Amelioration of Chromium-Induced Growth, Physiological, and Biochemical Alterations in Sunflower (Helianthus annuus L.) through Foliar Application of Zinc Oxide Nanoparticles

Abstract

The present study investigated the ameliorative effects of foliar-applied ZnO-NPs on sunflower (Helianthus annuus L.) subjected to Cr stress. Sunflower plants were exposed to 50 and 150 ppm Cr, with foliar application of ZnO-NPs at 50 and 100 ppm. The effects of these treatments were evaluated through photosynthetic pigments, root and shoot length, proline, soluble sugars, protein content, membrane stability index (MSI), fresh weight, and dry weight. Chromium stress significantly (P ≤ 0.001) reduced most growth, physiological, and biochemical attributes. Total chlorophyll decreased by 58.45% and 62.69%, MSI by 77.37% and 84.16%, fresh weight by 42.42% and 54.22%, and dry weight by 68.00% and 74.00% under 50 and 150 ppm Cr, respectively. In contrast, Cr stress increased proline and soluble sugar accumulation, indicating activation of stress-adaptive responses. Foliar ZnO-NP application substantially alleviated Cr-induced damage, with 100 ppm ZnO generally producing greater improvements than 50 ppm. At 100 ppm ZnO, total chlorophyll, chlorophyll b, shoot growth, protein content, and MSI showed pronounced recovery, while MSI amelioration reached 135.85%. Under severe Cr stress (150 ppm), total chlorophyll recovery increased from 22.80% with 50 ppm ZnO to 60.49% with 100 ppm ZnO, whereas chlorophyll b recovery increased from 31.69% to 70.93%. Correlation analysis demonstrated positive associations among growth, biomass, chlorophyll, protein, and MSI, while proline and soluble sugars were associated with stress-related responses. Hierarchical clustering and principal component analysis further separated Cr-stressed plants from ZnO-associated recovery responses, with the first two principal components explaining 91.2% of total variation. Overall, foliar ZnO-NPs, particularly at 100 ppm, substantially improved the physiological and biochemical performance of sunflower under Cr stress.