The Response of Cowpea (Vigna unguiculata L) Plants to Foliar Application of Sodium Selenate and Selenium Nanoparticles (SeNPs)
The present study describes the synthesis and rapid production of selenium nanoparticles (SeNPs) by reducing selenate in the presence of Ascorbic Acid (AA) as a reductant, coating, and stabilizing agent. The formation of nanosized selenium at 10 mm sodium selenate (Na2SeO4) and 1.5% AA was confirmed by the appearance of the characteristic surface plasmon absorption peak at 296 nm in UV-vis spectra. Transmission Electron Microscopy (TEM) indicates that SeNPs was mostly spherical with a mean diameter of approximately 33.4 nm. X-RAY Diffraction (XRD) pattern confirmed crystalline shape indicating particle size of approximately 42.92 nm. The particle size distribution of SeNPs was approximately 45.9 nm by Dynamic Light Scattering (DLS). Fourier Transform Infrared (FTIR) spectroscopic analysis indicates the coating of selenium nanoparticles with ascorbic acid bonding of SeNPs with the COO˗ group of ascorbic acid. A pot experiment at the Experimental Farm of Helwan University, Cairo, Egypt during the season of 2016 was performed to evaluate the effects of foliar applications of both Na2SeO4 and chemically synthesized SeNPs (≈ 33.4 nm) each at 0.0, 6.25, 12.5, 25 and 50 µM on vegetative growth, yield and some physiological activities of cowpea (Vigna unguiculata L) plants. Foliar application of Na2SeO4 and SeNPs up to 25 µM significantly increased growth criteria (i.e. length of roots and stem, the root, stem and leaves fresh and dry weights, No of leaves and total leaves area cm2/plant), weight and quality of seed compared to the corresponding untreated control plants. Application of Na2SeO4 and SeNPs, especially at 6.25 µM concentration increased the Total Photosynthetic Pigments (TPP), Total Carbohydrate (TC), total soluble proteins (TSP), and different minerals in leaves accompanied by decrease in Total Soluble Sugars (TSS). SeNPs at 6.25 µM increased the levels of the growth hormones Indole Acetic Acid (IAA), Gibberellic Acid (GA3) and Cytokinins (CKs) of cowpea leaves, with a relatively lower Abscisic Acid (ABA) content and higher GA3/ABA ratios, followed by Na2SeO4 at 6.25 µM which explains the increase in growth parameters and seed weight in SeNPs and Na2SeO4 treated plants compared to control plants. Atomic Absorption Spectroscopy (AAS) study unveiled the residual accumulation of selenium nanoparticles in leaves and seeds of cowpea plants at 50 µM. Transmission electron microscopy showed small, dark deposits in leaf cells exposed to SeNPs, which probably originated from the nanoparticles absorbed onto the leaves and transferred to seeds. In conclusion, application of SeNPs and Na2SeO4 at 6.25 µM improved vegetative growth, seed weight, nutritional value and quality of cowpea plants and seeds.