Look closely at a sliced tomato and you will see the beginning of many new tomato plants hidden inside the juicy red fruit (see Figure 1). Every tomato seed contains a tiny tomato plant that is alive but dormant. This means that it is not growing, it is just waiting. When environmental conditions are right, the seed will germinate. This is when the tiny plant sprouts from the seed and begins to grow.
Figure 1: Seeds inside a tomato.
Source: Pixabay
Flowering plants (including tomatoes) make more of themselves by producing seeds. The job of the tiny plant that is inside every seed is to break out and continue growing into a mature plant. The tiny plant is called an embryonic plant; embryonic means it is at an early stage in development. The embryonic plant has an embryonic root called a radicle and one or two embryonic leaves called cotyledons. The immature plant also has a plumule, which will develop into the shoot (Not visible in Figure 2).
Figure 2: Diagram of the inside of a tomato seed
Adapted from: Seedbiology.com
A protective shell, called a seed coat or testa, surrounds the embryonic plant. Many embryonic plants, including tomatoes, get the energy they need to germinate from stored energy inside the seed called the endosperm. This energy is usually starch, a complex carbohydrate made of many sugar molecules linked together. In other kinds of seeds, called non-endospermic seeds, the stored energy is in the cotyledons. Peas and green beans are two examples of plants with non-endospermic seeds. Flowering plants are classified by how many cotyledons they have in their seeds. When a flowering plant has seeds with one cotyledon, or embryonic leaf, it is called a monocotyledon, or monocot for short.
When a flowering plant has seeds with two cotyledons, it is called a dicotyledon, or dicot for short. Tomato plants are dicotyledons (See Figure 2).
Seeds are alive, but they do not live forever. Some seeds, like onions, only keep their germinating ability for one year. Others, like cucumber and lettuce, can still sprout after five years or more. Tomato seeds will live for at least four years.
In nature, seeds travel through the environment so that plants can grow in new locations where they will not have to compete with their parent plants. Seeds can travel by wind, water or by hitchhiking on the fur of animals. Tomato seeds can use animals for transportation in a different way. They can survive digestion and be “planted” in the animals’ waste. Some Tomatosphere™ seeds have even experienced transportation on a space shuttle!
Like all seeds, tomato seeds stay dormant until the environment has all the conditions needed for germination. Seeds need water, oxygen and warmth to start growing.
Dry seeds are dormant seeds (see Figure 3A). Water softens the testa to make it easier for the embryonic plant to sprout. Water is also taken into the cells of the embryonic plant and makes the cells bigger.
Figure 3: Photos of tomato seeds before and during germination. A. This is a dry, dormant tomato seed showing the hairs (called trichomes) on the testa. B. After 2 days in a warm, moist environment the radicle has started pushing through the weakened testa. C. After 4 days the radicle has grown several millimetres. D. After 6 days, the cotyledons (green) have emerged from the testa and the radicle has continued to grow (not shown). Note that 1 millimetre (1 mm) is marked in each photo
Source: Let's Talk Science
Oxygen also enters the seed. Just like animals, plant cells need oxygen for respiration, which is the process of getting energy from food. At this stage, the tomato seed gets energy from the endosperm.
Different seeds germinate best at different temperatures. In general, very cold temperatures prevent germination and warmer temperatures cause faster germination. Tomato seeds will not germinate below 10°C and their preferred range is 16-30°C.
When a tomato seed has water, oxygen and warmth, the embryonic plant begins to grow, using energy from the endosperm. The radicle is the first part to push through the softened testa (see Figure 3B and 3C) and the cotyledons emerge next (see Figure 3D). As long as environmental conditions stay favourable, the plant will keep growing, with the roots extending downward and the shoot and cotyledons pushing upward.
Environmental factors, such as water, oxygen and the right temperature are all needed for germination, but processes inside the seed are important too. For example, embryonic tomato plants produce growth hormones called gibberellins during germination. Gibberellins help weaken the endosperm and testa so that the embryonic plant can sprout.
As the tomato seedling grows, it stops using stored energy from the endosperm and starts using its cotyledons to make its own energy through photosynthesis. This is when the plant starts to need light and carbon dioxide in addition to water, oxygen and warmth. Light, carbon dioxide, and water are all needed for photosynthesis to occur.
Tomato seeds do not need soil to germinate (see Figure 3). However, the seedlings will eventually need to be planted in soil to get the nutrients and minerals they need to become healthy tomato plants.
One of the main kinds of nutrients in food and a source of energy for plants and animals; sugar is a simple carbohydrate and starch is a complex carbohydrate. Complex carbohydrates are made of many sugar molecules linked together.
The part of an embryonic plant that will become the first leaf (or leaves). These are also known as seed leaves.
A period of reduced activity that conserves energy in a living thing. Seeds contain dormant embryonic plants that will not grow until the right environmental conditions are found.
The tiny immature plant inside a seed.
The part of seeds that stores food in the form of starch (large molecules made of sugar) for use by the embryonic plant during germination.
The process of a plant starting to grow from a seed. This usually happens after a period of dormancy. Good environmental conditions are important to start germination.
Plant growth hormones that are essential for many steps in plant growth and development, including embryonic plant growth and weakening the endosperm during seed germination.
The process used by plants to change light energy into biochemical energy (sugar). Light energy is used to turn carbon dioxide and water into oxygen and sugar.
The part of an embryonic plant that will become the shoot.
The part of an embryonic plant that will become the root.
The process used by plants and animals get energy from food (specifically sugar molecules). It is often called cellular respiration to distinguish it from breathing (inhalation and exhalation).
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