Development of guar gum composite hydrogels for Cr(VI) adsorption and stress mitigation in Nicotiana tabacum L. seedlings

Abstract

Chromium (Cr(VI)) accumulation in Nicotiana tabacum L. poses severe phytotoxicity and health risks. To mitigate Cr(VI) phytotoxicity, a guar gum-acrylamide (G-AM) base hydrogel was synthesized and subsequently functionalized by incorporating biochar or humic acid, preparing two composite hydrogels designated as guar gum-acrylamide-biochar (G-AM-B) and guar gum-acrylamide-humic acid (G-AM-H), respectively. Characterization revealed that G-AM-B and G-AM-H possessed enhanced porosity and functional groups, leading to superior water retention (G-AM-H retained 94.3% water at 25 °C) and Cr(VI) adsorption capacity. At pH 7, G-AM-B exhibited an adsorption capacity of 89.9 mg/g, significantly higher than that of the base G-AM hydrogel (80.7 mg/g). Pot experiments under Cr(VI) stress demonstrated that both composite hydrogels markedly alleviated phytotoxicity. Compared to Cr-stressed controls, G-AM-B and G-AM-H treatments increased enhanced net photosynthetic rate by over 30%. They also effectively reduced oxidative stress, decreasing leaf malondialdehyde (MDA) content by 36.6-45.2% and root superoxide anion levels, as visualized by histochemical staining. Fluorescence imaging confirmed a reduction in stress-signaling molecule (HNO) accumulation. The mitigation is attributed to a synergistic mechanism where the hydrogels simultaneously adsorb Cr(VI) and improve the root-zone microenvironment. This work presents a novel, effective strategy for remediating heavy metal stress in crops using functional biopolymer-based hydrogels.

Keywords: Chromium stress; Guar gum composite hydrogel; Nicotiana tabacum L.