Plant Physiology - Musienko M. M. 2001

Physiology of Plant Growth and Development
Life Cycle of Higher Plants

The Development of higher plants is divided into the following stages: embryonic, juvenile (youth), reproductive (maturity), and senescence.

The embryonic stage of ontogeny characterizes the Development of the embryo from the zygote up to and including seed maturation (Fig. 158).

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Fig. 158. Life Cycle of angiosperms (Angiosperm)

The juvenile stage begins with the germination of seeds or vegetative propagation Organs (tubers, bulbs) and is characterized by the rapid accumulation of vegetative mass. At this stage, plants are incapable of sexual reproduction. Germination, in turn, is divided into the phases of imbibition, radicle emergence, heterotrophic growth of seedlings, and transition to autotrophic Nutrition.

The length of the juvenile period varies among different species, ranging from a few weeks (in annuals) to decades (shrubs, trees). The juvenile state is maintained by a specific balance of phytohormones.

The stage of maturity and reproduction is the period of preparation for flowering, the time of formation and growth of seeds and fruits, and the development of vegetative propagation organs.

The stage of Senescence and Death spans from the complete cessation of fruiting to the natural death of the Organism. These are the concluding Stages of Ontogeny in any organism, although the term "Aging" may refer not to the whole organism, but only to its individual parts (leaves, floral parts, fruits, etc.). During senescence, the content of chlorophyll, Proteins, and Nucleic Acids decreases in leaves, the intensity of Photosynthesis drops, and intracellular Organelles degrade. Concurrently, hydrolytic Enzymes are activated, ultimately leading to autolysis. The senescence of a whole plant organism is driven by the aging of its constituent parts and their correlative interdependence. H. Molisch explained aging as the outflow of nutrients to reproductive organs, whereas V.O. Kazaryan attributed it to a decline in ROOT system activity.

Throughout each of these stages in The life cycle of higher plants, new organs emerge. The process by which such organs are formed is called Organogenesis.

It must also be taken into account that young Tissues and Cells (Meristems) continuously arise and function in plants at any given moment, which in a sense "rejuvenate" the mature plant at the cellular, tissue, organ, and whole-organism levels.

The Study of age-related changes in plant organisms led M.P. Krenke to formulate the theory of cyclical aging and rejuvenation of plants. In his view, the physiological state of a new organ is influenced by the age of the maternal plant organism. The older the plant, the lower the physiological youthfulness of such an organ. Therefore, a distinction should be made between the calendar (or chronological) age and the overall (or physiological) age of an organ (e.g., a SHOOT or a leaf). The chronological or calendar age of an organ is the time elapsed from its initiation to the present moment. The overall or physiological age of an organ is defined as the calendar age of that organ taking into account the age of the maternal organism as a whole at the moment of the organ's initiation. Consequently, for instance, leaves of the exact same calendar age that originated on a young shoot versus an old shoot differ in physiological age. Thus, every part of a plant is characterized by the overall age of the maternal organism and its own individual age.

For the development of the most viable shoots, a plant must attain a certain physiological age. As the maternal plant ages, the viability of emerging organs increases progressively. Then, after reaching an optimal value, a further increase in age begins to adversely affect the viability of such organs. The ratio of aging to rejuvenation processes across the stages of ontogeny resembles a single-peaked curve, where rejuvenation processes prevail on the ascending slope, and aging processes on the descending slope. According to this theory, the transition of an organism from the Embryonic Stage to youth and maturity consists of gradual processes associated with aging. He believed that the entire developmental process is essentially an aging process that is cyclical and closely intertwined with the opposing process of rejuvenation. Different Cells and Tissues have varying juvenile periods, with meristems exhibiting the longest ones.

The tenets of this theory have proven valuable for crop production practices. For example, in vegetative propagation, it is crucial to accurately determine the physiological age of cuttings, as their viability depends on it. From Krenke's theoretical perspective, early and late maturity in most plant species are governed by the rate at which the aging process unfolds. His views provide a rationale for rejuvenation practices in fruit trees, The Role of pruning, The Use of tea leaves or mulberry leaves of a specific age for silkworm feeding, and the rooting of cuttings.