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Translocation:

Plants use sunlight, carbon dioxide and water to manufacture glucose, yielding oxygen as a by-product. Sunlight or radiant light is captured by the green pigment chlorophyll inside of chloroplasts to provide the energy for photosynthesis to occur. Once the food is manufactured in the leaves it needs to be distributed to the entire plant so that the glucose can be used by each cell for respiration and some of the photosynthetic products are then stored for later use.

The glucose is manufactured mainly in the palisade cells where there are more chloroplasts, and then passes into the phloem. Plants usually transport food in the form of the sugar sucrose because it is less reactive than glucose. Sucrose is transported to where it is needed in the the plant via phloem sap, and may be stored in roots, stems or fruit. Transport of food material from leaves to other parts of the plant is called translocation. Understanding the phloem structure is important to understanding how it transports food.

How the phloem functions

While the transport of water is usually unidirectional in xylem (upward or lateral), the movement of sugars in the phloem is multi-directional, and occurs by active transport, an energy-dependent process. Sucrose is actively transported against a concentration gradient into sieve-tube elements. The sieve-tube elements have no nuclei but the adjacent companion cells do. Companion cells are closely associated with sieve tubes and carry out all the cellular functions of the sieve tubes.

The cytoplasm of sieve tubes and companion cells is connected through numerous channels called plasmodesmata. These cytoplasmic connections allow the companion cells to regulate the content and activity of the sieve tube cytoplasm. The companion cells also help load the sieve tube with sugar and the other metabolic products that they transport throughout the plant. This lowers the water potential of the sieve-tube element, causing water to move in by osmosis, creating a pressure that pushes the sap down the tube. The metabolising cells of the plant actively transport sugars out of sieve-tube elements, producing exactly the opposite effect. The diagram below illustrates how the overall process works.

Diagram showing movement in the xylem and phloem vessels. Water movement is upwards in the xylem and lateral into and out of the phloem. Lateral movement also occurs into and out of the companion cells accompanying the phloem vessel.

Wilting and guttation 

We just discussed transpiration, and how leaves are constantly losing water vapour to the environment. However, what happens when there is not enough water in the soil to replace the water that was lost? Similarly, what happens when there is too much water? In the next section we discuss wilting, and why plants wilt and get `floppy’ in hot weather or after a long drought. We will also look at ways that plants can rid themselves of extra water when there is too much water in the environment and the plant has to cope with high root pressure and a low transpiration rate.

Wilting 

Plants need water to maintain turgor pressure. Turgor pressure is what provides the plant with much of its structural support. Have a look at figure  below which shows the effect of osmosis on the turgidity of cells.

Cells in solutions with different concentrations

Wilting refers to the loss of rigidity or structure of non-woody parts of plants . It occurs when turgidity of plant cells is lost. When a cell absorbs water, the cell membrane pushes against the cell wall. The rigid cell wall pushes back on the cell making the cell turgid. If there is not enough water in the plant, the large central vacuole of the cell shrinks and the cytoplasm decreases, resulting in decreased pressure being exerted on the cell membrane, and in turn, on the cell wall. This results in the cell becoming flaccid (floppy). When the cells of a plant are flaccid, the entire plant begins to wilt.

Wilting occurs due to lower availability of water which may be due to:

• Drought conditions: where the soil moisture drops below conditions that allow plants to grow.
• Low temperatures: which prevent the plants vascular transport system from functioning;
• High salinity(salt concentration): which causes water to diffuse from plant cells to the soil, thus inducing shrinking of cells.
• Bacterial or fungal infections: that block the plant’s vascular system.

Guttation 

Guttation is the “oozing out” or exuding of drops of water on the tips or edges of leaves of some vascular plants.

Below is an explanation of how guttation occurs:

• At night, when it is dark, less transpiration occurs since the stomata are closed.
• When soil moisture is very high, water will enter the plant roots because the water potential of the roots is lower than that of the surrounding soil.
• Thus, water accumulates in the plant, resulting in root pressure.
• The root pressure forces some water to exit the leaf tip or edge structures called hydathodesor water glands, forming drops.
• Root pressure is what drives the flow of water out of the plant leaves, rather than transpirational pull.

For guttation to occur there must be a high water content in the soil to create the root pressure. The transpiration rate must be slow in order for the root pressure to be higher than the transpirational pull. Guttation must not be confused with transpiration. Table 5.3 highlights the differences between guttation and transpiration.

Table 5.3: Table comparing guttation and transpiration.