Plant transport of solutes
Plant transport of solutes
The movement of organic and inorganic compounds through plant vascular tissues. Transport can take place over considerable distances; in tree species transport distances are often 100–300 ft (30–100 m).
This long-distance transport is necessary for survival in higher land plants in which specialized organs of uptake or synthesis are separated by a considerable distance from the organs of utilization. Diffusion is not rapid enough to account for the amount of material moved over such long distances. Rather, transport depends on a flowing stream of liquid in vascular tissues (phloem and xylem) that are highly developed structurally.
The movement of organic solutes occurs mainly in the phloem, where it is also known as translocation and where the direction of transport is from places of production, such as mature leaves, to places of utilization or storage, such as the shoot apex or developing storage roots. Organic materials translocated in the phloem include the direct products of photosynthesis (sugars) as well as compounds derived from them (nitrogenous compounds and plant hormones, for example). Some movement of organic solutes does occur in the xylem of certain species. Inorganic solutes or mineral elements, however, generally move with water in the xylem from sites of uptake in the roots to sites where water is lost from the plant, primarily the leaves. Some redistribution of the ions throughout the plant may then occur in the phloem.
The mechanism of phloem translocation is not known with certainty. Proposed mechanisms fall into two classes: one stresses the role of the conducting tissues in generating the moving force, and the other views the regions of supply and utilization as the source of this force. In the former group are mechanisms that depend on cytoplasmic streaming, electroosmosis, and activated diffusion in the sieve elements. The second group of theories, which has received more general acceptance in spite of a number of admitted limitations, includes a variety of mass-flow mechanisms. Theories of translocation must account for the important observations: polarity, bidirectional movement, velocity, energy requirement, turgor pressure, and phloem structure.
The model for the ascent of sap in the xylem which is correct according to all present evidence is called the cohesion hypothesis. According to this hypothesis, water is lost in the leaves by evaporation from cell-wall surfaces; water vapor then diffuses into the atmosphere by way of small pores between two specialized cells (guard cells). The guard cells and the pore are collectively called a stomate. This loss of water from the leaf causes movement of water out of the xylem in the leaf to the surfaces where evaporation is occurring. Water has a high internal cohesive force, especially in small tubes with wettable walls. In addition, the xylem elements and the cell walls provide a continuous water-filled system in the plant. Thus the loss of water from the xylem elements in the leaves causes a tension or negative pressure in the xylem sap. This tension is transmitted all the way down the stem to the roots, so that a flow of water occurs up the plant from the roots and eventually from the soil. The velocity of this sap flow in tree species ranges from 3 to approximately 165 ft/h (1 to 50 m/h), depending on the diameter of the xylem vessels.