Measuring Phloem Transport Velocity on a Tissue-Level Using a Phloem-Mobile Dye

Here, we describe an in vivo dye-tracking method for measuring phloem transport velocity in seedlings, leaves, and petioles and potentially other translucent plant tissues. The method requires measurement of the fluorescent signal of a phloem-mobile dye using sensitive photo-sensors placed externally to the plant. Following dye application, velocity is determined by either following a dye pulse or using laser fluorescence bleaching. Velocity is estimated by dividing the distance traveled by the dye by the time it takes to travel. This method can be used to measure phloem transport velocity on intact plants with minimal disturbance and has the potential to be used under a variety of growth conditions. Because there are large differences among species in their anatomy, this method should be optimized for individual plants and tissue types.