Without water life as we know it would not be possible. In space exploration it is the 'Holy Grail' and any hint of its presence on a planetary body causes a wave of excitement in the scientific world. It not only provides the medium in which life exists, but it is involved chemically in some metabolic pathways, eg. providing Hydrogen during photosynthesis.
Plants also use water for support. Most animals have a skeleton - either external or internal, to support their bodies. Plant tissue relies on water pressure with a support structure in the form of a cell wall. In most plant cells a vacuole which contains mainly water acts like a fluid-filled balloon to push outwards, keeping the cell turgid. If the pressure is lost due to a lack of water the cells become flaccid and the plant wilts. In woody stems the cells which are holding the plant upright are mostly dead so the vacuole is absent and a rigid cell wall remains to provide the support, eg. the heart wood of a tree.
The transfer of water into and out of cells is by a process called osmosis. This works across a concentration gradient where water moves from a dilute solution of salts and sugars through a semi-permeable membrane to a solution with a higher concentration. Plant cells contain salts and sugars in a higher concentration than their surroundings so water is drawn in - the cell membrane and the cytoplasm within the cell act as the semi-permeable membrane. On the fine roots the outer cells have very thin walls and elongated projections or root hairs which sit between the soil particles. The vacuole extends into the root hair and contains the higher concentration of salts to draw in the water.
A diagrammatic plant cell showing some of its contents.
Water flows through a plant carrying nutrients upwards from the soil and after photosynthesis, it carries the products back around the plant for direct use or storage. This takes place in a vascular system made up of the xylem and phloem in the stems and leaf veins. The xylem vessels take water from the roots to the leaves and the phloem moves some of it containing sugars back in the opposite direction. Most of the water which travels upwards is lost from the leaves through pores called stomata - a process called transpiration. It is this loss that creates the negative pressure to move the water upwards, as atmospheric pressure alone could not force it to the top of a tall tree, which can lose many hundreds of litres per day - this is called the transpiration stream. As a water molecule is lost through a stoma, it is replaced at the root by another molecule and the column of water moves up the stem.
Water also accumulates in the cell wall and spaces between cells - the inter-cellular spaces, called the apoplast. This is were most water movement takes place since the movement through cells is relatively slow and the fow rate of the transpiration stream can be up to 45 metres per hour. When a stem is cut the water continues to rise and air is drawn in, so the column is broken and it dies.
The symbiotic mycorrhizal fungi which are found in asociation with most plants extend the reach of the roots so they increase the amount of water the plant can take up.
As the water evaporates from the surface of the leaf it takes heat away so it is a cooling system to prevent the temperature rising too high - the cells would stop functioning and the leaf would die. Scorching can occur if droplets remain on the surface in direct sunlight because they act as a magnifying lens to burn the leaf tissues.
Sometimes if a shrub is pruned severely when it is in full leaf and growing well, it can suffer greatly or even die due to the interruption of the transpiration stream. The foliage on the severed branches was drawing the sap along and when it was removed this flow stopped, so buds below the cut which would be expected to sprout, die as well. Some may sprout and grow weakly, but in many instances they don't even start. This is why heavy pruning should be done in the dormant season or if it cannot be avoided, the branches should be thinned out leaving some to draw the sap. When new shoots appear lower down and are growing away strongly, the remaining long branches can be removed. Light pruning or thinning during the growing season should not be to detrimental, but at any time all cuts should be made just above a bud, as anything beyond there will die back and could allow disease to enter.
As well as allowing water to pass out of the leaves, the stomata are the route for carbon dioxide and oxygen to flow into and out of the plant for photosynthesis. The size of the pore determines the passage of water or gasses and this is controlled by the guard cells which lie side-by-side with the pore between them. The opening and closing occurs when the vacuoles of these cells expand and contract. In daylight the pores remain open due to the build-up of sucrose in the vacuole which holds more water so is turgid and causes the guard cells to form the opening. During the night starch accumulates in the cytoplasm, the vacuoles lose their turgor and the stoma closes. Some plants have a reversed stomatal rhythm and close during the day to cope with desert conditions.
