PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

11. PLANT GROWTH AND DEVELOPMENT

The Influence of Environmental Factors on Cell Growth

Plant growth is influenced by a variety of environmental factors. These include physical factors such as light, Temperature, gravity, gas composition, magnetic fields, humidity, nutrients, and mechanical stress.

Furthermore, when growing within plant communities, plants are subjected to METABOLISM/18.html">The Influence of metabolic products from other plants (allelopathy), as well as physiologically active substances produced by microorganisms (Antibiotics, growth promoters).

Light. Plant growth can occur both in the presence and absence of light. Green Organs of higher plants develop specific morphological traits in the dark that distinguish them from those grown in light. Such plants are referred to as etiolated. In dicots, darkness causes significant elongation of internodes and underdevelopment of leaf blades, while in maize seedlings, the mesocotyls become elongated. Etiolated plants typically exhibit underdeveloped mechanical Tissues and Stomata. These plants lack chlorophyll and appear pale yellow due to the presence of carotenoids.

The phenomenon of etiolation is not caused by a lack of chlorophyll or insufficient Nutrition: it is observed in many Fungi and in conifer seedlings, which are capable of turning green even in the dark.

To reverse etiolation, a brief (5-10 min) daily exposure to light is sufficient, which does not necessarily trigger chlorophyll synthesis. This process (the Prevention of etiolation) is regulated by Phytochrome (the red/far-red light response) or the phytochrome-flavoprotein system (the far-red/blue light response). The blue-violet part of the spectrum exerts the most potent morphogenetic effect.

Etiolation is an essential adaptive response for seedlings developing underground: through the rapid elongation of the mesocotyl, hypocotyl, or epicotyl, the SHOOT is quickly pushed to the surface toward the light. When etiolated seedlings are exposed to light, the Inhibition of Growth by Cell elongation occurs within minutes, while other light-induced effects (unfolding of the first leaf, initiation of subsequent leaves, growth of ROOT hairs) take several hours to manifest.

Temperature. Plant growth is also temperature-dependent. Within the 0-35 °С range, the Effect of temperature on growth follows the van 't Hoff rule.

For thermophilic plants, the minimum growth temperatures are above 10 °С, with optima between 30-40 °С; for cold-resistant plants, these ranges are 0-5 °С and 25-30 °С, respectively. The maximum temperatures for the growth of most plants lie between 35-45 °С. The optimal temperature is defined as the one at which growth is most rapid. However, high growth rates are not always beneficial for plants. During forced growth, stems are often weak and leaves are thin. In field conditions, such plants are prone to lodging.

Gas composition. Oxygen is essential for plant growth. However, short-term decreases in oxygen levels do not significantly affect growth. Even during prolonged oxygen depletion in the root zone due to flooding, growth continues, albeit at a slower rate. In such cases, adaptive mechanisms are activated, allowing the plant to utilize oxygen from nitrates, aerenchyma, etc.

An excess of CO₂ in the air leads to increased Cell wall stretching and a temporary stimulation of tissue growth (the "acid growth" effect). The influence of CO₂ on growth is based on its ability to lower the pH of cell walls, thereby inducing cell growth. The CO₂ effect is independent of oxygen availability. Along with shading in cereals, it causes excessive elongation in dense stands, which is a common cause of plant lodging.

Water regime. The process of plant growth by elongation is carried out through cell vacuolization, provided There is a sufficient supply of water. Therefore, water deficit slows down growth. Roots can only grow in sufficiently moist soil, nearly saturated with water vapor, with an osmotic pressure not exceeding 1-1.5 MPa.

The aerial parts of plants are always exposed to drier air with a humidity of 50-70%. They are protected from water loss by the cuticular-epidermal layer. Consequently, the water vapor pressure in the leaf mesophyll usually does not drop below 98% relative humidity. During prolonged water deficit in the tissues, the elongation phase concludes prematurely, leading to stunted stems and roots, smaller leaves, reduced cell size, etc.

Water shortages during the stem elongation stage in cereals result in significant yield losses.

Mineral nutrition. High levels of mineral nutrients, especially N, promote growth. This leads to the expansion of vegetative organs and is necessary for increasing the green mass of forage crops. However, high mineral levels delay differentiation processes and flower initiation, which reduces fruit and grain yields. In hydroponic culture, better fruit and vegetable yields are obtained by periodically subjecting plants to nutrient stress, particularly nitrogen starvation.