Rice-husks synthesized-silica nanoparticles modulate silicon content, ionic homeostasis, and antioxidants defense under limited irrigation regime in eggplants*

The utilization of nanoparticles in agricultural land is widely increasing worldwide. The present study takes the advent from converting the rice husk waste to silica nanoparticles (SiNPs) to be used in two years of field experiments, as fertigation treatment, against reducing irrigation by drip system. In this respect, the experiments comprised of three drip irrigation regimes (i.e., 60, 80, and 100 % of crop evapotranspiration, ETc) and four levels of SiNPs (0, 100, 200, and 300 mg L-1). The results indicated that limited irrigation reduced the plant fresh weight, plant height, and yield of eggplants by 15, 25, and 30 %, an average of two years, at the level of 60 % ETc compared to 100 % ETc. However, these negative impacts were alleviated by SiNPs via improving the photosynthetic pigments and relative water content where the plants supplemented with 300 mg L-1 SiNPs had the highest improving effects to be 3.8 mg g-1 FW and 76 % compared to 2.2 mg g-1 FW and 63 % (average of two years), respectively at the drought level of 60 % ETc. Furthermore, SiNPs ameliorated the oxidative damage induced by different water regimes by lessening lipid peroxidation and hydrogen peroxide (10.9 and 85.4 mu M g- 1 FW) compared to their drought level 60 % ETc (14 and 113 mu M g- 1 FW). This was associated with stimulating the antioxidant enzyme system by promoting the activities of peroxidases (ascorbate- and guaiacol-dependent types), catalase, and superoxide dismutase as the concentration of silica NPs increased to be (173.84, 12.2, 49.5, and 41 unit/mg/min, respectively) at 60 % ETc and 300 mg L-1 SiNPs compared to only 60 % ETc (100.5, 7.9, 32, and 30.5 unit/mg/min, respectively). Also, efficiently increasing SiNPs levels highly significantly restrained the ionic homeostasis in the leaves of eggplant via retaining the reduction of silicon, potassium, calcium, and magnesium contents at 300 mg L-1 silica NPs especially at 60 % ETc (33, 19, 4.5, and 6 mg g-1 DW, respectively) compared to the corresponding drought level (17.33, 7, 2.1, and 3.7 mg g-1 DW, respectively) which reflected on the upregulation of water status. Such amelioration effects of SiNPs were recommended during the two studied seasons. Altogether, the synthesized SiNPs efficiently mitigated the negative impacts of limited drip irrigation levels on ionic homeostasis, pigments, oxidative stress, and antioxidant system especially at the level of 300 mg L-1.