Thiourea-Capped Nanoapatites Amplify Osmotic Stress Tolerance in Zea mays L. by Conserving Photosynthetic Pigments, Osmolytes Biosynthesis and Antioxidant Biosystems

Abstract

Salinity is one of the most prevalent abiotic stresses which not only limits plant growth and yield, but also limits the quality of food products. This study was conducted on the surface functionalization of phosphorus-rich mineral apatite nanoparticles (ANPs), with thiourea as a source of nitrogen (TU-ANPs) and through a co-precipitation technique for inducing osmotic stress tolerance in Zea mays. The resulting thiourea-capped apatite nanostructure (TU-ANP) was characterized using complementary analytical techniques, such as EDX, SEM, XRD and IR spectroscopy. The pre-sowing of soaked seeds of Zea mays in 1.00 µg/mL, 5.00 µg/mL and 10 µg/mL of TU-ANPs yielded growth under 0 mM, 60 mM and 100 mM osmotic stress of NaCl. The results show that Ca and P salt acted as precursors for the synthesis of ANPs at an alkaline pH of 10-11. Thiourea as a source of nitrogen stabilized the ANPs' suspension medium, leading to the synthesis of TU-ANPs. XRD diffraction analysis validated the crystalline nature of TU-ANPs with lattice dimensions of 29 nm, calculated from FWHM using the Sherrer equation. SEM revealed spherical morphology with polydispersion in size distribution. EDS confirmed the presence of Ca and P at a characteristic KeV, whereas IR spectroscopy showed certain stretches of binding functional groups associated with TU-ANPs. Seed priming with TU-ANPs standardized germination indices (T50, MGT, GI and GP) which were significantly declined by NaCl-based osmotic stress. Maximum values for biochemical parameters, such as sugar (39.8 mg/g at 10 µg/mL), protein (139.8 mg/g at 10 µg/mL) and proline (74.1 mg/g at 10 µg/mL) were recorded at different applied doses of TU-ANP. Antioxidant biosystems in the form of EC 1.11.1.6 catalase (11.34 IU/g FW at 10 µg/mL), EC 1.11.1.11 APX (0.95 IU/G FW at 10 µg/mL), EC 1.15.1.1 SOD (1.42 IU/g FW at 5 µg/mL), EC 1.11.1.7 POD (0.43 IU/g FW at 5 µg/mL) were significantly restored under osmotic stress. Moreover, photosynthetic pigments, such as chlorophyll A (2.33 mg/g at 5 µg/mL), chlorophyll B (1.99 mg/g at 5 µg/mL) and carotenoids (2.52 mg/g at 10 µg/mL), were significantly amplified under osmotic stress via the application of TU-ANPs. Hence, the application of TU-ANPs restores the growth performance of plants subjected to induced osmotic stress.

Keywords: APX; POD; SOD; characterization; nanoapatite; osmolytes; thiourea.

Figure 1

Infrared spectroscopy of TU-ANPs representing various vibrational stretches and bending of functional groups associated with thiourea and apatite.

Figure 2

X-ray diffraction analysis of TU–ANPs showing intensities at different two theta levels matching a standard spectra of apatite with JCPDS: 00-009-0432.

Figure 3

SEM–EDX analysis of TU–ANPs showing intense agglomeration due to Ostwald ripening where the size ranges from 40 nm to 220 nm, and characteristic signals of P and Ca of apatite at given KeV.

Figure 4

Effect of TU–ANPs on mean germination time (MGT) and time to 50% germination (T50) of Zea mays under various levels of NaCl-based osmotic stress.

Figure 5

Effect of TU–ANPs on germination index (GI) and germination percentage (GP) of Zea mays under various levels of NaCl-based osmotic stress.

Figure 6

Effect of TU–ANPs on seedling growth (plumule and radical) of Zea mays under various levels of NaCl-based osmotic stress.

Figure 7

Effect of TU–ANPs on photosynthetic contents (chlorophyll a, b and carotenoids) of Zea mays under various levels of NaCl-based osmotic stress.

Figure 8

Effect of TU–ANPs on osmolytes content (protein, proline and soluble sugar) of Zea mays under various levels of NaCl-based osmotic stress.

Figure 9

Effect of TU–ANPs on activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX) of Zea mays under various levels of NaCl-based osmotic stress.

Figure 10

Effect of TU–ANPs on activities of peroxidase (POD) and catalase (CAT) of Zea mays under various levels of NaCl-based osmotic stress.