Growth and Development Physiology in Plants
Like every living thing, plants are born, grow and differentiate and eventually die. The phenomenon of increase in volume caused by the addition of new substances to the plant or plant organs is called growth. With the new substances gained as a result of growth, there is an increase in volume as well as differentiation. The growth event consists of two successive phases. These:
Growth
Development and differentiation
These two events occur in all tissues and organs, from the cells, which are the smallest units of plants, to the most differentiated organs.
Germination
After the seeds are separated from the mother plant, they go through a resting phase for a certain period of time, either free or in fruit. Meanwhile, the seed is in a structure to resist external effects such as cold and heat and other mechanical impacts. The amount of water in the cytoplasm of the cells that make the seed has been reduced to a minimum, and its vital functions have also decreased so much that it cannot be distinguished. ready for a journey. When the seed finds a suitable environment for germination, the signs of life begin to accelerate immediately. This biological event that will enable the development of the embryo made of divisible, embryonic cells in the seed is called germination. It is possible to collect the environment required for germination in two parts as internal and external conditions. The internal condition is the ability of the seed to develop in terms of formation and structure. External conditions are mainly water, heat and oxygen of the air. Also, light is needed in advanced circuits.
Germination first begins when the seed swells by absorbing water from the environment. This is followed by physiological events such as the dissolution of the substances in the nutrient tissues of the seeds by means of special enzymes, and their use by sending them to the necessary places. In the third stage, the intake of nutrients and the growth and development events provided by these are followed.
In the process of these events, the young, dividing cells of the embryo divide rapidly on one side and the number of cells in the embryo increases. On the other hand, some cells elongate, expand, and eventually the various organs of the embryo enlarge and become prominent. The first organ to emerge from the seed in germination is the root of the embryo. In the meantime, the hard shell of the seed is cracked as a result of swelling and the embryo parts are allowed to extend outward. The rootlet extends into the soil and develops rapidly.
The root is the organ that will take the necessary nutrients from the soil for the growth of the young plant. After the development of the root, it is seen that the stem and leaflets rise above the soil. A young plant consisting of roots, stems and leaves from a seed embryo is called a seedling.
Growth of Plants
In the seedling that develops with seed germination, each cell of various organs first expands in volume, develops, differentiates and increases the number of cells by dividing continuously. Thus, the entire growth of the embryo is ensured. With the repetition and continuous formation of all these events, the seedling becomes more and more developed. The various organs that make the seedlings and the cells that compose them form the mother plant, which will reach higher stages of development and differentiation by continuing the same growth and development stages without stopping. The structural growth and development period of the plant ends with the complete formation and differentiation of organs such as roots, stems and leaves.
Factors Affecting Growth
The growth and development events of plants depend on various factors of the environment and the internal characteristics of each plant.
External Factors
Various external factors have different effects on growth and development. Especially the effects of light, temperature, water and gravity are the most important external factors.
Light
Green plants require a certain amount of light for their growth and development. However, some primitive plants do not want light. Excess and intense light often has a stopping effect on growth. This is why plants are stunted in overly sunny environments. On the other hand, lack of light accelerates growth. It causes abnormal growth of the plant. A plant forced to live in a dark environment will grow too tall, its color will be yellow and its leaves will be small.
The light required by each plant for its structural growth is variable and light intensity is an important factor in growth. The demand for light intensity varies according to the plant species. In this respect, plants are distinguished as shade-loving and sun-loving plants.
Heat
In order for each plant to grow and develop, there must be a certain temperature in its environment. In general, plants can develop and grow between 0 and 40 0C. Under and above the appropriate temperature, which varies according to each plant, development and vitality come to an end. Each plant has different resistance to cold and heat.
Water and Humidity
Water is one of the essential substances for plants to live. A certain amount of water is needed for plants to grow and develop. Excess water generally accelerates growth. On the other hand, growth retardation is observed in plants grown in arid environments. Therefore, stunting is noticeable. Water scarcity also negatively affects differentiation.
