Using fluorescence recovery after photobleaching to calculate viscosity and velocity in the phloem sap of roots in Arabidopsis thaliana (594.2)

Abstract

The sieve tube system (STS) of phloem is the conduit by which energetic molecules for plant growth and storage are transported from the leaves, where they are constructed (sources), to non‐photosynthetic regions of the plant (sinks). A clear understanding of the physics and physiology of this system is crucial, as seeds and other storage organs of plants are staple foods for most human diets, and plant growth is dependent on this phenomenon. The pressure flow hypothesis, presented in 1930 by Ernst Munch states that a pressure differential built from a concentration gradient of photosynthate (mainly sucrose, in the case of A. thaliana) causes sap to flow from sources to sinks in vascular plants. If this is the case, in vivo measured parameters of this system should conform to the Hagen‐Poiseuille equation for pressure flow. Using confocal laser scanning microscopy (CLSM) to image young root phloem in Micro‐ROCS© (Rhizosphere Observation Chambers) has allowed measurement of several geometric features of phloem relevant to the Hagen‐Poiseuille equation. By making films of the SES refilling with dye‐loaded sap after photobleaching in specific regions, translocation can be observed and measured. Analyzing intensity recovery at regions along this refilling pathway generates curves that reveal not only translocation, but also diffusive properties of the fluorophore. Small capillary tubes were used as “simulated phloem” to verify this method with known concentrations of the fluorophore dissolved in solutions of known viscosity. Similar FRAP methods were used to analyze parameters of conditions in the tubes, and compare them with the measurements obtained from plants.