Background and Objectives: Irrigation with non-conventional water sources, such as wastewater, can have significant negative impacts on the growth, yield, and health of crop plants due to the presence of various dissolved substances and pollutants. Although the reuse of wastewater in agriculture is considered a cost-effective strategy for conserving freshwater resources, the risks associated with toxic elements, pathogens, and physiological stresses necessitate the use of mitigation methods. In this context, nanotechnology and soil amendments like zeolite and biochar have emerged as novel tools to reduce adverse effects and improve plant growth under stress conditions. Sunflower (Helianthus annuus), as one of the most important oilseed crops globally, holds a special place in agricultural production due to the high economic and technical value of its seeds and oil. This study aimed to investigate the effect of foliar application of silver nanoparticles (AgNPs) at levels (0, 60, and 120 ppm), along with the application of biochar (9 tons per hectare) and zeolite (9 tons per hectare), on the physiological, morphological traits, seed yield, and oil content of sunflower irrigated with urban wastewater. The experiment was conducted as a factorial design based on a randomized complete block design with three replications in Varamin during the 2018-2019 growing season.
Materials and Methods: The analysis of variance results indicated that the AgNPs treatment had a significant effect on physiological traits, including chlorophyll a and b, carotenoids, relative water content (RWC) of leaves, and proline content. Increasing the concentration of AgNPs, especially at the 120 ppm level, significantly reduced the amount of chlorophyll a and b, indicating damage to the plant's photosynthetic apparatus and the induction of oxidative stress. Additionally, proline content, as a protective compound against stress, decreased with AgNPs application, which could indicate disruption in protein expression patterns and metabolic pathways activated by ethylene. These findings suggest that AgNPs at high concentrations act as a stress factor themselves, affecting the plant's tolerance capacity. In contrast, the individual application of biochar and zeolite had positive effects on growth traits. The main effect of biochar and zeolite on plant height was significant at the 5% and 1% levels, respectively, and the application of these materials improved the physical and chemical conditions of the soil, enhancing root access to water and nutrients.
Results and Discussion: In the analysis of interaction effects, it was found that the combined application of these treatments plays a crucial role in modulating the effects of AgNPs. Although AgNPs had a negative effect on chlorophyll, this effect was relatively reduced in the presence of biochar and zeolite, indicating the protective role of these soil amendments. Regarding yield-related traits, the impact of the treatments was less pronounced. The number of seeds per head, 1000-seed weight, and seed yield were not significantly affected by the experimental treatments. However, the most significant positive finding of the research concerned product quality. The AgNPs treatment acted as an "elicitor" and caused a significant increase in seed oil percentage. This increasing effect was highest in combined treatments, particularly in the combination of 120 ppm AgNPs with biochar. Zeolite also showed an increasing effect when combined with AgNPs. These results indicate that stress induction by AgNPs may activate biosynthesis pathways of secondary metabolites, such as lipids, leading to increased oil accumulation in the seed.
Conclusion: Overall, the results of this study indicate that although the use of AgNPs under wastewater irrigation conditions can have undesirable effects on the plant's physiological traits, the simultaneous application of soil amendments like biochar and zeolite can mitigate these negative effects. At the same time, AgNPs, as a quality-enhancing agent, by increasing seed oil percentage, can boost the economic value of the sunflower crop. Therefore, a combined strategy of using AgNPs alongside biochar and zeolite could be an effective approach to improving product quality in sunflower cultivation with wastewater, albeit with consideration of its side effects on plant growth.