Nearly all crop and pasture plants begin their growth as seeds.
A seed consists of:
(1) A testa or seed coat,
(2) An embryo or small plant, and
(3) A supply of food.
The embryo of a seed is a minute plant which has a shoot or plumule with small leaves, a root or radicle, and a central portion bearing special leaf-like structures called cotyledons. The region just above the cotyledons is called the epicotyl, and the region just below the cotyledons is the hypocotyl.
There are two kinds of seeds:
1. The seeds of grasses, cereals and other monocotyledons have only one cotyledon, and the food is stored outside the embryo in tissue called the endosperm.
2. Seeds of beans, pumpkins and other dicotyledons have two cotyledons attached to the embryo, and the reserve food is absorbed by the embryo and stored in these swollen cotyledons.
In Figure 20.2 we see a wheat grain and a bean seed compared.
The embryo or little plant inside the seed is able to pass safely through periods of drought or very cold weather which might kill a mature plant.
With the proper conditions, the embryo inside the seed begins to grow and it emerges from the seed. This is called germination. Before germination can occur, a seed needs the right temperature, a little oxygen, some water, and of course the embryo must still be alive.
If seeds are kept dry, the embryos will live for a time, but will eventually die. The higher the water content of seeds the shorter will be the life of the embryos. In fact it has been found that the life of seeds can be extended considerably if their water content can be reduced below the normal amount. Normally most cereal seeds will remain alive for three or four years, but after this time more and more of the seeds will die and fail to germinate.
When a seed germinates, its greatest need is for water, and for this reason the radicle is usually the part of the embryo which emerges first. The seed roots or seminal roots develop root hairs, and are then able to absorb water faster from the moist soil. The next part of the embryo to grow is either the epicotyl or the hypocotyl.
If the hypocotyl grows rapidly, the cotyledons and the plumule will be carried up to the surface, as in the case of french beans and most of the clovers. The cotyledons will then be seen attached to the stem above the ground. This is called epigeal germination.
If growth occurs above the cotyledons in the epicotyl region, only the plumule will be carried up to the surface and the cotyledons will remain in the soil. This is hypogeal germination, and occurs in broad beans, grasses and most of the field crops.
Once the young leaves of the plumule have reached the light and turned green, the plant begins to manufacture its own food and there comes a time when it is no longer dependent on the stored food in the seed.
In wheat, maize and many other plants, the seminal roots are not sufficient for the needs of the plant, and as the young seedling grows other roots appear. The main roots or “crowns” develop between the seed and the ground surface. In wheat these crown roots are not formed unless the soil is moist around the stem about 3 cm below the surface.
The function of roots is to anchor the plant in the soil, and to absorb water and nutrients. Plants need large amounts of the elements carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulphur, calcium, iron and magnesium. Small amounts of boron, manganese and zinc are also needed. The plant obtains its supplies of carbon by absorbing the gas carbon dioxide into its leaves from the air. All the other elements needed must be absorbed by the roots.
The water taken into the roots supplies the hydrogen and oxygen, while the roots must obtain all the other elements needed by absorbing these nutrients from the soil. Plants need large supplies of nitrogen, phosphorus and potassium in the soil, and continuous cropping may soon exhaust the soil of these nutrients. Mixed fertilisers usually contain nitrogen, phosphorus and potassium in varying proportions. Small amounts of special fertilisers may be used if one of the so-called “trace” elements, such as zinc, is lacking in the soil.
The important part of a root is near the tip. If we examine a slice of a young root under the microscope, we see that the cells of the growing region at the tip are protected by cells of the root cap. Further back there is the region of elongation where the root is lengthening. Just behind this zone is the root hair region. Root hairs are very delicate single cells which grow out from the skin of the root and intertwine amongst the soil particles. It is the root hairs which are responsible for the absorption of most of the water and nutrients needed by the plant.
In each crop the roots grow in a special way, and this is important information. For instance, in maize and many of the tall grasses the crown roots first run out sideways into the surface soil, especially if the soil surface is shaded, and later on other roots grow downwards. Therefore unless cultivation is shallow, many of these early-developing roots will be pruned off.
But any cultivation at all will cut some of the surface roots and this is why some farmers try not to disturb the soil around crop plants, and instead use chemical sprays to control weed growth. In some crops like millet, nearly all the roots are shallow, whereas in lucerne the tap roots go downwards in the soil.
