Plant Physiology - Musiyenko, M. M. 2001

Water Regime of Plants
Features of the Water Regime of Plants from Different Ecological Groups

The adaptation of plants to varying Water supply conditions has significantly influenced their morphological, anatomical, physiological, and biochemical traits. Depending on the ecological niche they occupy, plants are primarily divided into aquatic plants (hydatophytes) and terrestrial plants. These, in turn, are subdivided into poikilohydric plants (Bacteria, cyanobacteria and certain other lower Algae, Lichens), which have adapted to withstand moisture deficits without substantial disruption to their vital activity, and homoiohydric plants (terrestrial ferns, gymnosperms, angiosperms), representing the vast majority of land plants. They are characterized by sophisticated regulatory mechanisms governing stomatal and cuticular Transpiration.

Plants from different zones are categorized into respective ecological types, among which mesophytes, hydrophytes, and xerophytes are the most prominent.

The mesophyte group mainly comprises cultivated flora capable of thriving under conditions of adequate water supply. This group is diverse, widely distributed across the globe, and of paramount importance to human life. Mesophytic plants are extensively utilized in agriculture, encompassing cereal crops, forage grasses, vegetables, industrial, oilseed, fodder, fruit, fiber, and other crops.

Hydrophytic plants are those that thrive under conditions of abundant water supply, typically inhabiting low-lying areas with a high water table, marshes, lakes, and similar wetlands.

The third group consists of so-called xerophytes, which differ markedly from the first two types in their unusual Morphology and Structure. This group includes plants of deserts, semi-deserts, and arid steppes, where water availability is severely limited. Based on METABOLISM/2.html">THE CONCEPT OF drought resistance as an adaptive response, xerophytes can be defined as plants of arid habitats that have developed the capacity during their ontogeny to adapt effectively to drought, thanks to a suite of traits and properties shaped by evolutionary pressures in their natural environments.

Among these types, there is an immense variety of forms, particularly within xerophytes. Water scarcity has profoundly impacted the Anatomical and physiological traits of plants, driving the evolution of unusual forms and structures. A universal feature of all xerophytes is the maximum reduction of their evaporating surface, which has consequently led to a poorly developed aerial portion. This explains why most xerophytes are herbaceous plants or low-growing shrubs whose underground systems are far more developed than their above-ground parts. Examples include wormwood, steppe alfalfa, camelthorn, and others.

Xerophytic plants are exceptionally diverse and comprise several groups—cacti, succulents, fine-leaved xerophytes, sclerophylls, and ephemerals. These encompass plants from various taxonomic groups, yet uniform water supply conditions have fostered parallel adaptations to their life environments.

Cacti are desert plants whose evolution has trended toward the maximum reduction of the evaporating surface. Consequently, their leaves have lost their assimilatory and transpiratory Functions. The ROOT System of cacti also exhibits unique features: it spreads across the superficial soil layers, an adaptive strategy that ensures the rapid absorption of moisture precipitated on the soil surface, which is swiftly transported into the plant. During periods of drought, root hairs in cacti wither away, while the remaining roots become covered with a protective cork layer. A distinctive characteristic of cacti is that their bulky bodies present a minimal surface area.

In addition to cacti, succulents include plants such as aloe, stonecrop, biting stonecrop, and others. They are characterized by significant water reserves stored primarily within their leaves, giving them a fleshy appearance and a high Cell sap content. Succulent leaves feature a well-developed water-storage parenchyma, and their cell sap exhibits relatively low osmotic pressure. The root system of this plant type is confined to surface soil layers. They typically inhabit rocks, stone walls, and sandy soils, and are distinguished by high heat resistance.

Fine-leaved xerophytes include wormwood, steppe alfalfa, camelthorn, and others. This group of plants possesses an extensively developed root system that penetrates deep soil layers to extract life-sustaining moisture. Their bodies are covered with fine white hairs that form

a semi-transparent screen protecting Chloroplasts from the damaging effects of intense light. Thorns and spines are Characteristic Features of these plants. Due to specific metabolic processes, their Tissues accumulate substances with a bitter taste and strong odor. The Cell sap of these plants maintains a high concentration, endowing it with immense suction force that facilitates water absorption from the soil. The leaf blades of fine-leaved xerophytes are deeply dissected; under severe soil moisture deficits, these plants may enter a state of dormancy (anabiosis).

The substantial loss of water via transpiration is compensated for by absorption through a deep-reaching root system. Along with alfalfa and camelthorn, fine-leaved xerophytes include the wild watermelon and certain species of wormwood, whose leaves wilt rapidly when detached from the plant. A common property of these plants is the disproportion between their root system and aerial parts.

Sclerophylls represent another xerophytic type, exemplified by certain steppe grasses such as feather grass (Stipa), sheep fescue, and various umbellifers, notably tumbleweed. Their most crucial adaptation is The ability to endure prolonged wilting. During such periods, their leaves roll into tight tubes, ensuring economical water use by trapping Stomata inside the tube, where they are isolated from the surrounding environment. Unlike succulents, sclerophyllous xerophytes are characterized by a high concentration of cell sap.

There is a group of plants known as false xerophytes, which feature an exceptionally short growing season and are termed ephemerals. They possess a weakly developed root system restricted to the uppermost soil layers. Their entire life cycle lasts three to four weeks, during which they flower and produce mature seeds. This phenomenon occurs in early spring, when surface soil layers contain sufficient moisture to support their rapid development.

Plant Water Balance. Water uptake, translocation, and loss by the Organism constitute the plant water balance. A plant's water balance can manifest in various ways. Theoretically, three scenarios are possible: 1) water uptake exceeds water loss; 2) uptake equals loss; 3) water loss exceeds uptake. Throughout different times of day and across the growing season, the ratio between Water uptake and loss frequently fluctuates, resulting in a water deficit within plant tissues. This deficit begins at the onset of uncompensated water loss, when root absorption lags behind The rate of transpiration. The formation of a water deficit is governed more significantly by the distribution of water among various hydrophilic cellular components than by its total content, with water redistributing among Organs under deficit conditions. Water deficit exerts a profound impact on a range of physiological processes (Photosynthesis, Respiration, cellular redox balance, Enzymatic Catalysis, etc.), ultimately reducing overall plant productivity. The Theory of plant water relations serves as the foundation for irrigated agriculture.