Plant Physiology - Musienko M. M. 2001

Physiology of Plant Growth and Development
Growth periodicity. Circadian rhythmicity. Biological clock

Growth processes, like other physiological phenomena, are inherently periodic. Plants possess mechanisms for perceiving changes in Temperature, day length, and light quality, which allows them to coordinate THEIR life cycle with seasonal climatic variations.

In temperate climates, the growth of perennial plants resumes every spring. The initial vessels formed during this period are quite broad and thin-walled, enabling them to easily transport the Water and nutrients essential for early growth. Later on, these vessels become fewer in number, narrower, and thicker-walled. Consequently, autumn wood differs markedly from the adjacent spring wood of the following year. In winter, the cambium enters a state of dormancy. As a result, annual rings are formed, their width depending on the climatic conditions of a given year. Dendroclimatology studies past climates using the data from such rings, whereas dendrochronology determines the age of wood based on the number and size of annual rings.

When periodicity persists even in the absence of external factors, the rhythm is referred to as endogenous, such as the "Sleep" movements observed in certain legumes (e.g., mimosa, bean). The leaves of these plants are positioned horizontally during the day and vertically at night—a phenomenon known as nyctinasty. Plants most commonly exhibit so-called circadian rhythms (from Latin circa — about, and dies — day), which have a period close to 24 hours (21–27 hours). Endogenous rhythms include mitotic activity in Meristems, growth rates, Photosynthesis and Respiration rhythms, and more. A circadian rhythm Functions as a self-regulating oscillator.

The cellular mechanism driving this rhythm is called the biological clock. The biological clock is a conventional term denoting the ability of living organisms, including plants, to orient themselves in time. This orientation is underpinned by the strict periodicity of all physicochemical processes, that is, endogenous biological rhythms. Thanks to endogenous rhythms, plants successfully adapt to specific, predictable weather conditions independently of fluctuations.

In most organisms, rhythms are entrained by daily cycles of light and temperature. Some theories suggest that The Nature of the biological clock stems from the ability of organisms to perceive cyclical fluctuations in geophysical factors, such as the daily and seasonal periodicity of Earth's electric and magnetic fields, as well as solar and cosmic radiation. Overall, the precise physical basis of endogenous circadian rhythms remains elusive, but most scientists believe that periodic changes in membranes constitute an integral part of the "clock mechanism." After all, membranes surround the entire Cell as well as individual cellular compartments. By rhythmically regulating the influx and efflux of metabolites into and out of The Cell and its Organelles, the biological clock generates rhythmic shifts in cellular biochemistry and physiology.

One of the most intriguing features of circadian rhythms is their temperature compensation, given that most metabolic processes and biochemical reactions are temperature-dependent. It is hypothesized that the clock involves two temperature-sensitive processes: one is accelerated by temperature, while the other is inhibited, resulting in their mutual cancellation. Above all, the biological clock enables plants to appropriately respond to changing seasons and adapt to their respective environmental conditions.