phaseolus vulgaris L. and Chenopodium quinoa Willd. against water deficit

Abstract

Selenium-deficient food negatively affects human health, as Se is an essential element for human biological processes. Crop biofortification has made it possible to enrich cultivated plants with selenium in soils containing low Se levels to ensure a proper Se intake in the human diet. Selenium is a beneficial element for plants but its essential metabolic role in plants has not yet been determined. The most prominent action of Se in plants is its role in the antioxidant protection mechanism, which promotes the activity of enzymatic and non-enzymatic systems to cope with abiotic stresses, such as water deficit. In this context, three greenhouse studies were performed to determine the effect of soil Se application on the enrichment of bean and quinoa grains with selenium and to evaluate the effect of applying Se on metabolic and physiological processes that are associated with tolerance to water deficit. By improving the nutritional quality of grains, while also increasing plant tolerance to water deficits we could contribute to minimize problems related to hidden hunger and water shortage, which are latent threats to food security. For the present study, sodium selenate was used as the source of Se and we evaluated physiological and metabolic parameters in plants subjected to two soil water conditions: irrigated and water deficit. The first study was installed in Brazil with Phaseolus vulgaris L. cultivar BRS Estilo, and five Se rates (0, 0.25, 0.5, 1.0, and 2.0 mg kg–1) were applied to an Oxisol (pH = 4.6; SOM = 32.7 g kg-1; clay = 610 g kg-1). Agronomic selenium biofortification enriched common bean plants with Se, and antioxidant enzyme activity showed positive responses to plant protection against water deficit when used at lower Se rates, mainly at 0.25 and 0.5 mg kg-1. The second study was conducted in Peru with Chenopodium quinoa W. var INIA Salcedo, and five Se rates (0, 0.25, 0.5, 1.0, and 2.0 mg kg–1) were applied to an Entisol (pH = 8.09; SOM = 11.5 g kg-1; clay = 320 g kg-1). Selenium biofortification enriched quinoa grains with selenium; meanwhile, antioxidant non-enzymatic activity might be one of the water deficit tolerance mechanisms promoted by Se on quinoa var INIA Salcedo and would be enhanced mainly at 0.5 mg kg-1 Se. The last study was carried out with Chenopodium quinoa W. cultivar BRS-Piabiru, and seven Se rates (0, 0.125, 0.25, 0.5, 0.75, 1.0, and 2.0 mg kg–1) were applied to an Oxisol (pH = 4.6; SOM = 32.7 g kg-1; clay = 610 g kg-1). Soil selenium biofortification enhanced quinoa grains' nutritional quality with selenium and antioxidant enzyme activity. Selenium also promoted free amino acid accumulation, which in turn enhanced the tolerance to water deficit. The integrated action of these defense mechanisms mitigated severe oxidative stress, thus regulating gas exchange that allowed the continuity of the photosynthetic process of quinoa cultivar BRS-Piabiru, particularly at 0.75 mg kg-1.