Where the xylem vessels end at the edges of leaves specialised structures called hydathodes can release excess water if there is a high soil-water content and high atmospheric humidity is preventing loss through the stomata. The water oozes out of the hydathodes in the form of small droplets, a process known as guttation. It may be noticed on greenhouse plants which have had copious watering followed by a cool night, the transpiration rate is low and the guttation ensures a good flow of water and nutrients to the plant tissues.
Leaf cells, showing a vertical section and a surface view of a stoma. The vacoules in the guard cells are distended to hold the pore open.
Root hairs extending into the soil.
The loss of water from leaves varies depending on the surrounding environmental conditions so in higher temperatures the loss can be greater. Plants have adapted to these conditions in different ways such as reducing leaf area, having a thick layer of cutin on the surface. Fine hairs create a microclimate at the surface and reduce the amount of evaporation also preventing winds blowing the water vapour away too quickly. Cutin is derived from fatty acids and is impermeable to water vapour. It forms the cuticle, an outer layer on the cell walls at the surface which makes a seal so that water loss occurs only through the stomata.
The cacti have gone to the extreme of reducing their surface area by not having any leaves and exist as a stout, fleshy stem. They can store all the water they need between periods of rainfall, and usually have sharp spines to protect this valuable resource. Some have a web-like covering to improve the microclimate close to the surface.
Pine trees have evolved to survive a wide range of conditions, in dry heat, at higher altitude and more northerly weather conditions, by having fine needle-like leaves to withstand strong drying winds and they contain a concentrated sap which has anti-freeze properties.
Most garden plants are Mesopyhtes and have relatively 'normal' water relations. The Halophytes have adaptations to deal with high salt concentrations in the surrounding water. Their cells have a higher osmotic potential so that water moves into the cells.
The Xerophytes are adapted to cope with desert conditions and tend to be succulents with fleshy leaves and stems. These are able to hold a large reserve of water and endure drought conditions. Some evade the drought by producing seeds or spores and die away until the returning wet conditions bring them back to life.
Evergreen plants usually have a thick cuticle on their upper leaf surfaces and most of their stomata on the lower surface. The sap also has the same anti-freeze properties of pine trees. They replace their leaves constantly, so lower ones overshadowed from above are dropped and during drought conditions some are shed to reduce the demand for water. Deciduous trees and herbaceous plants lose their leaves and topgrowth during the winter as a way of coping with the freezing and reduced light.
The xylem vessels become less porous as they age and some diseases can block them, so replacing them every year is a good way to keep the sap flowing freely. New vessels are being produced all of the time. They form at different rates throughout the year depending on growth rate of the plant. This gives rise to the annular rings in woody plants as production slows to a halt in winter. A layer of cambium cells lies between the xylem and phloem, these are the undifferentiated cells which divide on the inside to produce new xylem and on the outside to produce new phloem.
Some plant diseases cause death by blocking the vascular system, eg. Fireblight and Dutch Elm Disease. Also total removal of the bark causes a delayed death to a tree or shrub as the sugars and starch are unable to reach the roots so they die away and there is no stored starch to send up the trunk later to grow new tissue and leaves. This is sometimes used to reduce the vigour of fruit trees by partially ringing a trunk or branch, but careless mowing around trees and shrubs with a line trimmer can be fatal. This is also why planting too deeply kills a tree or shrub, some which naturally produce suckers are not usually affected, but with most the soil rots the bark which should be above ground, causing death.
| Self Rewinding Hose Reels No Batteries! No Electricity! No Springs. Receive a 10$ Discount Today! www.Garden-Gadget.com |
When and How to Water
Water is important to the gardener as the hotter drier summers now being experienced are leading to water shortages and hosepipe bans in some areas. Not all plants require watering and develop deep roots to reach well below the water table. Grass has relatively shallow roots so is usually the first to show signs of drought, but it usually recovers so should not be watered.
Plants in containers or planters are the most vulnerable and require the most attention, so use larger ones if possible. The foliage directs water to the outside of the canopy and if planted in the open the finer roots would be beyond this to gather the moisture, but in a pot they are prevented from reaching it. Palm trees are better suited for container growing as their foliage directs any rainfall towards the trunk, so it runs down to the roots.
