Interactions between aluminium, iron and silicon in Cucumber sativus L. grown under acidic conditions.

Aluminium (Al) is one of the major stressors for plants in acidic soils, negatively affecting plant growth and nutrient balances. Significant efforts have been undertaken to understand mechanisms of Al tolerance in plants. However, little is known of the relevance of iron (Fe) and silicon (Si) nutrition under Al stress conditions. The objectives of this study were to determine whether effects induced by Fe and Si are of importance for limitation of Al moving via xylem in plants (Cucumis sativus L.). Cucumber plants (cv. Phoenix and Solovei) were grown (i) hydroponically in a complete nutrient solution at pH 4.0, either with (+Fe) or in Fe-free (-Fe) nutrient solution, without (-Si) or with (+Si) supply of Si, without (-Al) or with (+Al) exposure of Al and (ii) in soil. Xylem sap concentrations of Al, Fe and Si were measured. To characterise the pattern of xylem sap transport of Al and Fe, metabolomic changes of root tissues were investigated. Although the growth of cucumber plants was not significantly affected by Al (Al-tolerant), Al exposure decreased xylem sap Fe (+Fe plants) and increased ferric chelate reductase (FC-R) activity of roots (-Fe plants). On the other hand, Fe supply greatly mitigated the Al-induced increase in xylem sap Al. The ameliorative effect of Fe depended on plant genotypes and was more pronounced in the more Fe-efficient cultivar Phoenix, which presented the highest level of xylem sap Fe. Xylem sap Fe was positively correlated with root serine, succinic and fumaric acids, suggesting that a probable underlying mechanism of Al tolerance might involve the chelation of Fe by biosynthesis of these chelating compounds. The Si-modulated root succinate increase appears to be of great importance for facilitating long-distance transport of Fe, thereby hindering Al transport from roots to shoots. The results highlight for the first time the importance of both Fe and Si supply in plant exclusion of Al under acidic conditions.