Rheology and Conductivity of Phloem

SIerra Beecher

The Pressure Flow Hypothesis, presented in 1930 by Ernst Münch, is the most accepted model for photoassimilate translocation by the phloem of plants. Net photosynthetic sugar production occurs in source tissues, often mature leaves. These photoassimilates are transported along the sieve tube system to consumptive sink tissues such as roots, meristems, and seeds. Münch’s hypothesis states that a sugar concentration gradient between sources and sinks osmotically generates a hydrostatic pressure differential that drives sap flow. If phloem sap is driven by a pressure differential, geometries of the sieve tube system, and the rheology of phloem sap should scale to equations describing pressure flow in low Reynolds number tubes. Imaging methods developed in this work provide robust geometric and rheological data, expanding the phloem biophysics toolkit. Additionally, insight into a mysterious phenomenon in which transient inhibition of translocation is induced by sudden localized stem chilling is presented. A novel application of fluorescence recovery after photobleaching (FRAP) allowed simultaneous measurement of sap velocity, and diffusivity of a phloem-mobile probe. This technique was used to investigate the cold shock phenomenon directly at the site of cold stimulus at higher resolution than was previously possible. Cold-induced sap velocity declines were not accompanied by similar reductions in sap viscosity. A Fluorescence Lifetime Imaging Microscopy (FLIM) method was also developed, providing direct in vivo measurements of viscosity in translocating phloem sap. Values obtained indicate phloem sap viscosities of less than 2 mPas, consistent with sugar concentrations estimated by previous time-intensive exudate analyses. Questions regarding pressure flow in trees, where tube lengths are very long and source phloem turgor pressures have been measured as surprisingly low were also addressed. Sieve element lengths, diameters, and sieve plate angles were measured in 3 positions of 22 different angiosperms. These conductivity factors were large, and scaled to stem length in trees. Because pressure flow equations indicate an inversely proportional relationship between conductivity and pressure, these data are consistent with pressure driven flow as the mechanism for translocation in trees.

Rheology and Conductivity of Phloem

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