Classification of plants by their water regime regulation methods and moisture requirements

Depending on the method of regulating their water regime, terrestrial plants are divided into two groups: 1) poikilohydric and 2) homeohydric (homoiohydric).

Poikilohydric plants do not possess the ability to independently, i.e., actively, regulate their water regime. The leaves of these plants have practically no stomata, have poorly developed protective boundary tissues, their transpiration is equivalent to simple evaporation, and the water content in the cells is in equilibrium with the vapor pressure in the surrounding air. Typical representatives of this group are many terrestrial algae, fungi, lichens, some mosses, and pteridophytes.

Homeohydric plants carry out effective regulation of the water loss process in their organisms using the stomatal apparatus of the leaves. Due to the protection of internal cells and tissues by waterproof coverings and leaf rolling, plant organisms maintain tissue hydration within a constant and necessary level. The relative stability of hydration is the result of the balance between water loss and water uptake processes in quantities that ensure the plant's needs. The bulk of currently existing plants belong to the homeohydric type. They inhabit territories with varying degrees of water factor stress, while possessing a number of structurally unique adaptive features at both the cellular and organismal levels.

In the process of vital activity, plants consume a huge amount of water. They partially use it for the synthesis of their biomass, but the majority is released into the environment. It is estimated that 1 hectare of maize crops consumes about 3 million 600 thousand liters of water during the growing season. A negligible part of it is used for phytomass synthesis – only 0.5–1%. The remaining water evaporates, i.e., it returns to the environment.

The water requirement of different plant species is not the same. According to their water requirements, they are divided into four groups: hydrophytes, hygrophytes, mesophytes, and xerophytes.

Hydrophytes are plants that live in an aquatic environment. Their bodies are partially or completely submerged in water. Due to the specifics of their living conditions, the main structural feature of hydrophyte leaves is the presence of large intercellular spaces and cavities that form a special air-bearing tissue – aerenchyma, which ensures the buoyancy of the organs. The vegetative organs of submerged hydrophytes differ from those of emerged ones by their thin, dissected leaves and the absence of a cuticle* and functioning stomata. These plants are characterized by the weak development of conducting and mechanical tissues, the reduction of root hairs, a low osmotic potential of cells, and heterophylly – a difference in the structure of aerial and submerged leaves on the same individual.

Cuticle is a thin outer film covering the epidermis of leaves and stems on the outside.

Hydrophytes are divided into the following subgroups: 1) floating on the surface, i.e., in contact with two environments – water and air; 2) submerged – "suspended" in water, in contact only with the aquatic environment; 3) submerged rooting, i.e., located simultaneously in water and soil; 4) floating on the surface, rooting, and in contact with three environments – water, soil, and air; 5) amphibious species, i.e., helophytes, constantly occupying shallow coastal and near-shore habitats. Their stems and leaves usually rise quite high above the water surface.

Hygrophytes are plants of moist habitats characterized by high precipitation and constantly high relative humidity of the air. The very name "hygrophytes" emphasizes their growth in air saturated with water vapor. Hygrophytes are an ecologically diverse group of plants, whose structure is influenced not only by high air and soil humidity but also by the temperature and light conditions of their habitat. In this regard, hygrophytes are divided into shade-loving and light-loving. Shade-loving hygrophytes grow in moist, shady tropical and dark coniferous forests of the temperate climate zone. Light-loving hygrophytes inhabit open, well-lit areas with excessive soil moisture and sufficiently high, though fluctuating, air humidity. Most often, they grow along the banks of water bodies, in river deltas, and in places where groundwater emerges.

The anatomical features common to all hygrophytes consist of the presence of a powerful system of air-bearing intercellular spaces in all their organs and high cell water content. In plants belonging to this ecological group, the roots are located in the surface layers of the soil, branch weakly, and have a negligible number of root hairs or lack them entirely. Hygrophyte leaves usually have large blades. Their epidermis** consists of thin-walled cells involved in photosynthesis. The mesophyll*** is few-layered, predominantly spongy. Some plants also have a developed columnar mesophyll, but it, like the spongy one, has many intercellular spaces. The stomata, located on the underside of the leaf blade, are few in number, always open, and transpiration in these plants is almost equal to physical evaporation. Due to weak stomatal regulation, hygrophyte leaves wilt quickly when humidity decreases. Many representatives of this ecological group of plants have hydathodes – stomata adapted for the passive release of liquid water droplets by the leaves (guttation) during very high air humidity. In light-loving hygrophytes, the leaf blades are denser and thicker than in shade-loving ones, the epidermis consists of thicker-walled cells, and the cuticle is better developed.

A special group of hygrophytes consists of certain horsetails and rushes, in which leaves are reduced, and the stems perform the functions of photosynthesis.

Mesophytes are plants adapted to life in conditions of sufficient, but not excessive, moisture. In terms of their moisture requirements, they occupy an intermediate position between hydrophytes and xerophytes. This is the most extensive ecological group in terms of species composition, including trees, shrubs, meadow and forest herbaceous species, weeds and crops, ephemerals and ephemeroids.

The following subgroups of mesophytes are distinguished:

1) evergreen mesophytes of tropical forests – trees and shrubs that remain in the growing season all year round, without a sharply defined seasonal rhythm. These plants possess large, dissected, and drooping leaves with a thick cuticle, which contributes to their preservation in conditions of frequent rains;

2) winter-green woody mesophytes – plants of the tropical zone, specifically its continental parts, where the alternation of dry and wet periods is already pronounced, causing leaf fall during the dry summer period;

3) summer-green woody mesophytes – representatives of the forest zone of the temperate belt with warm summers and cold winters, which the plants survive in a dormant state preceded by leaf fall. They are characterized by well-developed integumentary tissues and protection of buds from water loss;

4) summer-green herbaceous perennial mesophytes – plants of temperate forests, meadows, and northern steppes, whose above-ground organs, with the exception of protective buds, usually die off in winter.

