Combined application of 5-ALA and nitric oxide improves lead detoxification and antioxidant defense in barley

Two independent experiments were performed to investigate role of NO in 5-aminolevulinic acid-mediated resistance to lead toxicity in barley plants. Lead toxicity significantly resulted in reduction of plant growth, Fv/Fm, total chlorophyll, leaf water potential, and Ca²⁺ as well as K⁺ potassium levels. Concurrently, it resulted in elevated levels of leaf MDA, H₂O₂, EL, Pb, and NO in comparison to control group. Both ALA (50 µM and 100 µM; ALA1 and ALA2) treatments enhanced plant growth parameters and elevated leaf K⁺ and Ca²⁺ levels, while simultaneously decreasing leaf Pb, H₂O₂, and MDA concentrations in comparison to Pb-stressed plants. A second experiment was conducted to ascertain involvement of nitric oxide in mitigation of Pb stress in barley seedlings by ALA, utilizing nitric oxide scavenger C₁₄H₁₆N₂O₄.K (cPTIO) in conjunction with ALA treatments. ALA-induced tolerance to Pb stress was entirely negated by administration of cPTIO (C₁₄H₁₆N₂O₄.K), which significantly decreased concentrations of endogenous nitric oxide. The findings indicated that ALA improved resistance of barley seedlings to Pb toxicity via activating endogenous nitric oxide. This was corroborated by elevation of H₂O₂ and MDA levels, with a reduction in SOD, CAT, and POD activities. The application of cPTIO along with ALA, led to growth inhibition and a notable increase in leaf Pb concentrations. Both ALA and nitric oxide collaboratively enhanced Pb tolerance in barley.