Magnesium isotope fractionation reflects plant response to magnesium deficiency in magnesium uptake and allocation: a greenhouse study with wheat

Abstract

Abstract Aims Magnesium (Mg) deficiency is detrimental to plant growth. However, how plants respond to Mg deficiency via regulation of Mg uptake and allocation is yet not fully understood. In this study, we tested whether Mg isotope compositions (δ26Mg) associated with Mg mass balance of the plants could be used as an indicator to trace Mg uptake and subsequent translocation processes under sufficient and low-Mg supply conditions. We aimed at using stable isotope fractionation as a novel proxy for nutrient uptake and cycling in plants. Methods We grew wheat plants (Triticum aestivum) in a greenhouse under control (1 mM Mg) and low-Mg supply (0.05 mM Mg) conditions, respectively. The Mg concentrations and isotope compositions in roots, stems, leaves and spikes/grains at different growth stages were analyzed. Results Wheat plants were systematically enriched in heavy Mg isotopes relative to the nutrient solution regardless of Mg supply conditions. With crop growth, the δ26Mg of the whole plants, as well as each plant organ, gradually shifted towards higher values in the control. However, the δ26Mg value of the whole plants in the low-Mg supply did not vary significantly. In addition, the wheat stems and spikes showed continuous enrichment of lighter Mg isotopes in the low-Mg supply than those in the control. Conclusions As reflected from Mg isotope compositions, the Mg supply in the growth media could affect the Mg uptake and subsequent translocation processes in plants. Changes in δ26Mg indicated that wheat plants likely regulated their Mg uptake strategy by switching between active and passive pathways during their life cycle. When Mg supply was low, a more negative δ26Mg value of the spikes suggested a potentially enhanced remobilization of Mg from leaves to spikes. Our results showed that Mg stable isotopes can provide new insights into plants’ response to nutrient shortage.