Transport in plants

TRANSPORT IN PLANTS

Plant vascular system is made up of xylem and phloem vessels, which are the transport vessels of plants. They transport water and minerals from the soil and manufactured foods from the leaves to the rest of the plant. As we have already seen, the division of the apical meristems of roots, stems and leaves gives rise to plant vascular tissues as well as ground and epidermal tissues. These form the primary tissues of plants.

The arrangement of the vascular tissues in monocots is different from the arrangement in the dicots. Dicot tissues are arranged in discreet bundles around central pith, while monocots have no definite arrangement and are scattered.

 sections of a monocot and dicot showing arrangement of xylem and phloem

 In the dicots, the xylem is separated from the phloem by a vascular cambium that is absent in monocots. At maturity the cambium of the vascular system becomes active and begins to divide, adding more layers to the inner xylem and outer phloem. These new layers form the secondary xylem and secondary phloem.

The xylem is mainly wood and forms the woody part of a tree. Unlike the secondary xylem which accumulates new layers of wood, secondary phloem is pushed further to the epidermal wall as xylem increases, until it becomes part of the bark system that is eventually lost when the bark is shed.

Xylem consists mainly of transport cells called tracheids and vessel elements. Tracheids are thin-walled, long cells, tapered at the ends and with numerous pits on their surface, which allow water to move from one cell to another. Vessel elements are individual cells, which are linked to one another into long channels or tubes called xylem vessels.

The tracheids and the vessel elements are dead cells and therefore have no cytoplasm or protoplast, except a hollow space. They have cell walls that are strong and hard due to the deposition of lignin and the presence of fibre cells.

Tracheids and vessel elements

Phloem cells, unlike those of xylem, are alive and consist of fibre cells, sieve elements and companion cells. The companion cells have prominent nuclei. Their role is not quite known but they probably provide the sieve tube elements with some nutrients.

Transpiration

The movement of water and minerals from the soil through the xylem and phloem vessels and out through the leaves is called transpiration. During transpiration, water is absorbed by the root hairs by osmosis. Absorbed substances traverse the cortex and enter the vascular system through the endodermis. The endodermis regulates the substances that can enter the plant vascular system. However, some substances can get through the endodermis by active transport.

Transport through the Xylem and Phloem

The xylem and the phloem form transport channels that are continuous throughout the plant. Sugars manufactured in the leaves and water from the soil have to be distributed throughout the plant. The xylem transports water and minerals absorbed by the roots, while the phloem transport of glucose and other food substances that are manufactured by the leaves.

How do the xylem channels manage to transport water and mineral salts from the ground up to the tops of tall plants?

When the water that has been absorbed by the roots reaches the leaves, it is lost by evaporation through the stomata. A kind of a vacuum is created within the vascular system; more water from the roots moves to occupy the vacuum and is again lost by evaporation. Eventually a root pressure that pushes the sap (water and minerals) within the xylem upwards is created.

This can be demonstrated by attaching a glass tube to the end of a cut end of a plant. After some time, it is noted that the water level has risen beyond the cut end of the plant because of root pressure.

In grasses and other small plants, the root pressure is responsible for the presence of water drops on plants, which normally occurs at night when transpiration is not taking place. This loss of water by root pressure is called guttation.

Insert diagram showing root pressure.

However, root pressure alone is not sufficient to account for the movement of water in tall trees some of which are 60 m tall. Transport of water within tall trees is explained by what is known as the Tension Theory of Water Transport (TTWT). According to this theory, water molecules are usually cohesive and therefore tend to be attracted to one another and because of its cohesive nature water can flow easily in tiny tubes by capillary force. Water flowing through the vascular bundles is due to osmotic pressure, capillary force and the effect of the enviromental factors.

This cohesiveness of water makes the plant cells fill with water and become turgid. However, after the water is lost by evaporation through the leaves, the mesophyll cells of the leaf become less turgid, creating some kind of a vacuum. More water from the xylem vessels moves up to occupy the vacuum. The force needed to push water from the xylem to the leaves is called the transpirational pull.

How Stomata Open and Close: The Role of K⁺ ions

Each stoma is flanked by two kidney-shaped guard cells that control the diameter of the stoma. By changing their shape the guard cells widen or narrow the gap between them.

When guard cells take up water by osmosis, they swell and become turgid and the gap between the cells widens. If the guard cells lose the water and become flaccid, the space between them becomes smaller

The changes in turgor pressure therefore control the opening and closing of the guard cells.

 The changes in turgor pressure are due to the reversible absorption and loss of potassium ions (K⁺) by the guard cells.

