Binding of transition metals to monosilicic acid in aqueous and xylem (Cucumis sativus L.) solutions: a low-T electron paramagnetic resonance study

Abstract

The supplementation of monosilicic acid [Si(OH)4] to the root growing medium is known to protect plants from toxic levels of iron (Fe), copper (Cu) and manganese (Mn), but also to mitigate deficiency of Fe and Mn. However, the physicochemical bases of these alleviating mechanisms are not fully understood. Here we applied low-T electron paramagnetic resonance (EPR) spectroscopy to examine the formation of complexes of Si(OH)4 with Mn(2+), Fe(3+), and Cu(2+) in water and in xylem sap of cucumber (Cucumis sativus L.) grown without or with supply of Si(OH)4. EPR, which is also useful in establishing the redox state of these metals, was combined with measurements of total concentrations of metals in xylem sap by inductive coupled plasma. Our results show that Si(OH)4 forms coordination bonds with all three metals. The strongest interactions of Si(OH)4 appear to be with Cu(2+) (1/1 stoichiometry) which might lead to Cu precipitation. In line with this in vitro findings, Si(OH)4 supply to cucumber resulted in dramatically lower concentration of this metal in the xylem sap. Further, it was demonstrated that Si(OH)4 supplementation causes pro-reductive changes that contribute to the maintenance of Fe and, in particular, Mn in the xylem sap in bioavailable 2+ form. Our results shed more light on the intertwined reactions between Si(OH)4 and transition metals in plant fluids (e.g. xylem sap).

Calcium Carbonate Reduces the Effectiveness of Soil-Added Monosilicic Acid in Cucumber Plants

“…3). The findings regarding the improvement of Fe and Mn transport concur with some other cucumber studies under hydroponic conditions (Pavlovic et al 2013(Pavlovic et al , 2016Bityutskii et al 2014Bityutskii et al , 2017bStevic et al 2016). On the one hand, Si(OH) 4 can directly form complexes with Fe 3+ and Mn 2+ in water and xylem sap of cucumber, promoting the solubility and bioavailability of these micronutrients (Stevic et al 2016).…”

“…The findings regarding the improvement of Fe and Mn transport concur with some other cucumber studies under hydroponic conditions (Pavlovic et al 2013(Pavlovic et al , 2016Bityutskii et al 2014Bityutskii et al , 2017bStevic et al 2016). On the one hand, Si(OH) 4 can directly form complexes with Fe 3+ and Mn 2+ in water and xylem sap of cucumber, promoting the solubility and bioavailability of these micronutrients (Stevic et al 2016). On the other hand, Si treatment can enhance Fe translocation towards apical shoots by accumulation of some Fe-mobilising compounds in leaves and roots (Pavlovic et al 2013;Bityutskii et al 2014Bityutskii et al , 2017b.…”

Silicon Improves Chilling Tolerance During Early Growth of Maize by Effects on Micronutrient Homeostasis and Hormonal Balances

“…Bityutskii et al ( 2014 ) could not confirm this interaction, but in that study, plants were grown in nutrient solution without Zn supply, which may have prevented the accumulation of apoplastic Zn pools. Other studies suggest direct Si-metal interactions counteracting apoplastic metal immobilization and supporting metal transport in plants (Pavlovic et al, 2013 ; Hernandez–Apaolaza, 2014 ; Stevic et al, 2016 ). Effects of Si on the expression of metal acquisition and transport genes have been reported by Pavlovic et al ( 2013 , 2016 ), but the underlying mechanisms are unknown.…”

An ABC Transporter Is Involved in the Silicon-Induced Formation of Casparian Bands in the Exodermis of Rice

“…This agrees with previous findings that Si supply in the nutrient solution reduces the concentration of Fe, Mn and other nutrients by 20–50% in leaf DM (Ma and Takahashi, 1990; Dufey et al, 2013). Si, supplied as monosilic acid, can bind with Cu 2+ , Mn 2+ or Fe 2+ (Stevic et al, 2016). Such binding to monosilic acid may also occur with Fe EDDHA , as was supplied in this research, and may decrease the bioavailability in rice, a strategy II species.…”