Plant Physiology - Musiyenko M.M. 2001

Plant Water Regime
Transpiration

Leaf Veins, consisting of xylem and phloem, form such a dense network that virtually every Cell is located close to a Water source. Water moves from the xylem into The Cell walls of the mesophyll. Consequently, water in its liquid phase fills the continuous pathway from the soil, through the ROOT and stem Tissues, to the mesophyll Cells in the leaf. The flow of water is always directed towards a lower potential — moving from the highest potential in the soil to the lowest in the cells adjacent to the leaf epidermis. The low water potential in these cells is primarily caused by evaporation. The leaf mesophyll contains extensive intercellular spaces whose air is saturated with water vapor, part of which is released into the atmosphere. A large amount of water constantly flows through the plant in an unbroken stream, evaporating from The surface of its leaves and, in herbaceous plants, from the stem surface as well. Since the epidermis in plants is very often covered by a cuticle, water vapor escapes mainly through the Stomata (Fig. 31).

the outer walls are thin, whereas those facing the stomatal pore are thickened. This uneven Structure OF THE cell walls causes changes in both the volume and the shape of the stomata. It has recently been discovered that during the day in the light, guard cells accumulate potassium ions and accompanying osmotically active anions, which drive water influx; As a result, the outer walls stretch and the pores open. In the dark, however, potassium ions move out of the guard cells into the surrounding epidermal cells, which is correspondingly accompanied by an outflow of water, and the pore closes.

A stoma — is an opening (pore) bordered by two guard cells. Stomata are found in all terrestrial plant Organs, but most abundantly in leaves. The number of stomatal pores varies depending on the plant species, ranging from 10 to 600 per 1 mm² of leaf area. In many plants (75% of species), including the majority of tree species, stomata are located on the lower side of the leaf. The diameter of the stomatal pore is 3–12 µm. Stomata connect the internal space of the leaf with the external environment.

The process of water evaporation by plants is called transpiration. This specialized term is justified because we are dealing here not with a simple physical process of evaporation, but with a complex physiological mechanism. Transpiration plays a vital role in the life of a plant. First, it creates a continuous flow of water from The Root System to the leaves, uniting all plant organs into a single whole. Transpiration protects the plant Organism from overheating. Finally, soluble mineral and partially organic nutrients are transported along with the transpiration stream; the more intensive the transpiration, the faster this process occurs.

Transpiration can be stomatal (through the stomata), cuticular (through the cuticle), and lenticular (through the lenticels).

Stomatal transpiration occurs through the stomatal Pores in the epidermis, through which gas exchange takes place (Fig. 31). The rest of the leaf surface is covered by the cuticle, which is largely impermeable to water and gases. Stomatal transpiration is normally the primary pathway, but its significance relative to cuticular transpiration varies among plants of different ecological groups and depends on environmental conditions. The total area of stomata ranges from 1 to 2% of the entire leaf surface. Guard Cells of the stomatal apparatus differ from other epidermal cells of the leaf by containing METABOLISM/14.html">Chloroplasts. Furthermore, their cell walls are unevenly thickened — the outer walls are thin, while those facing the stomatal pore are thicker. This uneven structure of the cell walls causes changes in both the volume and the shape of the stomata. It has recently been discovered that during the day in the light, guard cells accumulate potassium ions and accompanying osmotically active anions, which drive water influx; as a result, the outer walls stretch and the pores open. In the dark, however, potassium ions move out of the guard cells into the surrounding epidermal cells, which is correspondingly accompanied by an outflow of water, and the pore closes.

Fig. 31. Structure of the stomatal apparatus: 1 — thin wall of the guard cell, 2 — thick wall, 3 — epidermal cell, 4 — guard cell, 5 — turgid guard cells, stoma open, 6 — guard cells have lost turgor, stoma closed

Cuticular transpiration takes place through the surface of the cuticle covering the leaf epidermis. As a rule, it is significantly lower than stomatal transpiration. However, young plant leaves exhibit a high rate of cuticular transpiration precisely because their cuticular layer is not yet as thick as in older leaves. While in an old leaf cuticular transpiration accounts for 5–10% of total transpiration, in a young leaf it often reaches 40–70%. Naturally, young plants are particularly sensitive to water supply and easily wilt.

Lenticular transpiration occurs via lenticels — clusters of loosely arranged periderm cells on perennial stems and roots that protrude onto the surface as small bumps or streaks through which gas exchange is carried out.

The transpiration process is largely determined by the Structural Features of the leaf, the state of its Cells and Tissues, as well as hydrometeorological factors. Plants are capable of regulating the intensity of their transpiration. By closing the stomata, a plant reduces transpiration while simultaneously raising its internal Temperature. However, with closed stomata, the plant cannot absorb carbon dioxide from the air for Photosynthesis. Therefore, the stomatal apparatus in plants responds quite complexly to changing environmental conditions, alternately opening and closing the stomatal pore.

Transpiration drives the passage of a tremendous amount of water through the plant body and has an adaptive significance closely linked not only to water exchange but also to other metabolic processes, notably photosynthesis, Respiration, and mineral Nutrition. Consequently, when investigating the water regime of various plants, it is extremely important to study transpiration metrics, such as transpiration intensity, the transpiration coefficient, transpiration productivity, and others.

Transpiration intensity is The amount of water evaporated by a plant (in grams) per unit of time (hours) per unit of leaf surface area (in dm²). This value ranges from 0.15 to 1.47 g per dm² per hour.

The transpiration coefficient is the amount of water (in grams) evaporated by a plant to accumulate 1 g of dry matter. For various plant species, its value ranges from 125 to 1000, most frequently being

around 300. In general, this indicator varies considerably depending on environmental conditions and can serve as an indicator of a plant's moisture requirements. For instance, in wheat plants, it can range from 220 to 750 units.

Transpiration productivity is the reciprocal of the transpiration coefficient and defines the amount of dry matter (in grams) accumulated by a plant during the period it transpires 1 kg of water.

According to N.A. Maximov, it ranges from 1 to 8, with an average of about 3 under temperate climate conditions. Thus, an average of about 300 g of water is used to synthesize 1 g of dry matter — which is only 0.2% of all the water passing through the plant body, while the remaining 99.8% is evaporated.

Understanding transpiration metrics makes it possible to track plant water supply requirements throughout ontogenesis and to substantiate agrotechnical measures aimed at providing plants with adequate water and creating favorable conditions for their growth, development, and high productivity.