Transport Of Food Substances Organic Solutes In Solution

One of the most important functions of water in the plant involves the translocation (transportation) of food substances in solution by the phloem, a process that has only recently come to be better understood. Many of the studies that led to our present knowledge of the subject used aphids (small, sucking insects) and organic compounds designed as radioactive tracers.

Most aphids feed on phloem by inserting their tiny tubelike mouthparts (stylets) through the leaf or stem tissues until a sieve tube is reached and punctured. The turgor

Water And Food Transport Plants
Figure Q.1S Droplets of guttation water at the tips of leaves of young barley plants.
Guttation Plants

Figure Q.16 An aphid feeding on a young stem of basswood (Tilia). A droplet of "honeydew" is emerging from the rear of the aphid, x10. From Martin H. Zimmerman, "Movements of Organic Substances in Trees" Science 133: 73-79, 1961, American Association for the Advancement of Science.

pressure of the sieve tube then forces the fluid present in the tube through the aphid's digestive tract, and it emerges at the rear as a droplet of "honeydew." In some studies, research workers anesthetized feeding aphids and cut their stylets so that much of the tiny tube remained where it had been inserted. Fluid exuded (sometimes for many hours) from the cut stylets and was then collected and analyzed (Fig. 9.16).

Carbon dioxide, a basic raw material of photosynthesis, can be synthesized with radioactive carbon. By exposing a photosynthesizing leaf to radioactive carbon dioxide, the pathway of manufactured food substances can be traced. The radioactive substances produce on photographic film an image corresponding to the food pathway. Data obtained from such studies reveal that food substances in solution are confined entirely to the sieve tubes while they are being transported. At one time, it was believed that ordinary diffusion and cyclosis (discussed in Chapter 3) were responsible for the movement of the substances from one sieve tube member to the next, but it is now known that the substances move through the phloem at approximately 100 centimeters (almost 40 inches) per hour—far too rapid a movement to be accounted for by diffusion and cyclosis alone.

The Pressure-Flow Hypothesis

At present, the most widely accepted theory for movement of substances in the phloem is called the pressure-flow (or mass flow) hypothesis. According to this theory, food substances in solution (organic solutes) flow from a source, where water enters by osmosis (e.g., a food-storage tissue, such as the cortex of a root or rhizome, or a food-producing tissue, such as the mesophyll tissue of a leaf). The water leaves at a sink, which is a place where food is utilized, such as the growing tip of a stem or root. Food substances in solution (organic solutes) are moved along concentration gradients between sources and sinks (Fig. 9.17).

direction of water flow direction of food solute flow direction of water flow direction of food solute flow

Water And Sugar Movement Plants

1. The source of the sugar molecules is the leaf cells

1. The source of the sugar molecules is the leaf cells

3. The increased sugar concentration causes water from the xylem to enter 0 the sieve tubes. ° 0 This increases water turgor pressure. molecules sugar molecules

The increased turgor pressure drives the fluid in the sieve tubes toward a sink, such as a storage parenchyma cell in a root.

As the sugar molecules are actively removed at the sink, the pressure in the sieve tubes there drops, causing a mass flow from the higher pressure at the source to the lower pressure at the sink. Most of the water then diffuses back to the xylem.

Figure Q.17 The pressure-flow hypothesis.

Chapter 9

First, in a process called phloem-loading, sugar, by means of active transport, enters the sieve tubes of the smallest veinlets. This decreases the water potential in the sieve tubes, and water then enters these phloem cells by osmosis. Turgor pressure, which develops as this osmosis occurs, is responsible for driving the fluid through the sieve-tube network toward the sinks.

As the food substances (largely sucrose) in solution are actively removed at the sink, water also leaves the sink ends of sieve tubes, and the pressure in these sieve tubes is lowered, causing a mass flow from the higher pressure at the source to the lower pressure at the sink. Most of the water diffuses back to the xylem, where it then returns to the source and is transpired or recirculated. The pressure-flow hypothesis explains how nontoxic dyes applied to leaves or substances entering the sieve tubes, such as viruses introduced by aphids, are carried through the phloem.

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