Plant roots alter their form according to the condition of the soil. In fairly dry soils, roots will produce many side roots (or primary laterals), but in moist soils they do not branch so much. In very dry soils root growth stops, and the parts above the ground will be stunted. Where crops are watered, it is important to water long enough to wet the soil deeply. Frequent light watering attracts the roots towards the surface, and these plants cannot then stand up to a dry spell.
If a soil is rich in plant nutrients, the roots tend to be shorter and more branched. Poor infertile soils lead to the growth of longer roots which do not have many branches.
The water and nutrients absorbed by the roots are taken up the stem of the plant to the leaves. The stem rising above the ground bears a number of slight swellings called the nodes. Leaves are special food-making structures which grow out from the stem at the nodes.
Most leaves are flat organs and are usually arranged on the stem so that sunlight falls on them. The green colour of leaves is due to a substance called chlorophyll which is contained in the cells of the middle or mesophyll layer of the leaf. The radiant energy of sunlight acts on chlorophyll enabling it to begin the process of changing water and carbon dioxide into sugar. This is the process called photosynthesis.
After the sugar has been formed, it may either be turned into starch, or to oils, or it may be combined with nutrients absorbed from the soil and formed into proteins.
It is important to realise two facts about food-making or photosynthesis in a plant:
1. Since food is made chiefly in the leaves, if the leaf area is reduced by hail, wind damage, by disease or by leaf-eating insects, the plant will yield less food.
2. If for any reason the amount of light reaching the leaf surface is reduced, then the manufacture of food will also be reduced.
Once sugar or other foods have been formed in a leaf, they are usually moved about the plant’s body from one place to another. This food movement is called translocation. In potato plants for instance, sugar is made in the leaves in the daytime and stored as starch in the leaf for a time.
At night this starch is turned back into sugar and translocated down the stem and into one of the tubers. Here the sugar is turned back again into stored starch. In the same way, food made in carrot leaves is taken to the root where it is stored. In sugar cane the food is stored in the stem as cane sugar.
When plants like wheat are making seeds, food which has been stored in the leaves and stems begins moving upwards towards the flowers. When this food has been stored in the seeds, the rest of the plant body is very poor in food and is called straw. Therefore in making hay, it is important to cut the plant down before it makes seeds, when the food is still found in the stems and leaves.
Like any other living thing, a plant needs supplies of energy to maintain its life. Now there is energy stored in food molecules, for chemical energy binds the atoms together in the molecule. The plant can be supplied with energy if some of these food molecules are broken down into smaller molecules, for then some of the chemical energy will be released.
Carbon dioxide and water are produced by this process of energy release which is called respiration. Respiration must go on in all living cells all the time, for no cell can continue to live without energy. The three factors which affect respiration most are temperature, water and supplies of phosphorus. Let us see how respiration varies during the life of a plant.
When a seed is formed, water is taken out of it until the seed is fairly dry. This means that the cells of the seed embryo are alive, but energy is only being released very slowly because of the lack of water. If the water content of a seed can be still further reduced, this will cause a further slowing in the rate of energy release, and the seed will remain alive much longer than normal.
On the other hand as soon as the seed lies in moist soil and has a little oxygen and the proper temperature, energy release begins to quicken and germination occurs. Young plants, like young animals, use large amounts of energy in making rapid growth.
These supplies of energy will depend chiefly on whether enough water can be obtained from the soil, but also whether the plant can absorb enough phosphorus. This is one reason why superphosphate should be given to a plant in its early stages of growth when its energy needs are greatest.
When a plant is mowed down for making hay, it should be properly dry before it is picked up and stored. If it contains too much water, bacteria from the soil will be able to use this water for energy release. The respiration of the bacteria will produce more water as a by-product, and this will make the hay even more moist, and it will spoil. Thus we see that plant products must be properly dried out before being stored or they will not keep well.
At a certain stage in their life most plants reproduce themselves by making flowers which in turn make seeds. Up to a certain stage in the life of the plant, the growing point or shoot apex at the top of the stem only makes new leaves. Then at a certain time it begins making buds which will later become flowers. Certain conditions in the environment cause the growing point to begin making flowers.
In some plants, such as the winter wheats, flower formation will not commence until the plant experiences low temperatures. In many other cases flowering is begun when the plant experiences a certain pattern of light and darkness. Short-day plants are those which require long nights before flower formation can begin. Long- day plants are those where flower formation is begun by the experience of short nights.
