Phloem transport in trees.

Phloem is like an enigmatic central banker: we know how important phloem is to plant function, but very little about how phloem functions as part of a whole-plant economy. Phloem transports carbohydrates, produced by photosynthesis and hydrolysis of reserve compounds, to sink tissues for growth, respiration and storage. At photosynthetic tissues, carbohydrates are loaded into phloem (Rennie and Turgeon 2009), a process that raises the solute concentration. This increased solute concentration then raises turgor pressure in the transport stream by drawing water from the xylem through osmosis. At growth and storage sinks, carbohydrates are actively unloaded or passively leak out of phloem, lowering the solute concentration. Water then moves back into the xylem from the phloem, lowering turgor, and the turgor pressure difference between the loading and unloading sites drives the mass flow of carbohydrates to the sink tissues. This simple mechanism of turgor-driven transport, first hypothesized by Münch in 1927 (Münch 1930), connects source and sink tissues, automatically delivering photosynthate to sink tissues with the lowest concentrations and thus the highest consumption rates and need. Turgor-driven phloem transport as simplified into a mathematical model can explain phloem transport for short distances (Christy and Ferrier 1973) and distances of up to 5–10 m (Thompson and Holbrook 2003). For a tree, these models would predict that phloem turgor pressure at the source would need to increase with canopy height to overcome resistances caused by transport distance (Thompson and Holbrook 2003). Consequently, the phloem of tall trees in the upper canopy would need higher turgor than herbaceous plants and a greater difference in turgor between the upper canopy and roots for effective carbohydrate transport. Though very few data exist to test them, these predictions are unsupported. Phloem turgor pressure is low in some trees (Sovonick-Dunford et al. 1981), too low to drive photosynthate out of the phloem (Turgeon 2010), and phloem turgor changes little to none with height in other trees (Hammel 1968, Lee 1981). More measurements are needed, but the turgor pressure itself complicates these measurements. Because phloem is pressurized, puncturing or damaging the phloem in an attempt to measure it causes a sudden release of pressure and alters phloem anatomy and thus the phloem pressure itself (Turgeon and Wolf 2013). Also, because phloem consists of live tissues, an attempt to measure it often induces wound reactions (Ehlers et al. 2000, van Bel 2003), contributing to the lack of data. However, tall trees exist and their roots are supplied with carbohydrates despite the fact that our understanding, models and measurements suggest that they should not. This disconnect between theory, data and tree behavior suggests that phloem transport in trees is a fertile area for research. Woodruff (2014) examined a problem in tree phloem physiology that is important for understanding transport in tall trees and also for understanding how drought might impact phloem anatomy, phloem sap viscosity and carbohydrate transport. The study examined how sieve cell radius, sap sugar concentration, phloem relative water content and sap viscosity might vary with mid day leaf water potential under well-watered soil water conditions in tree tops, with tree height varying from 2 to 57 m. Tree height generated a strong gradient in mid day leaf water potential from −1.2 MPa in 2-m-tall trees to −1.8 MPa in 57-m-tall trees. Because this gradient was assessed under well-watered conditions with low evaporative demand, we can assume that these water potentials are near the minimum that will be attained during a diurnal cycle under Commentary