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Why water fruit trees? | AG0297 |
Ian Goodwin, Tatura
September, 2000 |  |
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In most fruit growing districts in Victoria the average summer is so dry that the water required by an orchard exceeds the available water stored in the soil root zone. When this happens the trees come under water stress. Some level of water stress may be a good thing (see Agriculture Note AG0299: Irrigation scheduling for regulated deficit irrigation (RDI)) but when this is exceeded or occurs at critical fruit growth periods the crop will be irrevocably damaged. Irrigation can overcome this loss of production from water stress.
The role of water
Mature fruit consist of about 85% water, so a 200 kg crop on a tree contains only 170 litres (170 kg) of pure water. To grow this crop the tree would require a water supply over the growing season of around 17 000 litres. In other words about 1% of the water a plant uses is retained in the fruit, and less than 0.5% in the remaining parts of the tree, the leaves, shoots and roots. Knowing the role played by the other 98.5% will help us understand what happens if the water supply is somewhat less than the supposed 17 000 litres required for peak performance.
If we burn the 30 kg of material other than water that goes to make up the 200 kg of fruit on our tree, we find that only about 0.5 kg of ash is left. This ash is the fertiliser and nutrients other than nitrogen which have been taken from the soil. The other 29.5 kg is made up of sugars and other compounds derived from sugars which can be completely changed by burning to carbon dioxide and water. It is in the production of the fruit, not only for the sweet component of the fruit, but for the structural material that goes to make up the fruit, leaves, shoots, roots and trunk thickening, that water plays a vital role.
How water is used
Sugars are produced in the leaves from a chemical combination of carbon dioxide from the air with a small amount of water from the leaf. This reaction known as photosynthesis requires the presence of chlorophyll (the green pigment in the leaf) and sunlight. It also requires that a continuing supply of carbon dioxide is available within the leaf. The source of this carbon dioxide is the 0.03% present in air so it is necessary for air to move freely into and out of the leaf while photosynthesis is occurring.
This air movement takes place through pores, known as stomata, located mainly on the underside of each leaf. As carbon dioxide moves into the leaf through the stomata, water vapour can move out of the leaf into the air. The maximum rate of this movement of water vapour will vary from day to day and even from hour to hour depending on the drying capacity of the atmosphere.
It is this same drying capacity that is measured when we record the depth of water evaporated from the water tank, which explains why there is a close relationship between the evaporation measurement and the maximum water use by a plant on any day.
Guard cells protect plant
All too often, this maximum water supply is not available to the leaves as the soil dries out after irrigation or rainfall, because the demand is too great for the roots to be able to extract the required supply quickly enough. This is the start of water stress. If the stomata remained open under these conditions the leaves would dry out and die. However, each stomata is provided with two guard cells which are capable of closing the pore to prevent a degree of water loss that could cause permanent damage to the leaf. As soon as this happens, carbon dioxide can no longer enter the leaf and photosynthesis ceases.
Darkness closes guard cells
Darkness also causes the guard cells to close the stomata so that no water is lost from the leaves at night. This safeguard allows the plant to take up and hold water during the night to overcome the deficiency in the plant, even if the soil is almost dry and the rate of uptake is very slow. It also provides time for water in the soil some distance away from each root to move into the area closer to the root that may have been dried out excessively that day.
At sunrise the next morning, the stomata open under the influence of light and photsynthesis and loss of water will recommence. If no water is added to the soil the stomata will close at a progressively earlier time each day due to water stress and the production of sugars will be correspondingly reduced. The time at which the stomata will close on any particular day will depend on the evaporative demand for that day and on the ability of the tree to extract water from the soil at the required rate.
A short trip to close roots
If the roots are close together, as often happens in a light soil type, the water in the soil between the roots will have only a short distance to move before it comes under the suction influence of the roots. Under these conditions, the plant may be able to take up water at almost the maximum required rate until all the water remaining in the soil is held so firmly that the plant roots cannot extract any more. When this happens the plant is severely water stressed and leaves on the plant will often fall off.
The moisture content of the soil at which this happens is known as the wilting point. Obviously, an unwatered tree in a shallow light soil will show water stress symptoms early in the season and the soil will probably be called a 'droughty soil'. Without water in this type of soil, trees will certainly not produce a crop and may even die.
A long trip to sparse roots
In other more general cases the roots are not close together, particularly in the subsoil, and water has to move to them over relatively long distances. As the soil dries out, redistribution of the soil water at night will mean that sufficient water is available to ensure that stomata open at sunrise and allow photosynthesis to start. However, it is not long before the soil in the immediate vicinity of the sparse root system is depleted of available water and the stomata close for the rest of the day.
The next night a further redistribution of water occurs in the soil and the plant is able to repeat its short period of water use and photosynthesis on the following morning. In extreme cases on heavy clay soils this process may be repeated for a month or two before the whole of the soil volume available to the tree roots has reached wilting point when the leaves wilt and fall.
In the meantime the plant will have been producing sugars for only one or two hours each day. During the remaining hours of daylight it will be resisting loss of water by having its stomata closed. Where trees are under these conditions, the soil will probably be regarded as a type that enables trees to withstand drought. Without water in these types of soil, trees will certainly survive and may even be able to produce enough sugars to size a light crop of fruit.
The amount of water used
Since stomata are present on each leaf, in general terms the higher the number of leaves on the tree the greater the use of water. This statement can only be made in general terms because as the trees become large enough to intercept most of the sunlight falling on the orchard, their water use will tend to reach a maximum rate. Further increases in leaf numbers will have little effect on the water use.
Still, from time of planting until the spread of the limbs covers more than half the ground area available for each tree, the water use will be closely related to the leaf number or, more specifically, to the leaf area on the tree.
Therefore, as trees increase in size they will require increasing amounts of water to maintain maximum growth rates. Conversely, trees planted at wide spacings on dwarfing rootstocks, or trees which become dwarfed through some other effect, may be able to size a heavier crop of fruit without irrigation than larger trees planted at the same spacing.
The effect of a shortage of water
As mentioned earlier, the stomata on the leaves close in periods of water shortage to prevent permanent damage to the plant. Unfortunately they do not react instantly to a water shortage in the soil and it is common for some water to be extracted from the limbs, trunk and even the fruit before the stomata close completely for the day.
If there is a reasonably good water supply in the root zone, this may not matter since the losses will tend to be made up again overnight. If the soil in the root zone approaches wilting point, these deficits cannot be fully made up at night and a permanent reduction in growth of fruit results. Replenishing the water supply after such a period will improve the current growth rate but will not make up for the poor growth during the time the plant was droughted.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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