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

7. Plant Growth and Development
7.8 Influence of Environmental Factors on Plant Growth

Plant growth is influenced by various environmental factors, which include physical factors such as light, Temperature, gravity, gas composition, magnetic field, humidity, nutrients, and mechanical stress. In addition, when growing within plant communities, plants are affected by the metabolic byproducts of other plants (allelopathy) as well as physiologically active substances produced by microorganisms (Antibiotics and growth-promoting substances).

Light. Plant growth can occur in both light and darkness. In the dark, the green Organs of higher plants develop A number of morphological features that distinguish them from light-grown plants; such plants are referred to as etiolated. In dark-grown dicots, internodes elongate extensively while leaf blades remain underdeveloped, and maize seedlings exhibit elongated mesocotyls. Etiolated plants typically feature poorly developed mechanical Tissues and Stomata. Lacking chlorophyll, these plants appear pale yellow due to the presence of carotenoids. To prevent etiolation, brief daily illumination (5–10 min) is sufficient, even without affecting chlorophyll synthesis. This process (the Prevention of etiolation) is regulated by Phytochrome (responding to red to far-red light) or the phytochrome-flavoprotein system (responding to far-red to blue light). The strongest morphogenetic effect is exerted by the blue-violet region of the spectrum.

Etiolation is a vital adaptive response in subterranean plant seedlings: through the rapid elongation of the mesocotyl, hypocotyl, or epicotyl, the SHOOT is quickly brought to the surface and into the light. Upon illumination of etiolated seedlings, growth inhibition via Cell extension sets in within a few minutes, whereas other light-induced effects—such as the unfolding of the first leaf,

the initiation of subsequent leaves, and the growth of trichomes—take several hours to occur.

Temperature. Plant growth is temperature-dependent. Within the 0–35 °C range, the Effect of temperature on growth follows the van 't Hoff rule. For thermophilic plants, minimum growth temperatures lie above 10 °C and optima within 30–40 °C, whereas for cold-resistant plants, these ranges are 0–5 °C and 25–30 °C, respectively. Maximum temperatures for the growth of most plants fall within the 35–45 °C interval. The optimum temperature is defined as that which Supports the fastest growth rate; however, a high growth rate is not always advantageous for plants. Forced greenhouse growth ("forced cultivation") typically yields weak stems and thin leaves, making field crops prone to lodging.

Gas composition. Plant growth requires oxygen, although a short-term decrease in its concentration has little effect on growth. Moreover, during prolonged oxygen depletion in the ROOT zone caused by flooding, growth continues, albeit at a slower pace. Under these conditions, adaptive mechanisms are triggered, enabling the utilization of nitrate oxygen, aerenchyma, and other pathways. Excess CO₂ in the atmosphere promotes Cell wall extension and temporarily enhances tissue growth (the "acid growth" effect). METABOLISM/18.html">The Influence of CO₂ on growth is based on its ability to lower the pH of cell walls, thereby inducing cell expansion. This CO₂ effect is independent of oxygen availability. Combined with canopy shading, it causes excessive elongation in dense stands, leading to plant lodging.

Water regime. Plant cell growth via extension is driven by vacuolation as water enters the Cells; consequently, water scarcity slows down growth. Roots are only capable of growing in sufficiently moist soil saturated with water vapor, with an osmotic pressure not exceeding 1–1.5 MPa.

The aerial parts of plants are constantly exposed to drier air with a relative humidity of 50–70%. They are protected from water loss by a cuticular-epidermal layer, keeping the water vapor pressure in the leaf mesophyll typically no lower than 98–99% relative humidity. Prolonged water deficit in tissues accelerates the Termination of the elongation phase, resulting in shortened stems and roots, reduced leaf size, and smaller-celled tissues.

Mineral Nutrition. A high concentration of mineral nutrients, especially N, is favorable for growth, stimulating the proliferation of vegetative organs and proving essential for maximizing the green biomass of forage crops. However, high nutrient levels delay differentiation processes and floral initiation, thereby reducing fruit and grain yields. In hydroponic culture, superior fruit and vegetable yields are achieved through periods of nutrient deprivation, particularly nitrogen restriction.