create sulfate uptake assay and protocol

assays/SulfateUptake/protocols/SulfateUptakeProtocol.md

+## Sulfate uptake
+
+Sulfate uptake was measured in seedlings grown for 2 weeks in 12-well plates on a modified Long-Ashton medium under S-sufficient or S-deficient conditions. For uptake measurement, the medium was exchanged with 1 ml of the Long-Ashton medium with 0.2 mM sulfate supplemented with 12 mCi of [35S]sulfuric acid and incubated for 30 min in the light. Whole seedlings still on the mesh were washed thoroughly, blotted dry, and shoot and root samples were cut, weighed, and stored separately in liquid nitrogen until further processing on the same day. Samples were extracted in a 10-fold volume of 0.1 M HCl. A 10 μl aliquot of extract was used to determine sulfate uptake by scintillation counting (Dietzen et al., 2020).
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