Phloem transport in trees

Abstract

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.

Citation

Ryan, Michael G.; Asao, Shinichi. 2014. Phloem transport in trees. Tree Physiology. 34: 1-4.