Silicon-mediated enhancement of cucumber phytoremediation potential for manganese.

Manganese (Mn) is a naturally occurring element which is ubiquitous in the environmentcomprising 0.1% of the Earth’s crust. Mn is an essential micronutrient for plant growth with awide range of important physiological functions. Its availability in soil is directlyproportional to pH and redox potential, so the reducing conditions (poorly drained and acidicsoils) increase Mn bioavailability, uptake and tissue accumulation leading to toxicitysymptoms such as chlorosis and brown spots, initially visible on older leaves. Although it iswell established that silicon (Si) greatly improves Mn tolerance of many plant species, thephysiological and molecular mechanism of this effect is poorly understood. In this regard, theeffect of Si, when supplied as 1.5 mM silicic acid, on the plant growth and theappearance of Mn-toxicity symptoms in the leaves of cucumber plants (Cucumis sativusL. cv. Chinese long) grown at Mn excess (100 μM) was studied. In Mn treatedplants, Si improved biomass production and alleviated symptoms of Mn toxicitycompared with non-Si treated plants. Although bulk tissue Mn concentrations weresimilar in Si treated and non-treated plants, the apoplastic concentration of freeMn2+ and H2O2 of Mn-treated plants was signi?cantly decreased by Si treatment.These results suggest that Si nutrition does affect neither the metal uptake nor itstranslocation from root to shoot. Inert depositions of Si in the leaf cell walls of cucumber(known as Si-accumulating species), that enhanced cell wall stability, simultaneouslyare Mn2+-binding sites which contribute to decrease of harmful free Mn activitywithin the plant tissue. The effect of Si nutrition on the compartmentation of leaf Mnwas also veri?ed by the less destructive electron paramagnetic resonance (EPR)spectroscopy. Application of Si slightly decreased an EPR signal of free Mn2+ fromthe apoplastic ?uid and bulk tissue of plants subjected to high Mn concentrations.Hydroxyl radicals (OH) were recorded by EPR spin-trapping method using a reagentDEPMPO, where intensity of the EPR signals of DEPMPO/OH adducts of Si treatedplants was lower regardless of the Mn supply. Additionally, Si supply led to a rapidsuppression of guaiacol-POD activity under excess Mn in the leaf apoplastic ?uid whichwas visually confirmed by isoelectric focusing. Taking into account this Mn-Siinteraction, phytoextraction process on sites contaminated with Mn could be significantlyimproved by Si application. Unfortunately, most metal hyperaccumulator plantsare characterized with slow growth rate and low biomass. Addition of Si couldelevate their biomass production combined with metal-toxicity protection withoutaffecting metal uptake and the long-distance root to shoot translocation, so theexcess Mn from the soil could be easily removed by harvesting shoot biomass. [ABSTRACT FROM AUTHOR]