Structural, biochemical, physiological and nutritional responses in soybean plants under progressive salt stress.

Abstract

Soybean is a legume that is widely cultivated in many countries due to the high levels of proteins and oils contained in its grains. It is used in human and animal nutrition or for the production of medicines, industrial products and biofuel. On the other hand, salt stress is a limiting factor in crop production and it is estimated that more than 800 million hectares are affected by salinity. In this sense, the aim of this research was to evaluate the structural behavior, using root, stem and leaf variables, detailing the possible anatomical changes involved in these organs, in addition to understanding the nutritional behavior, the photosynthetic apparatus, gas exchange, antioxidant system and oxidative damage in soybean plants submitted to progressive salt stress. For this, the experiment was randomized into five treatments (0, 50, 100, 150 and 200 mM NaCl). In the root, increases in the epidermis and endoderm reveal the protective roles of these structures in plants subjected to 100 mM Na+, which favor the reduction of the influx of Na+. With the increase in salinity, the higher increase in the lysigenous aerenchyma minimizes the absorption of toxic ions by replacing dead cells with air spaces. In relation to the stem, increases in the cortex and pith, in the first internode in concentrations of 100 mM Na+, alleviate the damage and oxidative stress generated by salt in the meristematic regions. In all root and stem regions analyzed in soybean plants subjected to concentrations of 50-200 mM Na+, the metaxylem is reduced to prevent cavitation and loss of functionality of vessel elements and, these changes, maximizes the impermeability of this tissue preventing ionic flux by increase the thickness of the cell wall. In relation to leaves, progressive salt stress negatively interferes in K+ /Na+ homeostasis, nutritional content, photosynthetic apparatus and gas exchange, also increases oxidative damage and, to some extent, induces the antioxidant system and harms photosynthetic pigments. On the other hand, the impacts of salinity promote leaf anatomical changes to minimize the deleterious effects associated with Na+. Effects such the increase of epicuticular wax in saline concentrations of 50 mM Na+ favor a lipophilic protection that prevents the loss of water through transpiration and the direct incidence of solar radiation in the epidermal cells. In addition, the improvements observed in the number of stomata, in their most elliptical form, as well as the increase in the thickness of the epidermis, up to 100 mM Na+, evidence a strategy for the efficient use of water. Finally, this research showed that soybean plants subjected to progressive salt stress exhibited anatomical changes to minimize the deleterious effects associated with Na+.