Water

Water is the major constituent of any living organism and the maintenance of a plant with optimum water content is a very important part of plant growth and development (see Soil water). Probably more plants die from lack of water than from any other cause. Minerals are also raw materials essential to growth, and are supplied through the root system.

Functions of waters
The plant consists of about 95 per cent water, which is the main constituent of protoplasm or living matter. When the plant cell is full of water, or turgid , the pressure of water enclosed within a membrane or vacuole acts as a means of support for the cell and therefore the whole plant, so that when a plant loses more water than it is taking up, the cells collapse and the plant may wilt. Aquatic plants are supported largely by external water and have very little specialized support tissue. In order to survive, any organism must carry out complex chemical reactions, which are explained, and their horticultural application described, in Plant growth. Raw materials for these chemical reactions must be transported and brought into contact with each other by a suitable medium; water is an excellent solvent. One of the most important processes in the plant is photosynthesis, and a small amount of water is used up as a raw material in this process.

• Diffusion is a process whereby molecules of a gas or liquid move from an area of high concentration to an area where there is a relatively lower concentration of the diffusing substance, e.g. sugar in a cup of tea will diffuse through the tea without being stirred – eventually! If the process is working against a concentration gradient, energy is needed.
• Osmosis can therefore be defined as the movement of water from an area of low salt concentration to an area of relatively higher salt concentration, through a partially permeable membrane. The greater the osmotic pressure then the faster water moves into the root cells, a process which is also affected by increased temperature.

Movement of water
Water moves into the plant through the roots, the stem, and into the leaves, and is lost to the atmosphere. Water vapour moves through the stomata by diffusion from inside the leaf into the air immediately surrounding the leaf where there is a lower relative humidity.

The pathway of water movement through the plant falls into three distinct stages:

• water uptake;
• movement up the stem;
• transpiration loss from the leaves.

Water uptake
The movement of water into the roots is by a special type of diffusion called osmosis. Soil water enters root cells through the cell wall and membrane. Whereas the cell wall is permeable to both soil water and the dissolved inorganic minerals, the cell membrane is permeable to water, but allows only the smallest molecules to pass through, somewhat like a sieve. Therefore the cell membrane is considered to be a partially permeable membrane.

A greater concentration of minerals is usually maintained inside the cell compared with that in the soil water. This means that, by osmosis, water will move from the soil into the cell where there is relatively lower concentration of water, as there are more inorganic salts and sugars. The greater the difference in concentration of inorganic salts the faster water moves into the root cells.

If there is a build-up of salts in the soil, either over a period of time or, for example, where too much fertilizer is added, water may move out of the roots by osmosis, and the cells are then described as plasmolyzed. Cells that lose water this way can recover their water content if the conditions are rectified quickly, but it can lead to permanent damage to the cell interconnections. Such situations can be avoided by correct dosage of fertilizer and by monitoring of conductivity levels in greenhouse soils and NFT systems (see Alternatives to growing in the soil).

Movement of water in the roots
It is the function of the root system to take up water and mineral nutrients from the growing medium and it is constructed accordingly, as described in Plant cells and tissues. Inside the epidermis is the parenchymatous cortex layer. The main function of this tissue is respiration to produce energy for growth of the root and for the absorption of mineral nutrients. The cortex can also be used for the storage of food where the root is an overwintering organ.

The cortex is often quite extensive and water moves across it in order to reach the transporting tissue that is in the centre of the root. Movement is relatively unrestricted as it moves through the intercell spaces and the lattice work of the cell wall. The central region, the stele, is separated from the cortex by a single layer of cells, the endodermis , which has the function of controlling the passage of water into the stele. A waxy strip forming part of the cell wall of many of the endodermal cells (the Casparian strip) prevents water from moving into the cell by all except the cells outside it, called passage cells. In this way, the volume of water passing into the stele is restricted. If such control did not occur, more water could move into the transport system than can be lost through the leaves. In some conditions, such as in high air humidity, more water moves into the leaves than is being lost to the air, and the more delicate cell walls in the leaf may burst. This condition is known as oedema , and commonly occurs in Pelargonium as dark green patches becoming brown, and also in weak-celled plants such as lettuce, when it is known as tipburn , because the margins of the leaves in particular will appear scorched. Guttation may occur when liquid water is forced onto the leaf surface.

Water passes through the endodermis to the xylem tissue, which transports the water and dissolved minerals up to the stem and leaves. The arrangement of the xylem tissue varies between species, but often appears in transverse section as a star with varying numbers of ‘arms’ (see Figure 6.11).

A distinct area in the root inside the endodermis, the pericycle , supports cell division and produces lateral roots, which push through to the main root surface from deep within the structure. Roots, as with stems, age and become thickened with waxy substances, and the uptake rate of water becomes restricted. Root anatomy is described in Plant cells and tissues.

