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

3. Water relations in plants. Diffusion, osmosis, chemical and water potentials, osmotic pressure
3.6 Upper end engine. Mechanisms of stomatal opening. Types of transpiration

Transpiration is the physiological process of Water evaporation by plants. The primary organ of transpiration is the leaf.

Plants possess a large leaf surface area, which facilitates the uptake of CO₂, light capture, and creates an evaporative surface. Water evaporates from the leaf surface through The Cell walls of epidermal Cells and protective layers (cuticular transpiration) and through Stomata (stomatal transpiration). As a result of water loss during transpiration, the suction pressure (tension) within the leaf cells increases. This drives the enhanced uptake of water by leaf cells from xylem vessels and the Movement of water through the xylem from the roots to the leaves. Consequently, the upper terminal driver involved in transporting water upward through the plant is governed by foliar transpiration. The upper terminal driver can operate even when the lower terminal driver is completely inactive; moreover, it utilizes not only metabolic energy, like The ROOT System, but also environmental energy—such as Temperature and air movement.

Transpiration protects the plant from overheating. The temperature of a heavily transpiring leaf can be approximately 7 °C lower than that of a wilted, non-transpiring leaf. In addition, transpiration helps establish a continuous stream of water bearing dissolved mineral and Organic compounds from the root system to the aerial Organs of the plant.

Transpiration is typically expressed in the following units. Transpiration rate is The amount of water (in g) evaporated by a plant per unit of time (in hours) per unit of surface area (in dm²). This value generally ranges from 0.15 to 1.5. Transpiration coefficient is the amount of water in g evaporated by a plant per gram of dry matter accumulated. Transpiration productivity is the reciprocal of the transpiration coefficient, equal to the amount of dry matter in grams accumulated by the plant during the period it transpires 1 kg of water. Relative transpiration is The ratio of water evaporated by a leaf to the water evaporated from a free water surface of the same area over the same period. Transpiration economy is the amount of water (in mg) evaporated per 1 kg of water contained within the plant.

The Leaf as an organ of transpiration. Water moves from the stem into the leaf through the petiole along the Veins. As the veins branch, the number of conducting elements decreases, and the finest veinlets consist of isolated tracheids forming a very dense network. In C₄ plants, bundle sheaths are present. Tracheids terminate between the mesophyll cells. Externally, a single-layered epidermis with a cuticle (and sometimes trichomes and scales) creates a barrier to water movement. Between the parenchyma cells, the inner surface area of the leaf is typically an order of magnitude larger than its external surface area. Water within the intercellular spaces evaporates from all exposed mesophyll surfaces.

Stomatal transpiration. Stomata form a pore leading into the substomatal cavity, bordered by two crescent-shaped guard cells. Their number and distribution vary among different plant species. Stomata play a crucial role in gas exchange between the leaf and the atmosphere, serving as the main pathway for water vapor, carbon dioxide, and oxygen. On average, stomatal density ranges from 50 to 500 per mm². Stomata are predominantly located on both sides of the leaf, though in some plant species they are found exclusively on the lower surface. Transpiration through stomata proceeds at a rate nearly comparable to that from a free water surface, in accordance with Stefan’s law: The rate of gas diffusion is proportional not to the area of the aperture, but to its diameter or circumference. Therefore, although the area of stomatal pores is small relative to the total leaf area (0.5–2%), water evaporation through the stomata occurs very intensively.

Transpiration consists of two processes:

1) the movement of water along the veins to the cell walls of the mesophyll;

2) the evaporation of water into the intercellular spaces followed by diffusion through the stomata, or the evaporation of water from the cell walls directly into the atmosphere via cuticular transpiration. Water moves toward the evaporating surfaces along the cell walls, where it encounters less resistance than through the symplast. Water molecules leave the plant by moving (just as they do inside the plant) toward a lower water potential (which is lower when relative humidity is lower). If The water potential of the air is lower than that of the substomatal cavities, water molecules evaporate outward.

Regulation of stomatal transpiration. The primary factor influencing stomatal opening and closing is the water content within the leaf, particularly within the guard cells. The cell walls of guard cells are unevenly thickened: the inner wall bordering the stomatal pore is thicker, while the outer wall is thinner. As a guard cell osmomatically absorbs water, the thinner, more elastic portion of its Cell wall stretches and pulls the inner wall outward. The guard cells assume a semi-circular shape, and the stomata open. In the event of water scarcity, the guard cells straighten, and the stomatal pore closes (Fig. 3.3). Furthermore, as water deficit increases in plant Tissues, the concentration of the growth inhibitor Abscisic acid rises. It suppresses The activity of H⁺ pumps in the Plasmalemma of guard cells, leading to a drop in turgor pressure and the closure of stomata. Abscisic acid also inhibits the Synthesis of the enzyme α-amylase, which results in reduced starch Hydrolysis. Unlike low-molecular-weight CARBOHYDRATES, starch is not an osmotically active substance; consequently, the suction pressure of the guard cells decreases, causing the stomata to close. Unlike other epidermal cells, guard cells contain chlorophyll.

Stomatal status is also influenced by leaf age, the plant's developmental phase, and endogenous circadian rhythms.

Stomatal movement is heavily affected by the plant's water status. With an adequate water supply in the light, stomata open wider as light intensity increases. Photosynthesis also affects stomatal movement: the intensive production of carbohydrates increases the suction pressure of cells, causing the stomata to open.

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Figure 3.3 - Stomata in open (top) and closed (bottom) states. A - dicotyledonous plant, B - cereal (after S. I. Lebedev)

Stomatal status also depends on CO₂. If the CO₂ concentration in the substomatal cavity drops below 0.03%, the turgor of guard cells increases and the stomata open. This partly explains the morning opening of stomata (due to a decrease in CO₂ resulting from intensified photosynthesis). At night, the concentration of CO₂ in the intercellular spaces rises due to Respiration coupled with reduced or absent photosynthesis, causing the stomata to close.

In succulents with a specific circadian rhythm of organic acid METABOLISM, stomata open at night—when the partial pressure of CO₂ in the intercellular spaces decreases due to the intensive synthesis of malate—and close during the day, when CO₂ is released through the decarboxylation of malate.

Thus, multiple factors participate in regulating the functional activity of stomata through direct and feedback mechanisms. The simultaneous action of these pathways results in minor oscillations in stomatal aperture.

Cuticular transpiration. Externally, leaves feature a single-layered epidermis whose outer cell walls are covered with a cuticle and Waxes, forming an effective barrier against water movement. Trichomes (hairs) often develop on the leaf surface, which also influence the plant's water relations by reducing air movement over the surface and scattering light, thereby decreasing Water Loss via transpiration. The intensity of cuticular transpiration varies among plant species. In young leaves with a thin cuticle, it can account for about half of all transpiration. In mature leaves with a thicker cuticle, cuticular transpiration drops to about 1/10 of the total. In Aging leaves, damage to the cuticle may cause it to increase.

Consequently, cuticular transpiration is regulated primarily by the thickness and integrity of the cuticle and other protective surface layers on the leaves.

The rate of cuticular transpiration varies significantly among species, ranging from negligible losses up to 50%. Conifers and magnoliids possess a thick cuticular layer and lose very little water through the epidermis.

A certain fraction of water is lost through buds and reproductive organs. Occasionally, these losses can be substantial (e.g., sunflower heads, poppy capsules, pepper fruits). Winter transpiration from branches frequently induces water deficits, leading to plant death by desiccation.