Antioxidant Responses in Chromium-Stressed Maize as Influenced by Foliar and Root Applications of Fulvic Acid

Abstract

Maize (Zea mays L.) faces significant challenges to its growth and productivity from heavy metal stress, particularly Chromium (Cr) stress, which induces reactive oxygen species (ROS) generation and damages photosynthetic tissues. This study aimed to investigate the effects of fulvic acid (FA) application, via foliar spray or root irrigation, on mitigating chromium stress in maize by evaluating its impact on antioxidant activity and growth parameters. Two maize varieties, P3939 and 30Y87, were subjected to chromium stress (CrCl₃·6H₂O) at concentrations of 300 µM and 100 µM for a duration of 5 weeks. The experiment was conducted in a wire house under natural environmental conditions at the Seed Centre, Institute of Botany, University of the Punjab, Lahore, Pakistan. Physiological assessments included electrolyte leakage, chlorophyll pigment content, malondialdehyde (MDA) levels, and activities of antioxidant enzymes such as catalase (CAT), ascorbate peroxidase (APX), and guaiacol peroxidase (GPX) in maize leaves. Growth parameters were also monitored. The results revealed that chromium stress significantly reduced chlorophyll content and increased oxidative stress, as evidenced by elevated MDA levels and electrolyte leakage. However, FA application notably mitigated these effects: chlorophyll content improved by 15%, and MDA levels decreased significantly. Irrigation with FA was particularly effective, reducing MDA levels by 40% compared to the 300 µM chromium treatment. Furthermore, while chromium stress enhanced antioxidant enzyme activities, FA application further boosted total soluble protein levels and antioxidant enzyme activities under stress conditions. In conclusion, FA application demonstrates potential in improving maize tolerance to heavy metal stress by enhancing the antioxidant defense system and preserving photosynthetic pigments. These findings highlight FA's promise as a practical strategy for mitigating the negative impacts of chromium stress on maize, promoting sustainable agricultural practices in contaminated environments.

Keywords: Antioxidant enzymes; Chromium toxicity; Maize growth; Oxidative damage; Reactive oxygen species; Stress mitigation.

Fig. 1

The experimental layout includes two maize cultivars, P3939 and 30Y87, subjected to different chromium (Cr) stress levels and treatments with fulvic acid. The figure depicts the arrangement of the pots and highlights the visual symptoms in maize plants exposed to Cr, such as leaf discoloration.

Fig. 2

Effects of fulvic acid applied as a foliar spray (FFA, 0.5 L/A) and through the rooting medium irrigation (IFA, 300 L/A) on Chlorophyll content of maize cultivars P3939 (A) and 30Y87 (B) under chromium stress. Values represent mean ± standard error (SE), with lowercase letters indicating significant differences among treatments. -FFA indicates no fulvic acid application (control). + FFA represents foliar application of fulvic acid, sprayed on leaves. IFA refers to fulvic acid application through irrigation, delivered via the rooting medium.

Fig. 3

Effects of fulvic acid applied as a foliar spray (FFA, 0.5 L/A) and through the rooting medium irrigation (IFA, 300 L/A) on Electrolyte leakage (EL) of maize cultivars P3939 (A) and 30Y87 (B) under chromium stress. Values represent mean ± standard error (SE), with lowercase letters indicating significant differences among treatments. -FFA indicates no fulvic acid application (control). + FFA represents foliar application of fulvic acid, sprayed on leaves. IFA refers to fulvic acid application through irrigation, delivered via the rooting medium.

Fig. 4

Effects of fulvic acid applied as a foliar spray (FFA, 0.5 L/A) and through the rooting medium irrigation (IFA, 300 L/A) on MDA content of maize cultivars P3939 (A) and 30Y87 (B) under chromium stress. Values represent mean ± standard error (SE), with lowercase letters indicating significant differences among treatments. -FFA indicates no fulvic acid application (control). + FFA represents foliar application of fulvic acid, sprayed on leaves. IFA refers to fulvic acid application through irrigation, delivered via the rooting medium.

Fig. 5

Effects of fulvic acid applied as a foliar spray (FFA, 0.5 L/A) and through the rooting medium irrigation (IFA, 300 L/A) on Total Soluble protein (TSP) of maize cultivars P3939 (A) and 30Y87 (B) under chromium stress. Values represent mean ± standard error (SE), with lowercase letters indicating significant differences among treatments. -FFA indicates no fulvic acid application (control). + FFA represents foliar application of fulvic acid, sprayed on leaves. IFA refers to fulvic acid application through irrigation, delivered via the rooting medium.

Fig. 6

Effects of fulvic acid applied as a foliar spray (FFA, 0.5 L/A) and through the rooting medium irrigation (IFA, 300 L/A) on Catalase enzyme (CAT) of maize cultivars P3939 (A) and 30Y87 (B) under chromium stress. Values represent mean ± standard error (SE), with lowercase letters indicating significant differences among treatments. -FFA indicates no fulvic acid application (control). + FFA represents foliar application of fulvic acid, sprayed on leaves. IFA refers to fulvic acid application through irrigation, delivered via the rooting medium.

Fig. 7

Effects of fulvic acid applied as a foliar spray (FFA, 0.5 L/A) and through the rooting medium irrigation (IFA, 300 L/A) on Ascorbate enzyme (APX) of maize cultivars P3939 (A) and 30Y87 (B) under chromium stress. Values represent mean ± standard error (SE), with lowercase letters indicating significant differences among treatments. -FFA indicates no fulvic acid application (control). + FFA represents foliar application of fulvic acid, sprayed on leaves. IFA refers to fulvic acid application through irrigation, delivered via the rooting medium.

Fig. 8

Effects of fulvic acid applied as a foliar spray (FFA, 0.5 L/A) and through the rooting medium irrigation (IFA, 300 L/A) on glutathione peroxidase (GPX) of maize cultivars P3939 (A) and 30Y87 (B) under chromium stress. Values represent mean ± standard error (SE), with lowercase letters indicating significant differences among treatments. -FFA indicates no fulvic acid application (control). + FFA represents foliar application of fulvic acid, sprayed on leaves. IFA refers to fulvic acid application through irrigation, delivered via the rooting medium.

Fig. 9

Score (a, c) and loading plots (b, d) of principal component analysis (PCA) on various studied parameters of maize cultivars grown in Cr stressed soil. Score plot represents separation of treatments as (1) Control (without Cr contamination); (2) FFA (3) IFA (4) 100 µM Cr level, 5) 100 µM Cr + FFA 6) 100 µM Cr + IFA, 7) 300 µM Cr, 8) 300 µM Cr + FFA 9) 300 µM Cr + IFA respectively. The abbreviations of parameters are as follows: Plant H: plant height; NL: number of leaves; LW: leaf width; T chl: total chlorophyll; EL: electrolyte leakage; MDA: Lipid peroxidation; T pro: total protein; CAT: catalase; APX: Ascorbate peroxidase GPX: Guaiacol peroxidases.

Fig. 10

Correlation between growth and physiological parameters of maize cultivars grown in Cr stressed soil.