Movement of water up the stem
Three factors contribute to water movement up the stem:

• root pressure by which osmotic forces push water up the stem to a height of about 30 cm. This can provide a large proportion of the plant’s water needs in smaller annual species;
• capillary action (attraction of the water molecules for the sides of the xylem vessels), which may lift water a few centimetres, but which is not considered a significant factor in water movement;
• transpiration pull is the major process that moves soil water to all parts of the plant.

Transpiration
Any plant takes up a lot of water through its roots; for example, a tree can take up about 1000 litres (about 200 gallons) a day. Approximately 98 per cent of the water taken up moves through the plant and is lost by transpiration; only about 2 per cent is retained as part of the plant’s structure, and a yet smaller amount is used up in photosynthesis.

The seemingly extravagant loss through leaves is due to the unavoidably large pores in the leaf surface (stomata) essential for carbon dioxide diffusion (see Figure 8.8). However, two other points should be considered here:

• water vapour diffuses outward through the leaf stomata more quickly than carbon dioxide (to be used for photosynthesis) entering. However, the plant is able to partially close the stomata to reduce water loss without causing a carbon dioxide Deficiency in the leaf;
• the diffusion rate of water vapour through the stomata leads to a leaf cooling effect enabling the leaf to function whilst being exposed to high levels of radiation.

The plant is able to reduce its transpiration rate because the cuticle (a waxy waterproof layer) protects most of its surface and the stomata are able to close up as the cells in the leaf start to lose their turgor (see leaf structure). The stomatal pore is bordered by two sausage-shaped guard cells, which have thick cell walls near to the pore. When the guard cells are fully turgid, the pressure of water on the thinner walls causes the cells to buckle and the pore to open. If the plant begins to lose more water, the guard cells lose their turgidity and the stomata close to prevent any further water loss. Stomata also close if carbon dioxide concentration in the air rises above optimum levels.

A remarkable aspect of transpiration is that water can be pulled (‘sucked’) such a long way to the tops of tall trees. Engineers have long known that columns of water break when they are more than about 10 m long, and yet even tall trees such as the giant redwoods pull water up a hundred metres from ground level. This apparent ability to flout the laws of nature is probably due to the small size of the xylem vessels, which greatly reduce the possibility of the water columns collapsing.

A further impressive aspect of the plant structure is seen in the extreme ramifications of the xylem system in the veins of the leaf. This fine network ensures that water moves by transpiration pull right up to the spongy mesophyll spaces in the leaf, and avoids any water movement through living cells, which would slow the process down many thousand times.

If the air surrounding the leaf becomes very humid, then the diffusion of water vapour will be much reduced and the rate of transpiration will decrease. Application of water to greenhouse paths during the summer, damping down, increases relative humidity and reduces transpiration rate. While the air surrounding the leaf is moving, the humidity of air around the leaf is low, so that transpiration is maintained and greater water loss is experienced.

Windbreaks reduce the risk of desiccation of crops. Ambient temperatures affect the rate at which liquid water in the leaf evaporates and thus determines the transpiration rate.

A close relationship exists between the daily fluctuation in the rate of transpiration and the variation in solar radiation. This is used to assess the amount of water being lost from cuttings in mist units (see misting); a light-sensitive cell automatically switches on the misting. In artificial conditions, e.g. in a florist shop, transpiration rate can be reduced by providing a cool, humid and shaded environment. Plasmolyzed leaf cells can occur if highly concentrated sprays cause water to leave the cells and result in scorching.

The evaporation of water from the cells of the leaf means that in order for the leaf to remain turgid, which is important for efficient photosynthesis, the water lost must be replaced by water in the xylem. Pressure is created in the xylem by the loss from an otherwise closed system and water moves up the petiole of the leaf and stem of the plant by suction (see transpiration pull). If the water in the xylem column is broken, for example when a stem of a flower is cut, air moves into the xylem and may restrict the further movement of water when the cut flower is placed in vase water. However, by cutting the stem under water the column is maintained and water enters at a faster rate than if the plant was intact with a root system.

Anti transpirants are plastic substances which, when sprayed onto the leaves, will create a temporary barrier to water loss over the whole leaf surface, including the stomata. These substances are useful to protect a plant during a critical period in its cultivation; for example, conifers can be treated while they are moved to another site.

Structural adaptations to the leaf occur in some species to enable them to withstand low water supplies with a reduced surface area, a very thick cuticle and sunken guard cells protected below the leaf surface (see Figures 9.2 and 9.3). Compare this cross-section with that of a more typical leaf shown in Figure 8.8. In extreme cases, e.g. cacti, the leaf is reduced to a spine, and the stem takes over the function of photosynthesis and is also capable of water storage, as in the stonecrop (Sedum).