When flowers are first formed they are often completely hidden by the leaves surrounding the growing point. Later they will grow and become flower buds which can be seen.
Flowers may be either male or female or both.
1. The stamens are the organs in flowers which make the male pollen cells. Stamens consist of a stalk or filament and an anther or pollen box.
2. The female organs of flowers are the carpels. Each carpel consists of a stigma which receives male pollen, a style or stalk, and an ovary or seed box. Inside the ovary are formed one or more ovules which later become the seeds.
Before a flower can become fertilised, male pollen grains must be transferred to the stigma. This carrying of pollen from one flower to another, or from the anther of one flower to the stigma of the same flower is called pollination. The brightly coloured petals attract insects in those flowers like lucerne which are insect-pollinated. Wind-pollinated flowers like the cereals have no need for bright petals, and they are usually green.
When pollen grains are transferred to their own kind of stigma, they may germinate and make a delicate tube called a pollen tube. This grows down the style and into the ovary. The pollen tube enters an ovule and fertilises it. The fertilised ovule then begins to grow rapidly and becomes a seed.
Pupils studying crops should know that the life cycle of a plant can be divided into four stages, and in each stage growth is either weak or strong.
The stages are:
1. In the seedling stage growth is weak, and unless it has good conditions, the plant may not survive. This is the best stage to kill weeds which may grow up with a crop or which may germinate in clean cultivation.
2. Once they have passed the seedling stage, the active growth stage follows in which plants are strong and able to stand up to hard conditions. If weeds are allowed to reach this stage they are hard to kill.
3. In the flowering stage most plants can be damaged by unfavourable conditions. Thus frost may damage the flowers of wheat, hot dry winds may cause pollen blast in maize, and frost may cause the flowers of fruit trees to drop off. Weeds are often easier to kill with poison sprays at this stage of their life.
4. Once plants have reached the seed or resting stage, the seed embryos are able to pass through unfavourable conditions such as drought or periods of intense cold. It is therefore important to prevent weeds from seeding.
Some of the processes which take place during the life of a plant are described. These are the processes which normally take place in single plants growing by themselves. But the farmer has to deal, not with single plants, but with crop or pasture plants growing together in large numbers. The farmer has to deal with plant populations.
It is clear that if we are to understand crop and pasture production, we must make a study of plant populations and the effects which plants may have on one another when growing close together. We saw that photosynthesis or food manufacture in green plants depends on the presence of sufficient light, enough carbon dioxide in the air, enough oxygen in the soil for the roots, and the provision of enough water and mineral nutrients from the soil.
In other words, a plant can only manufacture food efficiently if conditions in its environment will allow this. But it is clear that if many plants are growing thickly in a dense population, each plant will be living in a very different environment from that which it would have if it was growing alone with no others near it.
Let us think for a moment about conditions inside a thick crop of tall plants on a still day. Figure 21.1 is a picture of the middle of a maize crop and we may consider the environment of the smaller plant in the centre of the picture.
The taller maize plants in the crop will obviously keep some light from the plant in the centre, so that it may be living in an environment of insufficient light. Being surrounded by so many other leaves, each one absorbing carbon dioxide from the air, perhaps the plant we are considering lives in an environment which contains too little of this gas to allow for rapid photosynthesis.
When we consider the soil environment, the presence of so many roots in the soil may mean that there is a shortage of oxygen for the roots. Again, the plant we are considering may not be able to find sufficient water and mineral nutrients in the soil. It is clear that in such a crop each plant will have some effect on its neighbours because it will help to alter the environment in which other plants are growing.
The study of competition is the study of the effects which plants may have on one another when they have to share the same supplies of light, water, oxygen, carbon dioxide and soil nutrients.
Four effects of competition will be considered:
1. The effects of competition on the individual plant.
2. The effects of competition on the whole plant community.
3. Competition between pairs of plant species as in a pasture.
4. The effects of competition by weeds.
1. Competition and the Individual Plant:
The effects which plants may have on one another will tend to increase as the plants are grown closer together; that is, competition between plants depends mainly on the thickness or density of a plant population. We must begin by considering what effects the density of a crop may have on individual plants.
As plant density increases, the growth of the individual plant is reduced; the amount of food which the plant is able to manufacture becomes less, the number of flowers which it produces are fewer and the yield of seeds or grain which it forms grows smaller. For instance, as the sowing rate of wheat is increased, each plant weighs less, makes less starch and produces less grain.
