Susan Dunford, University of Cincinnati

Mass flow in the phloem as outlined above is the usual textbook description, in which pressure is emphasized as the driving force and the controlling factor for flow. Bulk flow in a simple tube is driven by a pressure difference between the ends of the tube; examples include water flow in xylem vessels and in a garden hose (see textbook Chapter 3). However, sieve elements are not simple tubes.

Recent analysis emphasizes the role of solute exchange by the sieve elements as controlling flow in the phloem and stresses the fact that pressure differences exist only as a result of resistances to flow. In this view, a good analogy for the phloem when sources are providing an ample supply of sugars might be one in which sinks exist along the pathway like pigs eating at a trough. If the trough is full, “a local disturbance created by a single hungry pig is quickly equalized by a small, bulk displacement toward that pig, accompanied by a small reduction in what is available to the others” (Thompson 2006).

A model is only as good as the assumptions made in its design and the measurements of the required parameters. The “pigs at a trough” model (or osmoregulatory flow) predicts that the phloem transport system will work most efficiently with relatively small turgor differences between sources and sinks (Thompson 2006). In such a homogenous system, turgor in all sieve elements in the path can be regulated simultaneously and independently. Information in the form of pressure waves would be rapidly transmitted, resulting in an efficient sieve tube that could rapidly transmit information on changes in the pressure or concentration of sap over long distances.

What kinds of measurements might enable us to distinguish between a mass flow with large pressure differences between sources and sinks and one with small gradients? One obvious answer is to measure pressure gradients between sources and sinks. Such studies have been relatively few in number and restricted to small herbaceous plants. More measurements on a wider variety of plants are needed, necessitating the development of techniques that can measure turgor differences along the same transport stream, ideally in large plants such as trees. Developing such techniques will be an enormous technical challenge.