Gravity
Gravity accelerates the secretion of hormones in plants. This event affects growth and development. Therefore, the effect of gravity takes place indirectly.
Internal Factors
The genetic characteristics of each plant are effective in growth and development. For this reason, regardless of the external environment, a plant produces an individual suitable for its own species as a result of its hereditary ability.
One of the factors affecting growth and development in plants is hormones. Hormones are organic substances that can be transported from the place where they are produced and have a lot of effects although they are very few. We can examine plant hormones in three different groups.
Growth hormones: Auxins, gibberellins, cytokinins
Organ builders: These hormones affect the formation of flowers, roots, stems and leaves.
Wound hormones: They are secreted when the plant is injured. It allows the wound to heal.
One of the factors affecting growth and development in plants is vitamins. They are organic compounds such as vitamins and hormones that affect growth and development even in small amounts.
Some Problems With Growth
Polarity in Plants
One of the important problems in plants is the formation of three-dimensional shape and differentiation related to growth. During development, all single-celled or multi-celled organisms acquire various forms at the stages of development from the embryonic stages. During the acquisition of these forms, various asymmetrical regions emerge.
Correlation Between Plant Organs
It consists of the sum of the features that make up the whole plant and the features of the various organs of that plant. Each of the different organs of a plant shows its own characteristics and behaviors. All of these behaviors form the unique behaviors of the plant. On the other hand, various organs with these different characteristics work in harmony in the plant. However, it is also known that there is a certain change in the plant as a result of the deliberate or accidental detachment of any of the different organs that make up a plant.
For example, if the blade of a leaf is cut off, the growth of the petiole is significantly reduced. On the other hand, the leaf petioles falling off from the stem (leaf shedding) accelerates. Although the lack of leaf blade had a reducing effect on the elongation of the petiole, it accelerated the development of fungal tissue that caused the stem to fall. This explains to us that when the palm is present, it has an effect that accelerates the elongation of the stem, whereas it also creates a stopping effect in the development of the fungus tissue that allows the stem to fall.
Dormancy
Plant organs enter the resting phase in some periods of the year in terms of growth and development. This is called sleepiness or dormancy.
The state of sleep is broken by processing different environmental factors and various chemical substances. Development and growth are awakened in that organ and the sleep state is shortened.
Abscession in Plants
One of the most visible signs of the growth and development of plants is that they leaf out in the spring and shed their leaves in the fall. The event (defoliation) that results in plants losing their leaves under the influence of certain external and internal conditions is defined as abscission. Defoliation is a characteristic feature, especially in woody dicotyledon plants. Defoliation, which can also be seen in many herbaceous plants, is sometimes quite sudden and sometimes occurs slowly and gradually.
Growth Periodicity in Plants
It has been observed that there are differences in the vital activities of trees in winter and spring in our daily lives. We cannot think that such a periodic development depends entirely on external factors. Certainly, the role of the external environment in such a transformation is great. However, views that only consider external conditions cannot always explain this transformation in development. Because, although there is no winter in a plant that we move to a warm climate, a decrease in development and abnormalities occur with defoliation. Therefore, not only external factors but also internal factors are involved.
Plants have an unlimited growth and a much longer lifespan than animals and humans, both because of this periodic development and because they have permanently divisible meristematic tissues. As a matter of fact, among plants that last a week, annual and perennial plants are easily distinguished. However, it is also known that many trees have a very long life.
Restitution in Plants
In plants, in an injured area, it is seen that the indivisible tissue cells of that area gain the ability to divide and divide rapidly and repair the wound. The event that cells begin to divide and close the wound is called regeneration. Sometimes, thanks to this ability, a missing organ or even a whole plant can be formed from a piece of tissue. The formation of an organ or organs with the help of regeneration is called restitution. The practical consequences of these events have revealed the technique of vaccination, which has found wide application in agriculture.