Measuring Esculin Export from Leaves as Proxy for Sucrose Loading Rates in the Phloem

Phloem loading of sucrose and the transport from source leaves to sink tissues is vital for plant growth and carbon allocation. Traditional methods to measure phloem loading are often time-consuming or require specialized equipment. Here, we introduce a rapid, cost-effective esculin-based fluorometric assay as a reliable proxy for sucrose loading. Esculin, a fluorescent coumarin glucoside, is specifically transported by sucrose transporters in plants with active apoplastic phloem loading. After application to source leaves, esculin fluorescence is measured in the extracted leaf sap, providing a sensitive and relatively high-throughput method for analyzing phloem loading dynamics. Validated against established techniques, the assay is accessible to nonspecialized laboratories and enables investigations into environmental and developmental regulation of phloem loading, offering insights into plant growth and stress responses.