Ii. Seeds and Seed Germination

UNIT 4 - PLANT APPLICATIONS

I. INTRODUCTION

II. SEEDS AND SEED GERMINATION

1. Seeds

2. Seed Germination Factors

3. Effect of Humic Acids on Germination

III. PLANT PROPAGATION

1. Seed Propagation

2. Propagation from Cuttings

3. Other Propagation Methods

IV. TRANSPIRATION STREAM APPLICATION

V. UNIT SUMMARY

INTRODUCTION

Organic vegetable growers use transplanted seedlings to grow many crops. The benefits of transplants include earlier planting, accurate spacing, and getting ahead of the weed seed that will germinate at the time of planting. Reasonably priced organic transplants are not readily available, so most farmers grow their own.

Most vegetable crops are grown from seed but many fruit crops are produced from asexually propagated plants. Knowledge of both methods of reproduction is important to establishing an organic farm.

SEEDS AND SEED GERMINATION

Objectives:

1. Describe the structure and formation of a seed.
2. List the three requirements for germination of most seeds.
3. State the effect of humic acids on germination and where they can be found.

SEEDS

Plants reproduce by making seeds. The seeds are a storehouse for the plant’s offspring, waiting for the right conditions to begin growing. They have a husk or outer covering, a germ or living embryo, and food stored to get the plant started.

Suzanne Foster @USDA-NRCS PLANTS Database

Seeds are formed inside the flowers of plants when pollen from the same plant or another plant lands on the part of the flower that contains the ovum or egg. This starts the growth of the embryo and the formation of the seed. The embryo grows to a certain point inside the seed covering and then stops. This cessation of growth is called dormancy. Dormancy begins when the seed has finished forming on the plant and the embryo has developed a radicle or embryonic root and one or two cotyledons or embryonic leaves. Many of the seeds used in agriculture have no dormancy requirement. They will sprout under the right conditions as soon as they mature.

SEED GERMINATION FACTORS

Germination is the resumption of growth of the plant embryo within the seed after a period of dormancy. The first visual sign of germination is when the radicle bursts through the seed coat and protrudes as a young root. Seeds germinate or sprout if the right conditions are present. Most seeds need three things to begin sprouting:

• Light
• The right amount of moisture
• The right temperature

Light is important in stimulating seed germination. All seeds need some amount of light exposure to start sprouting. For some seeds, all it takes is a flash of light. This can happen, in the case of weed seeds, when we plow or cultivate. The seeds are exposed to a flash of light when soil is disturbed. Others require constant light to stimulate germination. An example of this is lettuce seed.

A myth about seeds that need constant light to stimulate germination is that they have to be placed on the surface of the soil or planting mix and left uncovered. Enough light can penetrate through soil and soil mixes below the surface to stimulate seeds to sprout. If they are buried below this penetration zone, however, they will not receive enough light. One of the problems with surface sowing is that the seeds may dry out because there is not have enough moisture on the soil surface to stimulate germination.

The right amount of moisture is another requirement for germination, If there is too much or too little moisture present, most seeds will not germinate. Plants used as agricultural crops mostly fall into a group requiring the same amount of moisture for their seeds to germinate. This requirement is moist but not so wet that the seed has water all around it. They can also be moistened and begin the germination process even if they dry out again. Some seeds, like parsley and carrots, require a long period of a week or more of constant moisture to germinate. If the seeds are moistened and then dry out or if they go through a wet and dry cycle, they will not germinate well or at all.

The third requirement for germination is the right temperature. This varies from plant to plant. For instance, spinach has a much lower germination temperature than tomatoes. Temperature stimulates the seeds to germinate in the season to which they have become adapted.

There is an optimal temperature for each kind of seed. At this temperature more seeds germinate and at the same time. However, the temperature can be slightly higher or lower and still stimulate the seed. Some seed catalogues show this range as a chart, as well as the optimal germination temperature.

There are some seeds that need special conditions for germination, like scoring of the seed husk or a number of cycles of freezing and warm temperatures. Seed companies sometimes prepare seeds for fast germination by performing these procedures before the seed are sold.

EFFECT OF HUMIC ACIDS ON GERMINATION

Recent experiments have shown that humic acids stimulate faster germination of seeds. In these experiments, humic acids were extracted from organic matter and mixed with water. This solution of water and humic acids was used to water seeds planted in flats. These seeds sprouted faster than those planted in the same way but without humic acids in their water. The seeds treated with humic acids also had a higher germination rate. There is no explanation yet of how this works, but it is possible to use this information in starting seeds.