Typical mesophytes have a well-developed root system. The conducting and mechanical tissues in these plants are usually in optimal proportions, ensuring

Epidermis is the outer integumentary tissue of plants, consisting mostly of a single layer of cells and performing a protective function, as well as the function of gas exchange. ***

Mesophyll is the main chlorophyll-bearing parenchyma of the leaf blade, enclosed between the epidermal layers, ensuring normal transport of substances and strength of vegetative organs. Mesophyll can be homogeneous or differentiated into palisade and spongy tissue. The leaves of mesophytes acquire a light or shade structure depending on the lighting. These plants are very plastic, and in habitats with different humidity, features of either hygrophytes or xerophytes manifest in their structure.

Ephemerals are annual plants with a short (30-50 days), usually spring, period of development. They are characteristic of deserts, semi-deserts, and dry steppes.

Ephemeroids are perennial herbaceous plants with a short, usually spring, period of development. Ephemeroids of arid steppes and deserts grow and flower only in spring or only in autumn (during the rainy season), and for the rest of the year they persist in the form of bulbs and tubers.

It is typical for ephemeroids to lack "classic" signs of xeromorphosis, but their seeds are capable of withstanding severe desiccation and high ambient temperatures. Due to low leaf density, their photosynthesis intensity is high, and therefore, these plants are able to rapidly accumulate assimilants during a short wet period.

However, not all scientists agree with classifying desert ephemerals and ephemeroids as mesophytes, considering them xerophytes in a broad sense, since these species, despite the prevalence of mesomorphic structural features, still differ from mesophytes by at least a high intensity of transpiration and extremely high drought and heat resistance of seeds.

Xerophytes are plants of dry habitats capable of surviving long periods of soil and air dryness. They are typical for deserts, dry steppes, savannas, and subtropics that experience a lack of precipitation for long periods of time. The ability of xerophytes to endure unfavorable moisture conditions is associated with specific anatomical-morphological signs and physiological features. Adaptations of this kind are combined into three categories: a) limiting evaporation, b) enhancing water absorption when it is scarce in the soil, c) allowing for the creation of water reserves for long periods of water supply interruption.

The reduction of non-stomatal unproductive transpiration is achieved through partial leaf reduction, the development of a thick cuticle and numerous hairs, the deepening of stomata, the rolling of leaves with the stomatal side inward, and an increase in the number of sclerenchyma elements.

The increased ability of many xerophytes to extract water is associated with specific features of their root systems. Depending on the ecological situation, plants form different types of root systems:

• a deep, sparsely branched system reaching aquifers or their capillary fringe;
• a system that densely penetrates a significant volume of soil;
• a surface type that ensures efficient water absorption during scarce rainfall and reserves it for the rainless period.

In addition to anatomical-morphological features, physiological uniqueness is characteristic of xerophytes, manifesting in high values of osmotic potential, to which high water content of vegetative organs and suction force are closely linked. According to the features of xeromorphic signs, xerophytes are subdivided into sclerophytes and succulents.

Sclerophytes are drought-resistant plants with tough leaves possessing a thick cuticle and highly developed mechanical tissues. Sclerification is expressed in the development of robust areas of mechanical tissue—sclerenchyma, which leads to a reduction in intercellular spaces and the overall internal surface area for evaporation, as well as a reduction in cell size.

Understanding the mechanisms of plant water exchange helps the agronomist correctly assess their drought resistance and plan plant protection measures. Different species adapt to moisture deficits in various ways: by altering leaf structure, shedding leaves, or accumulating water in tissues. The most dehydration-tolerant plants are sclerophytes—plants with tough leaves and high internal cell pressure.

Bound water predominates in the cells of sclerophytes, and the suction force of the roots reaches several tens of atmospheres. This allows them to successfully extract moisture from parched soil. Depending on the method of adaptation, sclerophytes are divided into three groups:

• Euxerophytes — tolerate dehydration and overheating due to pubescence and reduced transpiration during the hottest hours. Their root system is highly branched but shallow.
• Hemixerophytes — evaporate water intensively due to powerful, deep roots. They are heat-resistant but sensitive to prolonged soil drying.
• Poikiloxerophytes — do not regulate their water balance and dry out to an air-dry state in the heat (mosses, lichens, wood-destroying fungi, algae). After the first precipitation, they fully restore their vital activity.

Succulents and plants of cold habitats

Succulents tolerate drought by storing moisture in fleshy tissues. They have few stomata, which remain closed during the day, and evaporation occurs through the primary epidermal cells. Based on their structure, they are divided into two types:

• Leaf succulents — accumulate water in thick leaves protected by wax and a thick cuticle. Their mesophyll is practically undifferentiated, and vascular bundles are located beneath the chlorophyll-bearing layer.
• Stem succulents — have reduced leaves (spines, as in cacti or spurges), and photosynthesis is carried out by the stem. The entire inner part of the stem is occupied by water-storage parenchyma.

A specific group of xerophytes includes plants that have adapted to moisture deficiency combined with low temperatures. Under such conditions, they suffer from physiological drought, where water is physically present in the soil but inaccessible to the root system due to the cold.

• Psychrophytes — grow on damp and cold soils. They have tough, sclerified leaves, combining features of drought-resistant and moisture-loving species.
• Cryophytes — adapted to dry and cold habitats (tundras, rocky screes). They withstand strong winds and sharp temperature contrasts, often adopting a cushion-like form.