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

Abstract

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.). 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. 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\xa0mM 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+. 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.