As plant density increases, the growth rate of individual plants is slowed down. When competition is strong, the growth of some plants is slowed down more than others and they are smaller plants than their neighbours. If competition is severe, the proportion of small plants in a population tends to increase during the growing season.
2. Competition and the Plant Community:
As the density of a plant community increases, the yield of plant material which can be harvested from a given area also increases up to a certain point. If the density of plants is increased still further, the yield of dry matter does not increase and may even tend to decrease slightly.
As the density of a crop increases, the production of grain from a given area increases until a certain crop density has been reached. If density is increased still further, grain production begins to fall.
It is important to understand that the greatest yield of grain from a given area of land is obtained when the density of plants is so high that the yield of each plant is much less than it would be if each plant had been given more room to grow.
3. Competition between Pairs of Plant Species:
In some cases competition occurs between two species of plants which are grown together in one paddock. For instance rape may be sown with a cereal crop such as oats, or a pasture may be sown down with two plant species such as H1 rye grass and white clover. In such cases it is usual to find that one of these plant species produces a much higher yield of plant material from a given area than the other species.
If two such plant species are grown together they compete with each other for the light, water, nutrients and other materials of the environment. The result is that the yield of plant material from a given area will be less than could have been obtained if the higher yielding plant species only had been grown.
At the same time the yield of plant material from such a mixture is higher than could be obtained if the lower yielding plant species only had been grown.
At one time it was thought that sowing two species of plants together was a good idea because the two species, having different requirements, could make a more complete use of the environment than if only one species were grown. For instance it was thought that sowing a shallow-rooted and a deep-rooted plant species together would result in a much greater yield than if only one species were grown.
Scientists have no proof that this is so, and when mixtures of plant species are sown together, it is not done to obtain a much higher yield of plant material but for some other reason. For instance rape may be sown with oats because, since it germinates and grows faster, it will provide shelter for the young oat plants. Legumes may be sown with grasses in a pasture because they provide more protein in the pasture material and because they may make the soil more fertile.
The competition between grasses and legumes in a pasture has been studied very closely. It has been found that whether grasses grow more strongly than the clovers or vice versa depends a great deal on changes in the environment brought about by grazing management or the addition of fertilisers.
For instance if animals are kept on a pasture until the tall grass growth is eaten down, this will let more light fall on the clovers which will then grow strongly. If nitrogen fertilisers are added to a pasture, this stimulates the grasses to grow very strongly, and they will outgrow the clovers.
4. Competition between Crops and Weeds:
Competition will occur between weeds or any other plants which may be growing with a crop. For instance, wheat may have to compete with lucerne which has been sown with the wheat. But the most serious competition is between crops or pasture plants and weeds.
A plant is a weed if it interferes with man’s use of land for special purposes, or with his well-being, or with the nature of his environment. Weeds are plants that come into places that man has disturbed, so that there would be no such things as weeds but for man.
Weeds have these features:
i. They are successful in growing in many different environments.
ii. Their seeds germinate easily in many soil conditions.
iii. The seeds may not germinate all at once, and some may live on a long time.
iv. The seedlings grow rapidly.
v. They make flowers and seeds very quickly.
vi. They keep making seeds for a long time.
vii. They make very large numbers of seeds.
viii. They grow together thickly quite well.
ix. They are able to spread their seeds very well.
x. If they are perennials, they can grow from pieces broken off the stems or roots.
xi. They compete very well with other plants.
Weeds compete with crop and pasture plants for water, soil nutrients, light and living space. In addition, they may choke harvesting machinery, injure or poison farm animals, or have other undesirable effects.
Because of these effects, weeds:
i. Increase the cost of producing crops or pasture,
ii. Reduce the quantity or quality of the plant product.
Weeds are commonly thought to be the greatest enemies of crops.
Because they are so successful in growing in disturbed places, people have come to realise two things about them:
i. It is not possible to eliminate weeds altogether; so instead of talking about weed control, we should speak rather of weed management by which means weeds are kept down to small numbers.
ii. It is unlikely that any one single control method will be successful, so several control methods are usually needed.
The chief methods of weed control are these:
1. Good Management:
Crops and pastures that are growing strongly compete much better with weeds. Anything that can be done to make crops and pastures grow better will reduce the effects of weeds.
Important things that can be done include:
(a) Choosing a strong-growing variety of crop or pasture plant;
(b) Choosing the best planting time and sowing rate;
(c) Making good use of fertilisers;
(d) Making good use of irrigation;
(e) Using good control of insects and diseases;
(f) Using good grazing management.
