MASS FLOW

also called Pressure Flow, in botany, the most widely accepted explanation for the movement of sugars and other nutrient solutes through the phloem. The mass-flow hypothesis explains how foods move from source areas, where they are manufactured (mainly in the leaves) or stored (such as in the storage tissues of stems and roots), to sink areas, where they are metabolized or stored. (Storage tissues, it should be noted, can serve as either a source or sink, depending on the direction of the nutrient flow.) According to the mass-flow hypothesis, dissolved nutrients (primarily sugars) move from a source into the sieve tubes of the phloem via active transportthat is, by a process that involves the expenditure of energy. This inflow of solutes causes the contents of the sieve elements (sieve-tube cells) in the region of the source to become hypertonic, and water flows into these cells by osmosis. This influx of water raises the osmotic pressure in the sieve elements; eventually, the increased pressure causes the fluid contents of the cells to flow into adjacent sieve elements. This mass flow carries the nutrient solutes with it, and, thus, the receiving sieve elements now become hypertonic and the process repeats itself. The mass flow proceeds along the length of the sieve tube until the sink is reached; at this point, nutrient molecules are removed from the sieve tube by active transport. Their removal makes the sink tissue hypertonic, and water osmotically flows out of the phloem into the sink. The mechanism of food translocation as envisioned in the mass-flow hypothesis thus involves active transport at each end of the sieve tube, with the passive movement of foods through the tube by means of osmotic pressure build-up and resultant bulk flow. The mass-flow hypothesis does not satisfy objections that much higher turgor pressures (internal pressure in plant cells caused by osmosis) should exist within the sieve tubes than has been shown; that pressure gradients from sources to sinks should be more clear-cut; that the observed rates of transport are higher than mass flow can explain; that transport is often simultaneously bidirectional; and that low temperature and lack of oxygen depress transport. These observations suggest that the movement of substances in sieve elements is under more control by the living cytoplasm of the cell than the mass-flow hypothesis allows for. Despite these objections, mass flow remains the best explanation for nutrient movement in plants.