Strontium as a tracer for calcium: uptake, transport and partitioning within tomato plants

Purpose: Calcium (Ca²⁺) is a major structural plant nutrient whose low mobility in the phloem causes deleterious nutritional disorders in non-transpiring organs. Since strontium (Sr²⁺) and Ca²⁺ share many chemical properties, Sr²⁺ is frequently used as a tracer to study Ca²⁺ cycles in ecosystems. However, the level of agreement between Sr²⁺ and Ca²⁺ distribution pattern in plants is debatable, and several studies have reported toxic effects of Sr²⁺. Therefore, we investigated Sr²⁺ and Ca²⁺ uptake rates and distribution pattern to determine how reliably Sr²⁺ can be used as a tracer of Ca²⁺ in tomato plants (Solanum lycopersicum L.). Methods: We conducted six independent experiments of various duration: from a few hours to several weeks, in hydroponic and perlite substrate. We treated plants with either Ca²⁺ or Sr²⁺ at equivalent concentrations and monitored their accumulation in shoot and fruits. Results: Under short-term exposure (hours), Ca²⁺ and Sr²⁺ uptake and distribution within the plant were comparable, while the long-term exposure (days and weeks) to 4 mM Sr²⁺ reduced transpiration and biomass accumulation. The toxic effect of Sr²⁺ was more prominent when growth conditions were favourable. Nonetheless, Sr²⁺ accumulated similarly to Ca²⁺ in shoot and fruit. Surprisingly, Sr²⁺ deposition in tomato fruit cell walls prevented blossom end rot (BER) to the same degree as Ca²⁺. Conclusion: Sr²⁺ can credibly be used as a tracer of Ca²⁺ uptake and allocation in the short-term, making Sr²⁺ a powerful tool to study the factors governing Ca²⁺ allocation to plant organs, primarily fruit Ca²⁺ delivery.