Finished compost is high in humic acids if it has not been exposed to high rainfall that would leach them. Sifted compost can be used to cover seeds in flats to stimulate germination. Seeds sown in beds and rows can be covered this way, also. This puts the seeds in direct contact with the compost. The compost will also act as a mulch cover for the seeds. The soil surface where the seeds are sown will stay moist longer and this will also aid in germination.

SUMMARY

Seeds are the plants’ offspring. They develop when the ovum in the flower is fertilized by pollen. A mature seed contains an embryonic root, called a radicle and embryonic leaves called cotyledons. There is an outer protective cover around the embryo and stored food for it to live on until it starts to make its own food. Some seeds have a certain dormancy requirement before they can germinate, but most agricultural seeds do not.

PLANT PROPAGATION

Objectives:

1. List three benefits to starting seeds in flats in a greenhouse.
2. Describe three ways to get an acceptable organic soil mixture.
3. Identify the hormone important in stimulating rooting of cuttings and where it can be found.
4. List three other ways to asexually propagate plants.

Normal plant propagation takes place through sexual reproduction. The plant’s flowers produce seeds and these produce new individuals. There are also asexual propagation techniques that are used to produce more plants. These techniques use tissues or parts of plants to start new ones. Asexual methods produce plants that are identical to the original one. They are used primarily for woody perennials like berries and nursery crops or ornamentals, but some are used for agricultural crops.

SEED PROPAGATION

Almost all agricultural crops are started from seeds. These are planted either in fields or beds (direct seeded) or in flats and the young plants are transplanted later. Some plants do better if direct seeded, like root crops, beans and peas. However, most can be transplanted after starting in flats.

There are benefits to starting seeds in flats or containers indoors or in a greenhouse. The first of these is that the best conditions for germination can be created for each type of seed. There is no way to control soil temperature in fields unless some type of cover is placed over planted areas. In addition, it’s much easier to control the moisture in a greenhouse. Another benefit is that plants can be started before outdoor temperatures are suitable for the crop.

One of the challenges of starting seeds indoors in an organically certified operation is finding a “starting mix” that is acceptable under the standards. This is because most locally available mixes contain chemical fertilizers and other substances that are prohibited. Local sources for acceptable mixes are scarce in most areas. This means that shipping costs make acceptable mixes that are commercially available expensive.

However, organic growers can make their own mixes for low or no cost. Custom mixes can be made from acceptable materials like peat moss and perlite. Organic “starter” fertilizers and minerals can be added to these as solids when mixed or in dissolved form in water after planting the seeds. These types of “sterile” or “soiless” mixes are usually required for resale of plants.

If all the plants are for use on the farm where they’re started, a mixture containing soil can be used. To make a soil mix, select some soil from a fertile area or field. The soil can be used in flats and cells without additions. If used this way, it will help to cover the seeds with sifted compost. This will stimulate germination while acting as mulch to hold moisture in the flats or cells.

In the case of sandy soil, a mixture of 50% sandy soil and 50% sifted compost works well. This ratio will improve the mix’s water holding capacity, reducing the number of times seedlings will need to be watered each day. A high clay content soil may require addition of some organic matter to help it drain. This can be either compost or peat moss. If the soil is rocky or contains clods, it should be screened or sifted before using for plant starting.

A sifter of any size for both soil and compost can be made by constructing a wooden frame. Wheelbarrow-scale sifters can be made from 1x2 or 2x2 boards. These can be nailed or screwed together and hardware cloth attached as the screen.

The main disadvantage of using a mix containing soil is that weed seeds will be in the soil and will sprout soon after the crop seeds. Hand weeding can eliminate these easily, but takes time and some care. Also, the weeding needs to be done before the weed sprouts grow so much root mass that pulling them will disturb the crop roots.

PROPAGATION FROM CUTTINGS

Plants have three basic structures with different tissues: roots, stems, and leaves. However, most plants have the ability for one of these structures to grow the tissues of another. This ability enables us to grow plants from different tissues removed from the parent plant.

An example of this is the ability of plants to grow roots from their stem tissues. The plants do this when we place a cutting from a plant containing stems and leaves into soil or some other rooting medium. If the plant has this ability and there is sufficient water and nutrients available, the cutting will grow root tissues where there were none.