The climate experts predict that in the British Isles we will have hotter, drier summers and milder winters which may be wetter in some areas. This will affect the range of plants that can be grown. In summer more drought-tolerant plants will do well, but these may not survive the winter wet when the soil may become waterlogged. The condition of the soil will have greater importance; it will have to retain moisture in the summer, yet have better drainage for the winter. Trees will be the most vulnerable as they require most water and some have relatively shallow root systems, eg. Beech trees are already showing signs of stress in the summer and some have already been lost in Southern England. In these areas Hornbeam is a better choice and arboriculturalists have already decided to replace drought-damaged trees with species such as Walnut which are better adapted to drier conditions.
Digging in plenty of compost or other organic matter would be the best remedy as it retains moisture and opens up wetter soils to improve drainage. When planting trees which could be damaged by waterlogged soil, plant them in a mound of soil which has a better chance of draining water from the root ball. If drought could be a problem a thick mulch of organic matter over exposed soil will help to keep in the moisture as well as reducing weed seed germination.
The best time to water plants is after sunset as there will be less evaporation and it has all night to percolate down to the roots. So-called 'grey' water can be used, this is water that has already been used for washing or water which has been collected from the roof. Any soaps and detergents must be biodegradable to avoid a build-up of toxins in the soil. Irrigation systems which deliver the water at soil level are best; these can be dribbling nozzles or seep hoses which are made from reconstituted rubber tyres, and are laid around the garden close to the plants. They can be attached to a watering timer to operate during the night.
If planting a large tree or shrub it will assist in watering if a piece of perforated drainage pipe or a pierced plastic bottle is buried beside it to direct some water deeper down. Retain the top of the bottle to keep out debris, but it will have to be pierced as well to let the water flow out through the hole(s) in the bottom.
Using a spray over everything is the least efficient method and if done while the sun is out it can scorch the foliage. Also the plants may not receive enough water if the leaves direct it away from the base of the stems. It is suitable for more open ground such as a new lawn or seedbed. Placing a straight-sided container within the area to catch some water will indicate when enough has been supplied. About 2.5cm should percolate down to about 20cm in moist soil so if the time is noted to reach this level, this can be used for future waterings. A sample digging with a trowel should reveal how far down the water has gone as this can vary with different soil types and the degree of moisture in the soil at the time. However, it is important not to water too often as the plants can develop shallower roots, becoming less drought tolerant by being unable to find water lower down in the soil. So a good watering once a week is better than a dribble every day.
Overwatering can be just as harmful to plants as a drought. In fact more indoor plants are thought to be killed by too much water than by too little. The symptoms of overwatering are similar to those of a lack of water, the leaves wilt and turn yellow before being shed. The tips and edges are usually brown. Seeing the wilting can be taken as a lack of water and more water is applied, so check the compost first to see if it is wet - a wilting plant where there is plenty of moisture can also occur if the roots are being eaten as in a Vine Weevil grub attack. The roots also suffer and begin to rot as there is no air left in the waterlogged soil - this is probably what is causing the wilting as there are insufficient roots functioning to supply the plant. Usually the watering is too frequent or the drainage holes in a pot are blocked - in open ground the drainage needs to be improved by digging in plenty of grit and organic matter.
Irregular watering can damage a plant by causing it to split. This can be the bark of trees and shrubs or it is more common with developing fruit. The split itself is not usually fatal, but the wound allows diseases in, which can be. Try to keep watering regular and in containers with smaller amounts of growing compost such as hanging baskets, use water retaining gel to hold moisture for longer periods and prevent drying out.
With house plants feel the weight of the container after watering and check regularly by picking up the pot. After a while you will know by the weight when to add water. With heavier pots use a finger to test the planting medium for moisture, when it feels dry it is time to water.
Where irrigation is used on outdoor crops the salts washed down into the subsoil, can be brought up again by over-watering which raises the water table and the salinity rises at root level. This is a problem for commercial growing in drier regions where there is a risk of desertificaton due to irrigation salinity which poisons the soil and no plants will grow. The irrigation water also contains dissolved salts which can build up in the soil as the water evaporates and is used by the plants. If the levels become greater than those within the plants the concentration gradient is reversed and the plants cannot take up any water.
The overuse of fertilizers can produce the same effect as the levels of dissolved salts increase beyond where the plants can cope.
