Biological interaction levels of zinc oxide nanoparticles; lettuce seeds as case study

Abstract

Background: Seed germination is a critical stage in plant life, and recent practices use nanomaterials for the improvement of plant seed germination indices. This study was conducted to assess the effect of laboratory prepared zinc oxide nanoparticles on the physiological and biochemical changes of lettuce seeds.

Methods: Lettuce seeds were soaked in a suspension of moderately polydisperse zinc oxide nanoparticles at two different concentrations (25 ppm or 50 ppm) and shaken for 3 h at 25 °C. Seeds treatment was followed subsequently by two to three days drying at ambient conditions. Treated seeds were stored for 3-4 weeks, at ambient conditions and then tested for germination in petri dishes. Germination was observed on daily basis and seedling length was measured. After imbibition and before the start of the visible germination, seeds were examined for topography and surface analysis using the scanning electron microscope and zinc uptake was measured by using the atomic absorption spectrometry and the energy dispersive X-ray. The pattern of mobilization of biomolecules was analyzed to detect any differences among different seed groups.

Results: There was no loss of viability for the nanoparticles treated seeds. Indeed their germination was enhanced and their biomass increased. The activated performance of the nanoparticles imbibed seeds has been found to be correlated with an increased level of Zn inside lettuce seeds. The recorded measurements show a significant enhancement of seedling length. Interaction of zinc oxide nanoparticles with lettuce seeds mediates a variation in the biochemical processes. Changes detected in treated seeds were as following: reduced levels of the total carbohydrates (including simple saccharides and polysaccharides), higher capacity of protein synthesis, an elevated level of starch as well as an increased activity of antioxidant enzymes.

Discussion and conclusion: Lettuce seeds primed with ZnO nanoparticles were found not only to maintain seed viability but even to exhibit a detectable level of germination enhancement compared to the control seeds. Overall, the promoted response of lettuce seeds during early stages of seed growth is encouraging for the application of ZnO NPs for seed priming for better germination indices.

Keywords: Antioxidants; Biochemical analysis; Biochemistry; Biological sciences; Materials application; Nanomaterials; Nanoparticles; Plant biology; Proteins; Seedling length; Size.

Figure 1

SEM image of Zinc oxide nanoparticles.

Figure 2

Mean values of the number of germinated seeds over time after lettuce seeds treatment with ZnO NPs using 25 ppm and 50 ppm. The mean values of germinated seeds were calculated for a total of 300 seeds distributed into 6 plates (N = 6 plates) for each group.

Figure 3

Effect of two different concentrations of ZnO NPs (25 ppm and 50 ppm) on both lettuce seedling length (A) and fresh weight of lettuce seedling (B). Values are the means for each criterion and the vertical bars represent standard errors. Significant difference (p < 0.05) are marked with “∗”.

Figure 4

Scanning electron microscope examination of the surface and appearance of lettuce seeds. 1. Low to high magnification of untreated vs. treated. Left Panel: untreated seeds, Right Panel: seed treated with 25 μg/ml ZnO NPs.

Figure 5

SEM image right-hand side panel and EDX spectra left-hand side panels of lettuce seed treated with 25 ppm ZnO NPs. The EDX spectra sampled from SEM areas. EDX spectrum shows Zn peak of atomic zinc origin ZnL. Prominent peaks were detected;. C, O, Mg, Al, S, K, Ca, denote corresponding chemical elements. The ending letters K and L denote the characteristic binding energies of the electron shells.

Figure 6

The effect of two concentrations of ZnO NPs; 25 ppm and 50 ppm on the relative proportions of starch, protein and carbohydrates in lettuce seeds. The total of each bar is 100 and the measured concentrations of each of the following components; starch, protein and total carbohydrates are expressed as percentage of the respective totals. Labels inside the rectangle bars represent the measured concentrations (mg/ml) of biomolecules in each seed group.

Figure 7

The relative activities of catalase and amylase enzymes in lettuce seeds treated with two concentrations of ZnO NPs; 25 ppm and 50 ppm. The total of each bar is 100 and the measured activity of each one of the enzymes is expressed as percentage of the respective totals.