The stomata open when the guard cells actively accumulate K⁺ from the surrounding epidermal cells. The uptake of K+ increases the osmotic concentration within the guard cells, making the cells to become more turgid as water is drawn in by osmosis.

The influx of K⁺ increases the positive charge which is counterbalanced by the uptake of chlorine ions (Cl^-) by the pumping out of the cell H⁺ ions released by the organic acids within the cells and by the negative charges of these organic acids after losing their charges. Efflux of K⁺ from the guard cells causes the guard cells to close, due to an osmotic loss of water.

Generally, the stomata open at dawn and remain open during daytime and close at night. This prevents plants from losing water when photosynthesis is not taking place. At least three factors influence stomatal opening at dawn.

Firstly, light stimulates guard cells to accumulate potassium and become turgid. Secondly, the depletion of CO₂ within air spaces of the leaf which occurs when photosynthesis commences in the mesophyll (By placing a plant in a chamber devoid of CO₂ at night, it can open its stomata). Thirdly, there is an internal clock in the guard cells, as indeed in most biological systems, that ensure that the stomata will continue to open and close even when the plant is kept in a dark place.

Demonstrations of the Transpirational pull

Tanspirational pull can be demonstrated in the following experiment. Three glass tubes A, B, C are placed in three beakers, each containing mercury. Air is evacuated from tube A and the top end clamped. Tube B is attached to a plant twig; tube C is attached to a porous clay cup. They are allowed to settle for sometime.

Diagram showing tanspirational pull

Results show that the mercury in A rises according to the atmospheric pressure. When a plant twig is added, as shown in B, water vapour evaporates from the leaves and this creates a tanspirational pull. Together with the atmospheric pressure, the level of mercury rises above the level of A. Evaporation of water through the clay porous cup in C seems to exert even a higher tanspirational pull.

Translocation

Phloem sieve tubes conduct sugar, amino acids, hormones and even some mineral ions from the leaves to other parts of the plant. This is called translocation. As plants become older, the phloem sieve tubes are pushed farther away by the growth of the secondary xylem towards the outside until they become part of the bark. That is why the practice of girdling trees by removing strips of bark prevents manufactured substances from reaching the lower parts of the tree so that the plant is starved of nutrients and eventually dies.

It is possible to show that sugar is transported down the phloem by labelling leaves with radioactive ¹⁴ carbon. If a photosynthesising leaf if placed in a bottle as shown in the diagram and radioactive liquid CO₂ is introduced into the bottle using a syringe, it is discovered that only sugar within the phloem is labelled with ¹⁴ carbon. If a ring of phloem is removed from the stem before setting up the experiment, there will be no traces of radioactivity in the parts of plant below the ring. This shows that sugars are transported within the phloem vessels.

Diagram of the demonstration of phloem transport

Carbon dioxide enters the plant through the stomata by a diffusion gradient. The spongy layer of the leaf has air spaces between the cells, which CO₂ can occupy. Carbon dioxide diffuses easily through the membranes of the cell to reach the chloroplasts where photosynthesis takes place.

Factors that Influence Transpiration

(1) Relative humidity

(2) Wind and air currents

(3) Water supply,

(4) Light

Temperature

altitude

If there is much water vapour in the atmosphere, the air will be saturated with water and the relative humidity will therefore be very high. Transpiration will be very low. Wind increases humidity by blowing water vapour near the plant away, thus creating room for more water vapour. If water supply is scarce, the stomata close and reduce transpiration. Conversely, if water is plentiful the stomata open and transpiration increases.

Light intensity and temperature promote transpiration. Light is needed for photosynthesis, which uses CO₂ and water. Increasing light intensity makes the stomata to open to allow CO₂ to diffuse into the leaves. This increases evaporation of water from the leaves as well, thus promoting transpiration. Hot temperatures increase evaporation, which in turn increases transpiration.

To Demonstrate the Rate of Transpiration

Take two potted plants and cover one completely with a polythene bag and the other cover the pot including the roots only. Make sure that the polythene bags are properly secured so that no air or water can escape from the pots. Weigh the pots and continue weighing daily at the same time for at least one week. Record your results on a graph. The graph should show a loss of weight in the pot where the plant was not completely covered.

Insert diagram

Study Questions

1. With the use of diagrams show the arrangement of vascular tissues in monocots and dicots.

2. Draw and name tracheids, vessel elements, sieve elements and companion cells. Show how the structure of each is suited to its function.

3. What are the functions of the phloem and how are they different from those of the xylem? Trace the path of water and mineral ions from the root hairs to the xylem.

4. Define the following terms: guttation, girdling and translocation.

5.Discuss the factors that influence the rate of transpiration.