Raised pH conferred the ability to maintain a balance between production and detoxification of reactive oxygen species and methylglyoxal in aluminum-toxic Citrus sinensis leaves and roots

Highlights: • Enhancing nutrient solution pH from 2.5 to 4.0 mitigated Citrus Al-toxicity. • With Al-stress, high pH reduced root and leaf ROS and MG production and level. • With Al-stress, high pH enhanced root and leaf ability to detoxify ROS and MG. • High pH prevented Al-treated roots and leaves from oxidative damage. Little is known about interactive effects of pH-aluminum (Al) on reactive oxygen species (ROS) and methylglyoxal (MG) metabolisms in plants. Citrus sinensis seedlings were fertilized with nutrient solution at an Al concentration of 1 or 0 mM and a pH of 4.0, 3.5, 3.0 or 2.5 for 18 weeks. Thereafter, gas exchange and chlorophylls in leaves, H₂O₂ generation, electrolyte leakage, total soluble proteins, MG, malondialdehyde (MDA), antioxidants, sulfur-containing compounds, enzymes [viz., antioxidant enzymes, sulfur metabolism-related enzymes, ascorbate oxidase, phosphomannose isomerase, glyoxalase I and glyoxalase II] involved in ROS and MG detoxification in leaves and roots were measured. Effects of low pH and Al-toxicity on these parameters displayed obvious synergism. Without Al-toxicity, low pH increased H₂O₂ production, electrolyte leakage, MDA and MG concentrations by 45.7%–90.3% (52.4%–73.6%), 24.3%–74.5% (26.7%–86.2%), 18.6%–44.8% (35.6%–53.7%) and 16.3%–47.1% (13.8%–51.7%) in leaves (roots) relative to pH 4, respectively; low pH-induced upregulation of enzymes involved in ROS and MG detoxification and sulfur-containing compounds in leaves and/or roots could not protect them against oxidative damage. At pH 2.5–3.0, Al-toxicity increased H₂O₂ production, electrolyte leakage, MDA and MG concentrations by 34.2%–35.5% (23.9%–72.7%), 10.2%–29.5% (23.7%–56.8%), 15.6%–35.7% (27.5%–33.9%) and 21.5%–26.8% (21.0%–49.2%) in leaves (roots), respectively, and decreased total soluble protein concentration by 46.2%–47.4% (18.8%–20.8%) in leaves (roots); at pH 3.5–4.0, Al-toxicity did not affect significantly the five parameters in leaves and roots except for Al-induced increases in root MDA concentration at pH 3.5–4.0 and root electrolyte leakage at pH 3.5, and Al-induced decrease in root total soluble protein concentration at pH 4.0. Raised pH conferred the ability to maintain a balance between production and detoxification of ROS and MG in leaves and roots, thus protecting them against oxidative damage, and hence alleviating Al-induced increase in electrolyte leakage and decrease in total soluble protein level.