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

3. Water Exchange in Plants. Diffusion, osmosis, chemical and water potentials, osmotic pressure
3.8 Physiological characteristics of the water regime in various ecological plant groups

Moisture deficit in plants affects such processes as Water absorption, Transpiration, ROOT pressure, seed germination, Photosynthesis, Respiration, enzymatic activity, GROWTH AND DEVELOPMENT, and mineral ratio.

By altering METABOLISM, moisture deficiency impacts plant productivity, fruit flavor, wood density, fiber length and strength, and more. The moisture content required for germination varies among species. The Impact of water deficit on metabolic processes depends on its duration. Prolonged wilting accelerates the Breakdown of Proteins and Nucleic Acids. Consequently, protein content decreases in leaves while increasing in seeds. Under water-deficit conditions, sugar content initially drops due to reduced photosynthetic activity, then rises slightly as Polysaccharides in lower leaves undergo Hydrolysis, and subsequently decreases again across all forms.

Water deficit reduces The rate of photosynthesis and ATP synthesis, while also inhibiting the outflow of photosynthetic products from leaves.

Under water deficit conditions, the respiratory quotient decreases, and the respiration rate drops sharply, especially in young leaves.

In moisture-deficient conditions, upper leaves increase their concentration of osmotically active substances, drawing water away from lower leaves and maintaining normal synthetic processes for a longer period.

In arid zones, understanding all physiological plant traits and their alterations during water stress is crucial for determining accurate irrigation schedules and durations.

Aquatic plants regulate the constancy of their internal environment using mechanisms that protect against the continuous, excessive influx of water absorbed across their entire surface.

In terrestrial plants, water balance regulation mechanisms are geared toward preventing significant water loss. These mechanisms vary across plants of different ecological groups. Based on their ability to adapt Water Metabolism to fluctuating water supply, plants are divided into two groups.

Plants are classified into two ecological groups depending on their adaptations to environmental water shortages:

Poikilohydric plants (Bacteria, blue-green Algae, green algae of the order Protococcales, Fungi, Lichens, dry-steppe grasses, pollen grains, and angiosperm seeds) have adapted to withstand severe water shortages without losing viability. Under these conditions, their metabolic rate decreases. Based on the daily fluctuations of all water-regime indicators (osmotic pressure, transpiration rate, water content), they belong to hydrolabile plants.

Homoiohydric plants (terrestrial ferns, gymnosperms, flowering plants) possess sophisticated regulatory mechanisms for stomatal and cuticular transpiration, as well as root system activity. Their Cells feature a well-developed vacuolar system and lack the capacity for reversible desiccation. Their water-regime indicators characterize a hydrostable type.

Based on their water requirements, all plants are categorized into the following ecological groups: xerophytes, mesophytes, hygrophytes, and hydrophytes.

Xerophytes are plants of arid habitats capable of enduring prolonged atmospheric and soil droughts while maintaining physiological activity.

They are characterized by hard, rigid leaves with a thick cuticle and a multi-layered, thick-walled epidermis. Their leaves are often densely pubescent, covered with a waxy bloom, and tend toward reduction. A well-developed root system allows them to access moisture from deep soil layers. Examples of xerophytes include spurges, aloes, cacti, wormwood, feather grass, and others.

Mesophytes are plants growing on moderately moist, well-aerated soils, occupying an intermediate position between xerophytes and hygrophytes in terms of moisture demand. Morphologically and physiologically, mesophytes combine various xeromorphic and hygromorphic traits. Leaf Tissues are differentiated into palisade and spongy parenchyma, with medium-sized cells and intercellular spaces. Mesophytes include deciduous trees, forest and meadow grasses, and most cultivated crops.

Hygrophytes are plants that thrive in conditions of high air humidity and abundant soil moisture, typical of swamps, wetlands, lakeshores, and riverbanks. Their leaves exhibit a hygromorphic Structure, featuring large blade surfaces, cells, and intercellular spaces. They consist of loose spongy parenchyma, while palisade parenchyma is poorly developed or entirely absent; external protective tissues (epidermis and cuticle) are also underdeveloped. The osmotic pressure within their cells is low. The Root System is shallow, located in the upper soil horizons, and poorly developed. They possess open Stomata and hydathodes for water excretion, and they tolerate drought poorly. Representative hygrophytes include thin-leaved ferns, certain violets, marsh marigolds, and other plants adapted to high moisture and/or shade.

Hydrophytes are aquatic plants that live fully or partially submerged in water. They regulate the stability of their internal environment through mechanisms that prevent excessive water uptake. In monad forms of green algae inhabiting primarily freshwaters, Cell walls are incomplete due to cytoplasmic extensions called flagella, which facilitate locomotion. All monad forms possess contractile vacuoles that expel excess water and metabolic waste from cells. In hydrophytes with complete cell walls, wall counter-pressure is sufficient to prevent excess water influx. Algae represent primary hydrophytes, whereas aquatic flowering plants are secondary hydrophytes derived from terrestrial ancestors.