Neutron Imaging to Reveal Phloem Function in Trees

Understanding phloem function is key to understanding the regulation of plant function. The phloem distributes the main building blocks that support plant function and growth from the production sites to sites of consumption and storage. It also distributes important signaling molecules and interacts with the adjacent water-transport tissue, the xylem, to support hydraulic integrity of plants. Yet, studying phloem function is challenging because of the sensitivity of this tissue to mechanical stimulation that can lead to blockage of flow. Furthermore, its small size, adjacent location, and interactions with the much larger xylem tissue that operates under an opposite pressure gradient to the phloem make experiments of phloem function in opaque, woody tissues challenging. Here, we present a method to measure water transport rates within the phloem and between the xylem and the phloem using neutron imaging with heavy water, D₂O, as a tracer. The method is based on differences in neutron attenuation by hydrogen and deuterium atoms that create a contrast when imaging plant tissues or soil that are relatively transparent to neutrons. This method allows nondestructive visualization of water flow within the xylem and phloem tissues and calculation of flow rates at the spatial and temporal resolution determined by the experimental setup.