Silicon ameliorates iron deficiency in sour orange seedlings grown under different pHs

Iron (Fe) is an essential element for all plants while silicon (Si) is non-essential but has some beneficial roles for reducing iron deficiency stresses in plants. Soil pH affect iron solubility and availability and thus plant Fe nutrition however, little is known on how high or low pHs affect Si efficiency in alleviating Fe deficiency stress. In this research the interactive effects of root zone pH and Si nutrition on the alleviation of Fe deficiency in sour orange (Citrus aurantium) seedlings were investigated. Citrus seedlings were grown for three months in Fesufficient (control), Fe-deficient and Fe-zero media supplied with or without Si (0.7 mM), adjusted to pHs 6.5 and 8.0 and analyzed subsequently. Fe deficiency resulted in declined tissue Fe, proteins, chlorophylls, carotenoids and the activities of catalase, soluble and cell wall peroxidase associated with increased hydrogen peroxide and lipid peroxidation. Higher pH compromised plant growth associated with depleted tissue Fe in both Fe-deficient and Fe-sufficient plants and imposed chlorophylls and soluble proteins reduction. Si nutrition increased plant growth in Fe-deficient and Fe-sufficient plants under both pH treatments. It also improved tissue Fe content, soluble proteins, chlorophylls, carotenoids and the activities of antioxidant enzymes while decreased tissue hydrogen peroxide and lipid peroxidation levels. These results suggest that Si nutrition can reduce the harmful effects of soil Fe deficiency and higher pHs in sour orange plants via increase of Fe content and reduction of oxidative stress; however, in the absence of Fe, the applied Si cannot mitigate plant nutrient stress symptoms.