Auxin is the active ingredient in most rooting compounds in which cuttings are dipped during this type of propagation. This ensures that the plant being rooted has an abundance of this hormone to stimulate growth of root tissue. Plant hormones that stimulate this are abundant in willow tree tissue. This is why they’re so easy to root from cuttings. It’s possible to make an extract from willow branches by cutting them up and soaking in water. This “willow tea” is used to water cuttings after placing them in the rooting medium.

OTHER PROPAGATION METHODS

Other methods of propagation include layering, separation of bulbs and tubers and tissue culture. Layering causes roots to develop on shoots that are still attached to the parent plant. The stem is not cut from the main plant until it has rooted. This method is a good propagation choice when only a few plants are needed.

Simple layering is done by bending a branch to the ground and burying a portion of it, leaving the tip uncovered. Hold the branch in place with a rock or peg. Layering is usually done in early spring while plants are still dormant or in late summer on wood that has not become woody. Plants with flexible branches are particularly suited to this method.

Plants can be propagated from tuberous roots. This type of propagation involves cutting the tubers into pieces so that each piece has at least one eye or bud. An example of this is cutting up potatoes before planting to increase the number of plants from a quantity of seed potatoes.

Tissue culture is a type of asexual reproduction. Each plant cell has the potential to grow into a new plant exactly like the parent. This fact coupled with technical advances, specialized equipment and sterile laboratory conditions has led to tissue culture. In tissue culture, individual or small groups of plant cells are manipulated so they each produce a new plant. A tiny piece of bud, leaf, or stem can produce incredible numbers of new plants in a small space in a short time. However, there are some problems with spontaneous mutations that naturally occur. In tissue culture, the incidence of these mutations is much greater than through other methods of propagation.

Conditions for tissue culture are very exacting. Sterile conditions must be maintained. Temperature, light, humidity, and atmosphere are strictly controlled with electronic sensors and computerized controls. Such costly equipment rules this out for most farm operations.

SUMMARY

Seeds are used to start most agricultural crops. They may be directly sown in the field or started in the greenhouse in flats and later transplanted to the field. Organic soil mixes may be bought or made, but care must be taken to make sure they do not have chemical fertilizers added. Peat and perlite are basic ingredients for most mixes. Mixes containing soil can be used but weed seed may be a problem.

TRANSPIRATION STREAM APPLICATION

Objectives:

1. Describe why wilting occurs in young tender plants like lettuce, when there is water available to the roots.
2. Identify a method to prevent wilting.

Another application of plant physiology that we can use to enhance plant growth has to do with understanding the transpiration stream. Some plants wilt easily. These are usually tender plants that are grown for their leaves like lettuce. These tender plants don’t have the ability to tolerate heat as well as most other crops. They wilt even with sufficient water present near their roots. This is because they lose water through evaporation faster than their roots can absorb it in an attempt to cool themselves. This more rapid loss of water “breaks” the transpiration stream and stops the flow of water through the plant.

Wilting under these conditions can be avoided through the type of irrigation used. Wilting of crops when sufficient water is available is common when drip irrigation is used. Overhead irrigation, timed to cool the plants before temperatures become critical, will cool the plants’ tissues and prevent wilting. This will also rapidly revive already wilted plants. Overhead irrigation or misting can be combined with drip, using the drip to supply water to the roots and overhead to cool at timed intervals. Use of a gentle spray nozzle on a hose can be used for small operations.

SUMMARY

Some plants will wilt when there is plenty of water available to the roots because they are losing water so rapidly from the leaves that the transpiration stream is broken and water can no longer be brought up from the roots. Cooling the plants with overhead irrigation can prevent this problem.

UNIT SUMMARY

Seeds are the way plants reproduce and they contain the structures necessary to make a new plant within a seed coat filled with stored food. Some seeds need a period of dormancy before they can germinate. Germination starts with the absorption of water and the eruption of the radicle from the seed coat. Light, moisture and the right temperature are necessary for seeds to germinate. Humic acids seem to promote germination of seeds.

Plants can be propagated by sowing seeds in flats in a greenhouse or directly in the field. Care must be taken to use an organic seed starting mix if growing in the greenhouse. Some plants can be propagated using asexual methods such as layering, rooting stems, dividing tubers and tissue culture.

Knowledge of how the transpiration stream works allows for techniques to prevent wilting in plants that transpire very rapidly from large leaf surfaces.