If soil is well prepared and a crop is sown when there is sufficient moisture in the soil, and if the sowing rate is increased a little, the crop plants may be able to “get away” from the weeds and shade them. Good crop management also includes the use of rotations which will help to control the effects of weeds, insects and diseases as well as raising soil fertility.
Good pasture management can also reduce weed problems by control of stocking rates, application of fertiliser at important times, draining, or sowing new kinds of pasture plants.
2. Physical Control Methods:
These are the oldest methods of weed control and are still important in many cases:
i. Cultivation can destroy weeds when they are at the tender seedling stage. It is most important that the cultivation be done when the weed seedlings are at the very young stage when they are weak. Cultivation may have to be repeated to destroy most of the weed population. It is of course most important, if possible, to prevent the weeds from seeding, for this would only make the problem much worse later on.
ii. In some cases mowing is a useful method of weed control, provided that there are no low-growing weeds in that place.
iii. The use of flame guns or flame throwers has been successful in control of weeds in some situations.
iv. Mulches have been successfully used for weed control in such valuable crops as pineapples and strawberries. The commonest kinds of mulches consist of plastic sheets of polyethylene or sawdust.
3. Chemical Weed Control:
Chemicals that are used to control the growth of weeds are called herbicides or weedicides. Although many hundreds of these substances have been discovered and used, only a few dozen of them are used in large quantities.
The advantages of using chemicals to kill or control weeds are:
i. They may be cheaper than using expensive methods of cultivation.
ii. Some weedicides may be used without disturbing the soil at all. Any form of cultivation will usually prune off some roots of a crop and this has a bad effect on the crop, so some forms of chemical weed control are called “minimal tillage” methods, or “chemical ploughing”.
There are many kinds of herbicides or weedicides and they are used in different ways and for different purposes:
i. Total herbicides or “knock down” weedicides are those which are used to kill off all plant growth. Sodium chlorate was once used to do this, but now chemicals like Paraquat, Diquat and Bromacil and Roundup are used. It is often necessary to use such chemicals at high strength to get a killing effect.
ii. Provided that they are used at the right strength, some weedicides have a selective effect; that is to say, they kill weeds but no other crop plants.
There are several kinds of these substances:
(a) Pre-sowing herbicides are those like Trifluralin and Nitralin which are applied to the soil before the crop seeds are sown. They kill the germinating weeds but not the crop plants.
(b) Pre-emergent herbicides are applied to the soil after the crop seeds have been sown, but before the crop seedlings have appeared above the ground.
These chemicals, like Linuron and Chloramben, will kill weeds but not the crop seedlings.
(c) Post-emergents are selective herbicides which can be applied to a crop after it has emerged from the soil. There are many such chemicals as Diuron, Simazine, Atrazine, Propanil, Asulam and Chlorbufam, but a very widely used group of chemicals are the phenoxyacids. These, like 24D, MCPA and 245T are cheap to make, can be very selective, but their toxic nature worries many people and their use is banned in some states.
4. Biological Weed Control:
Occasionally it is possible to use other organisms to control the growth of weeds. The best-known example in Australia is the use of the insect Cactoblastis to control the growth of prickly pear. A recent example of biological control is the attempt to use a rust fungus and several insects to control the serious pest known as skeleton weed.
5. Weed Ecology:
Once a weed has become established at several sites in an area, it may be impossible to eliminate it by using one or other of the normal control methods. The only way of attacking such a problem is to learn as much as possible about the plant and its relationship with other plants, animals and the environment.
This study of the ecology of a weed may show that it has certain weak spots which may be used to control it. For example, no amount of cultivation or chemical control could stop the growth of a weed called the groundsel bush, since one plant may make 1 500 000 seeds. Then it was found that the germinating seedlings had a high need for light. Therefore, if a good ground cover of pasture or crop can be established, the weed seedlings will not grow properly.
It is well known that some farmers make a success of growing crops, but others often have a failure. In a country like Australia where rainfall is not dependable, there is a good deal of luck in growing a crop, but successful farmers are those who plan carefully and do everything needed to make a crop grow well.
There are many factors which affect the growth of a crop, and all of these must receive attention if a crop is to be a success. A barrel will only hold as much water as the height of its lowest stave. In the same way, crop production can only be high when all of these production factors are ideal.