Structure of alginate polysaccharide selenium-nanoparticles and the mechanism of promoting selenium accumulation in rice

Objectives The structure of alginate polysaccharide selenium-nanoparticles (APS-SeNPS) was characterized for explaining its efficiency in promoting Se absorption in rice. The optimum foliar spraying time and content of APS-SeNPS were studied for its efficient application.

Methods The preparation process of APS-SeNPs was optimized by a single-factor experiment and response-surface methodology. Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and surface tensiometer were used to characterize the APS-SeNPs. A field foliar spraying experiment was conducted on rice, with APS-SeNPs concentration of 0, 5 or 25 mg/L, respectively. Se accumulation in japonica rice ‘Luodao 998’ and indica rice ‘Chuanxiangyou 2’ were investigated.

Results 1) The optimized preparation conditions of APS-SeNPs containing Se 2800 mg/L were as follows: molar ratio of Na₂SeO₃ to ascorbic acid (Se∶Vc) was 19∶100, reaction temperature 32℃ and reaction time 20 min. The produced particle size of APS-SeNPs was 38.94 nm, and the Zeta potential was −52.17 mV. The Se—O chemical bond was identified in APS-SeNPs by FTIR. The surface tension of APS-SeNPs is 46.23 mN/m, which is significantly lower than that of Na₂SeO₃. The liquid holding capacity of APS-SeNPs on rice leaves was 6.28 mg/cm², which was much higher than that of Na₂SeO₃ (3.94 mg/cm²). 2) Field experiments revealed that Se contents in each organs of the two rice cultivars increased with the increase of spraying Se concentration. The Se content in both the rice cultivars was in order of leaf>rice husk>brown rice. The Se content of brown rice in APS-SeNPs treatment was 2.2%−34.5% higher than that in Na₂SeO₃ treatments. The organic Se proportion in brown rice treated with two Se sources was 70.9% to 85.3%, and the organic Se proportion in brown rice was decreased with the increase of Se spray concentrations, but the organic Se proportion in brown rice treated with APS-SeNPs was 2.1−7.6 percentage points higher than that treated with Na₂SeO₃.

Conclusions In the APS-SeNPs structure, Se—O bond was formed, which decreased the surface tension and lead to higher liquid holding capacity of APS-SeNPs on leaves, which was good for the absorption of Se. Therefore, the accumulation of Se in grain and the proportion of organic Se in grain were increased. Spraying 5 mg/L APS-SeNPs at grain filling stage of rice increases the brown rice Se content to the enrichment standard (0.04−0.3 mg/kg).