Sustainable Modeling and Optimization of Green-Synthesized ZnO Nanoparticles and Humic-Citric Amendments: Enhancing Growth, Nutrient Uptake, and Flowering of Solidago in Saline Soils

Abstract

The flowering habits of solidago is a significant and complex phenomenon and this thee event that determines the amount of floral branches used commercially in bouquets arrangement. Thus, improving the flowering attributes is so important to boost the solidago cultivation benefits especially under marginal soil conditions. The current research aimed to assess the modifications in physiological, morphological, floral and anatomical attributes of solidago plants grown in saline soil owing to application of humic-citric (HC) soil amendments and green-synthesized zinc oxide nanoparticles (ZnNPs). A split-plot randomized complete block design was used over two growing seasons (2021-22 and 2022-23). ZnNPs were synthesized using an eco-friendly approach with Pelargonium odoratissimum extract, applied at 0, 50, 100, and 200 mg L⁻¹ (ZnNPs0, ZnNPs50, ZnNPs100 and ZnNPs200, respectively). HC amendments were soil-drenched at 0, 25, and 50 L ha⁻¹ (HC0, HC25 and HC50, respectively). Physiological, anatomical, and flowering traits were measured alongside plant nutrient analyses.Findings illustrated that the highest membrane stability index and leaf water content were recorded from plants treated with HC50 × ZnNPs200, followed by that received HC25 × ZnNPs200 treatments, while the highest chlorophyll fluorescence, performance index, and leaf greenness were obtained from plants treated with combination of HC50 × ZnNPs200, and then next treated HC50 × ZnNPs100. HC50 × ZnNPs200 treatment increased the first floral branch height, floral branches number head⁻¹ and decreased days for flowering by 82.3%, 385.1%, and 48.9% respectively, compared to the salt stressed plants. HC-drenched at 50 L ha⁻¹ plus foliar spray with ZnNPs at 200 mg L⁻¹ increased nitrogen by 39.8 or 53.9%, phosphorus by 26.0 or 80.4%, potassium by 9.6 or 14.9%, calcium by 204.9 or 380.8%, potassium/sodium ratio by 14.8 or 64.0%, and decreased sodium by 22.2 or 33.9%, respectively, when compared to combination (HC50 × ZnNPs0), or control (HC0 × ZnNPs0) treatments. Application of HC50 × ZnNPs200 was the effective treatment for increasing the mid-vine thickness by 37.2 or 64.5%, leaf blead thickness by 24.2 or 64.6%, plastid tissue thickness by 30.7 or 100.0%, vascular bundle diameter by 10.4 or 114.1%, vascular bundle length by 14.7 or 121.8%, xylem vessels number by 32.4 or 96.0%, and xylem vessels diameter by 17.0 or 175.0%, in comparison with the HC50 × ZnNPs0 or control (HC0 × ZnNPs0) treatments, respectively. Meanwhile, the highest increment in spongy tissue thickness was recorded with HC25 × ZnNPs200, followed by HC50 × ZnNPs200, compared to the control treatment. Integrating HC amendments with ZnNPs represents a promising strategy to enhance solidago production in saline-alkaline soils. This approach improves nutrient uptake, enhances physiological and anatomical traits, and boosts flowering performance. Future research should focus on the long-term effects and molecular mechanisms behind these improvements.