Determination of the Phloem Mass Flow Velocity and Calculation of the Volumetric Flow Rate.

Here, we describe a microscopic approach to measure the mass flow velocity and to calculate the volumetric flow rate in the midrib of leaves using a phloem-mobile fluorescent dye. The method requires measurements of the carboxyfluorescein (CF) fluorescence using a fluorescent microscope and the determination of the sieve element (SE) cross-sectional area. The translocated CF within the vascular system is tracked in the leaf using cross-sections and the mass flow velocity is deduced. Additionally, SE cross-sectional areas are measured in the center of the leaf at a high resolution to calculate the volumetric flow rate per SE. This method can be applied to all higher plants. However, as the velocity depends on various parameters like plant species, age, and developmental stage, all data needs to be collected and analyzed in each experiment.