Plant Ontogeny: Life Cycle and Individual Development of Plants
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Plant ontogeny is the individual development of a plant organism. Individual development, or ontogeny, refers to a plant organism passing through its normal life cycle, from seed germination to the natural death of the plant.
What is the life cycle of plants?
The life cycle of plants varies in length. Most herbaceous plants live from one year to several years, while woody species such as ash and linden can live for several centuries; in addition, there are woody plants whose lives span thousands of years. All plants are divided into two groups:
- monocarpic — bearing fruit once;
- polycarpic — bearing fruit repeatedly.
What are monocarpic plants?
Monocarpic plants flower and produce seeds only once in their lifetime; once the seeds form, the plant dies. The lifespan of monocarpic plants varies considerably.
It is extremely short in ephemerals — small plants growing in arid zones. Germinating in spring during periods of rainfall, they quickly move to flowering, form seeds, and then die off.
The next group of monocarpic plants is the annuals, whose lifespan lasts only a few months.
The third group consists of biennials, with a lifespan of two years. In the first year they form only vegetative and storage organs; the following year the plant produces a flowering stalk, an inflorescence, and seeds (cabbage, carrot, turnip and others).
Some species of bamboo form only vegetative organs over a number of years, then flower, set seeds, and die. An even more striking example of how the life of a monocarpic plant depends on the onset of fruiting is the American agave, which flowers in its eighth to tenth year of life.
It produces a gigantic stalk with an inflorescence, sets seeds, and then dies off.
What are polycarpic plants?
Polycarpic plants flower and produce seeds many times during their life. All of these plants are perennials.
How are growth and development connected?
According to D. A. Sabinin's definition, growth is the process of forming new elements of the organism's structure, while development is the change in the formation of new structural elements caused by the organism passing through its life cycle — that is, qualitative changes in the plant organism as a whole.
Growth and development are interconnected and depend on one another. It is rather difficult to draw a boundary between growth and development, because a superficial observation may fail to notice the difference in the newly forming organs. It seems that winter wheat during tillering is only growing, not developing.
In reality, however, careful examination reveals that the formation of a new leaf is accompanied by definite changes in the process of forming new structural elements, which means the process of development is also at work here.
What are the main stages of growth and development?
The individual life cycle of a plant — ontogeny — consists of a series of basic stages of development and growth:
- embryonic,
- youth,
- maturity (sexual or vegetative),
- reproduction (sexual or vegetative),
- old age.
In seed plants:
- The embryonic stage begins at the moment of egg cell fertilization and continues until the seed embryo germinates.
- The youth stage lasts from the germination of the seed embryo until the first flower primordia appear on the plant. During this stage, leaves, stems and roots are formed.
- At the maturity stage, the formation of the generative organs is completed.
- The sexual reproduction stage begins with the formation of the embryo and lasts until the fruits and seeds ripen.
- The old age stage runs from the cessation of fruiting to the death of the plant.
Monocarpic plants pass through all these stages once in a lifetime. Polycarpic plants pass through the first two stages once, while the stages of sexual maturity and reproduction are repeated many times. Polycarpic plants are characterized by a longer duration of all the stages of ontogeny.
How do ecological conditions affect plant ontogeny?
Plant ontogeny depends on the prior history of the plant organism and on ecological conditions. By changing environmental conditions, the rate of an organism's development can be sped up or slowed down, and the timing of flowering and fruiting can be influenced.
By regulating light intensity and mineral nutrition, G. Klebs prevented ground ivy from flowering for a number of years. G. Gassner, by exposing winter plants and some biennials to lowered temperatures during the early stages of development, achieved their flowering and fruiting in the first year of life.
Many studies have established that some plants require a certain temperature at the beginning of development, and then a certain day length, in order to move to flowering and fruiting. An interesting fact is the relationship of winter and biennial plants to particular temperatures.
The level and duration of temperature required for different plant species to move to flowering are not the same. Plants whose heredity formed in southern latitudes require higher temperatures to move to flowering than plants whose heredity formed under low-temperature conditions.
