Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

7. Plant Growth and Development
7.6 Periodicity of Growth

Growth processes are cyclical in nature. In plants, the most common are circadian rhythms with a period of approximately 24 hours: mitotic activity in Meristems (roughly 20 hours), growth rate (roughly 24 hours), rhythms of Photosynthesis and Respiration, substance transport, flower opening, and so on. These rhythms are closely linked to daily fluctuations in light, Temperature, and other environmental factors. However, the pre-existing periodicity of physiological processes persists in plants for some time even when environmental conditions change, which is why these rhythms are termed endogenous. Through these rhythms, living organisms have successfully adapted to their native habitats and remain largely independent of random weather fluctuations.

In addition to daily periodicity, there is seasonal periodicity (such as annual growth rings in woody plants). During growth, at all Stages of Ontogeny, plants experience a dormancy period. This period occurs at the stage of embryonic development (dormancy of the zygote and seeds), and shoots remain dormant across various phases of vegetative development. A distinction is made between enforced dormancy, caused by environmental factors, and physiological dormancy, which refers to delayed germination due to Specific features of the embryo or its surrounding Tissues (a unique physiological state of the embryo, often involving morphological underdevelopment). The state of dormancy is regulated by a balance of phytohormones: indole-3-acetic acid, Cytokinins, Gibberellins, and ABA. The dormancy of buds and shoots is determined by climatic conditions to a much greater extent than seed dormancy. In this way, plants adapt to withstand adverse climates. In woody plants, SHOOT apices stop active growth in winter and become covered with bud scales. In their dormant state, plants exhibit significantly greater frost resistance than during active growth. Bulbs, tubers, and rhizomes serve the same adaptive purpose, allowing them to withstand overheating and drought as well.

Most species in temperate zones regulate dormancy through photoperiodic responses. Long days promote vegetative growth, whereas short days trigger growth cessation and The formation of dormant buds. Nevertheless, not all plants respond to changes in day length, and breaking dormancy is governed by factors beyond photoperiod alone. In many plants, buds break dormancy only after prolonged exposure to low temperatures or As a result of brief exposure to high temperatures (thermal baths, +30-40 °С).