Nuclear Magnetic Resonance imaging of sap flow in plants

Long distance transport in plants occurs in conduits that are mostly too small to see with the naked eye. The implications of the process however are large, and are evident on a global, ecological and evolutionary scale. The ability to internally distribute water and solutes with vascular tissues has been one of the key steps that enabled plants to conquer the land in the Silurian period, some 400 million years ago (Lösch, 2001, Westoby & Wright, 2006). Without vascular tissues plants would not have been able to reach a size larger than a few centimetres. Non-vascular terrestrial plants have also evolved and still exist today (e.g. mosses and liverworts), but in terms of biodiversity and biomass they have not been nearly as successful as the vascular ones. On a global scale, the availability of water is one of the major factors that limit plant production and yield. Plants need to take up CO2 for photosynthesis. Because they do not possess membranes that are permeable to CO2, while at the same time being impermeable to water, evaporative water loss is an inevitable consequence of taking up carbon dioxide (Sperry, 2004). As many as 100 to 1000 water molecules are lost per molecule of CO2 assimilated (Maseda & Fernandez, 2006). Transpiration in higher plants accounts for about one eighth of all the water that evaporates to the atmosphere all over the globe, and for about three-quarters of all water that evaporates from land. Long distance transport in plants thus directly affects the global water cycle, and with that, global climate (Sellers et al., 1997).

Long distance transport occurs along two parallel pathways, the xylem and the phloem (Fig. 1). The xylem is primarily responsible for the transport of water and nutrients from the soil to the leaves, growth tips, fruits and flowers; while the phloem is responsible for the transport of photosynthates such as sucrose from leaves to the rest of the plant. A secondary function of these two vascular systems is to provide a pathway for signaling. Plants lack a nervous system, but use signaling compounds such as ions, phytohormones and even RNA molecules in combination with their vascular systems to achieve communication between plant organs.