Effect of foliar application of ZnO nanoparticles on yield components, yield and Zn efficiency indices in rice under water stress

Introduction Rice is the main food for more than half of the world population (Reis et al., 2018), which is one of the most sensitive plants to water stress (Yang et al., 2008). Among the various environmental stresses, drought stress is one of the most damaging due to a significant reduction in crop yield, which can reach billions of dollars annually worldwide (Tardieu et al., 2014). Water stress in different growth stages of plant decreases the growth and grain and straw yields of rice (Venkatesan et al., 2005). Water deficit stress after flowering stage has little effect on yield and yield components of rice (Wu et al., 2011). Zinc as a cofactor of antioxidant enzymes such as peroxidase and superoxide dismutase plays an important role in plant resistance to drought stress (Welch, 2001). Application of nutrients in the form of nanoparticles improves resistance of plants to biotic and abiotic stresses and increases the nutrient utilization efficiency (Alharby et al., 2016). Foliar application of nano-ZnO can be effective in improving the performance of total biomass and leaf area index under water stress (Kheirizadeh Arough et al., 2015). Therefore, the purpose of this study was to investigate the effects of doses of nano-ZnO in different stages of water stress on yield components, yield, Zn concentration and uptake in plant tissue, as well as Zn efficiency indices in rice (cv. Tarom Hashemi). Materials and methods The experiment was conducted as split plot in a randomized complete block design with three replications in Amol in 2016. Irrigation in four levels (I1: control or flood irrigation, I2: water stress at tillering stage for 15 days, I3: water stress at flowering stage for 15 days, and I4: water stress at grain filling stage for 15 days) were considered as main plots and foliar application of ZnO nanoparticles in four levels (Zn1: 0, Zn2: 25, Zn3: 50 and Zn4: 75 mg ZnO l-1) as sub-plots. Zn foliar application was performed at four stages of plant growth including early tillering, mid-tillering, booting stage and full heading stage. At the end of the season, some agronomic and physiological traits, as well as Zn efficiency indices in rice were determined. Data analysis was done by using MSTAT-C software and means were compared using the LSD test at the probability level of 5%. Results and discussion The results showed that the effect of irrigation levels was significant on all the yield components, yield and Zn concentration and uptake in grain and straw of rice. The effect of different doses of Nano-ZnO was significant on all the measured traits except for the 1000-grain weight. Also, the interaction effect of the experimental factors was not found to be significant on the studied traits. The yield, yield components, as well as the Zn concentration and uptake in grain and straw of rice were significantly reduced under water stress, especially at flowering stage. So, when irrigation was stopped at flowering stage, the grain yield decreased by 30.2% compared with flood irrigation condition, while the yield did not significantly decreased by water stress at grain filling stage. Water stress at flowering stage due to reduced the panicle length and number of filled grains per panicle led to a significant decrease in grain yield compared with flood irrigation conditions. The use of 50 and 75 mg.l-1 Nano-ZnO resulted in improved the panicle length, number of fertile tillers per hill, number of filled grains per panicle, Zn concentration and uptake in plant tissue, and finally the grain yield compared with control. There was a significant positive correlation between Nano-ZnO and grain yield at a probability level of 1%. The increase of grain yield by nano-ZnO foliar application have been reported in other crops such as wheat (Karimi, 2014), triticale (Kheirizadeh Arough et al., 2015), mungbean (Shojaei and Makarian, 2014) and foxtail millet (Davoodi et al., 2013). By increasing the application of ZnO nanoparticles at higher doses, especially in rate of 75 mg.l-1, decreased the Zn efficiency indices in rice. Conclusions The highest grain yield was obtained under flood irrigation, but water stress at flowering stage caused a significant decrease in yield and Zn uptake of rice plant tissue, which indicates the plant's high sensitivity to water deficit in this growth stage. Although by application of different doses of Nano-ZnO improved the yield components, yield and Zn uptake in grain and straw of rice compared with control, but application of 50 and 75 mg.l-1 Nano-ZnO produced the maximum yield and Zn uptake in plant tissue. Therefore, the flood irrigation during the plant growth period with application of 50 mg.l-1 ZnO nanoparticles is appropriate for improving the yield and Zn uptake in rice grains.