When studying the effect of lowered temperatures during the early stages of development of winter plants, it was established that exposing germinating seeds to temperatures of 0 to 5° for 35–60 days (depending on the variety) allowed winter wheat varieties to produce a harvest when sown in spring. In this way, plants were obtained that passed through their life cycle in the manner of spring (summer) plants.
What is vernalization?
The process of exposure to lowered temperatures that removes the delay in flowering in winter plants and biennials and brings the course of their development closer to that of spring plants was named vernalization.
For the vernalization process to occur, besides a certain temperature, good aeration and a high water content in the tissues are required. In cereal grasses and some grain legumes, vernalization can take place either in germinating seeds or in green plants; in biennials (beet, cabbage and others) it occurs only in actively growing plants or in overwintering vegetative organs.
After vernalization is completed, winter and biennial plants are prepared for photoperiodism reactions, which ultimately lead the plants to flowering.
What is photoperiodism in plants?
W. W. Garner and H. A. Allard established the dependence of the onset of plant flowering on day length. The influence on plants of the ratio between the length of day and night was named photoperiodism.
Plants that flower faster when the night is longer than the day during a 24-hour period were called short-day plants; conversely, plants that flower faster when the day is longer than the night were called long-day plants. Perilla, soybean, millet and southern varieties of maize are short-day plants; wheat, oats, barley, radish and others are long-day plants. There is also a group of neutral plants that flower at any day length.
These include, for example, certain varieties of tobacco and buckwheat. Research on photoperiodism was carried out by V. N. Lyubimenko, A. V. Doroshenko, N. A. Maksimov and V. I. Razumov. They established a connection between the geographic origin of plants and their reaction to photoperiod.
The researchers showed that plants of southern origin, where the transition to flowering occurred under the influence of a short day, are short-day plants, while plants of northern origin are long-day plants.
It was also found that for a photoperiodic reaction to appear, it is not necessary for plants to be at a particular day length throughout their vegetative period. Work by S. A. Egiz, V. N. Lyubimenko, N. A. Maksimov and V. I. Razumov established that the photoperiodic reaction arises after plants spend 10–15 days, and sometimes more, under the optimal photoperiod.
What is photoperiodic induction in plants?
Plants must be exposed to a particular photoperiod at the beginning of their vegetative period; moving plants, after the photoperiodic exposure ends, into a day length unfavorable for the photoperiodic reaction does not delay their flowering.
This phenomenon was called photoperiodic after-effect — induction. The duration of photoperiodic induction differs among plants: in wheat and oats — long-day plants — it amounts to 20–30 days; in perilla and cotton — short-day plants — it is 7 and 75 days respectively. After this, plants no longer respond to day length. Work by M. Kh. Chailakhyan and B. S. Moshkov established that the organs perceiving the photoperiod are the leaves.
A short-day plant grown the entire time under long days does not flower; the same plant whose leaves are kept under short days while the shoot tips are under long days does flower. With the reverse arrangement — leaves under long days and shoot tips under short days — the plant does not flower. When kept under short days, the plant flowers.
Biochemical studies of control plants and of plants that had passed through both stages of development showed that the metabolism of the latter changes. During vernalization, the activity of oxidative and hydrolytic enzymes increases, and the embryos become enriched with soluble sugars, water-soluble proteins and ascorbic acid.
The accumulation of nucleic acids, chiefly RNA, increases, and the position of the isoelectric point of proteins shifts toward a lower pH. In addition, the overall physiological state of the plant changes, and frost resistance and drought resistance drop sharply. Plants kept under a favorable photoperiod are distinguished by higher intensity of photosynthesis and respiration.
In them, the oxidative potential decreases and the reductive activity of the tissues increases. In long-day plants, the content of nucleic acids increases under long days; in short-day plants, under short days. Despite the large number of studies on the processes of vernalization and photoperiodism, the material obtained is still insufficient to fully characterize the metabolic changes that constitute the essence of these processes.
Knowing the conditions for plant flowering, one can speed it up or delay it, which has practical significance. Long-day plants that receive short days after vernalization do not move to flowering even under long days and accumulate a large vegetative mass. This technique can be used when growing lettuce, spinach and certain grasses to obtain green mass.
Short-day plants sown in the north (where the day is long) likewise delay their development and produce a large green mass. Maize grown under these conditions can be used for silage.
