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

8. Plant Adaptation and Resistance to Adverse Environmental Factors
8.2 Drought and Heat Resistance

EFFECT OF Water shortage on plants. A lack of water in plant Tissues occurs when Water Loss via Transpiration exceeds its uptake. A water deficit can develop during hot, sunny weather by midday, at which point the suction force of leaves increases, stimulating water uptake from the soil. Typically, in the case of leaf wilting, their water deficit recovers during the evening and night hours (temporary wilting). Deep wilting is observed in the absence of available soil moisture for the plant. Most often, it leads to plant death.

A characteristic sign of a persistent water deficit is its retention in tissues in the morning, as well as the cessation of bleeding sap from a cut stem. During prolonged wilting, The activity of synthesis Enzymes decreases and hydrolytic processes are activated, notably proteolysis, which leads to an increase in the content of low-molecular-weight Proteins in Cells. As a result of polysaccharide Hydrolysis, soluble CARBOHYDRATES accumulate in tissues, and their outflow from the leaves slows down. Under METABOLISM/18.html">The Influence of drought, The amount of RNA in leaves decreases due to reduced synthesis and the activation of ribonucleases. The breakdown of polyribosome complexes is observed in the Cytoplasm. Changes affecting DNA occur only during prolonged drought. As the volume of free water decreases, the concentration of vacuolar sap increases. The ionic composition of cells changes, and the processes of ion efflux from them become easier.

Overall Photosynthesis during moisture deficiency generally decreases, although sometimes (at the Initial Stages of dehydration) a slight increase in its intensity is observed. The decrease in The rate of photosynthesis can be a consequence of CO2 shortage due to stomatal closure; impaired chlorophyll synthesis; uncoupling of Electron Transport and Photophosphorylation; changes in photochemical reactions and CO2 reduction reactions; disruption of chloroplast Structure; and delayed outflow of assimilates from leaves during prolonged water deficit.

During dehydration in plants unadapted to drought, Respiration intensity increases significantly (possibly due to a large amount of respiration substrates—sugars), and then gradually decreases. In drought-resistant plants under these conditions, no significant changes in respiration are observed, or a slight increase is noted.

Under conditions of water deficit, Cell Division and especially cell elongation are rapidly inhibited, leading to The formation of small cells. As a result, the growth of the plant itself, particularly leaves and stems, slows down. ROOT growth at the beginning of drought is usually accelerated and decreases only in the event of prolonged absence of water in the soil. Roots respond to drought with A number of protective adaptations: suberization of the exodermis, acceleration of the differentiation of cells emerging from the meristem, etc.

Effect of overheating on physiological processes. During drought, In addition to dehydration, plants experience overheating. When exposed to high temperatures (35 °C and above), changes in cytoplasmic viscosity are observed: more often an increase, less frequently a decrease. An increase in cytoplasmic viscosity slows down its movement. High Temperature increases cell sap concentration and cell permeability to urea, glycerin, eosin, and Other Compounds. As a result of exosmosis of substances dissolved in cell sap, osmotic pressure gradually decreases. However, at temperatures above 35 °C, an increase in osmotic pressure is again noted due to enhanced starch hydrolysis and an increased content of Monosaccharides.

The process of photosynthesis is more sensitive to high temperatures than respiration. Under high-temperature stress, polymer hydrolysis is significantly activated. Protein breakdown is accompanied by the formation of ammonia, which can cause cell poisoning in heat-susceptible plants. In heat-tolerant plants, an increase in the content of organic acids is observed, which bind excess ammonia. Another method of protection against overheating is considered to be enhanced transpiration provided by a powerful root system. In other cases (succulents), heat tolerance is determined by high cytoplasmic viscosity and an increased content of tightly bound water.

In agricultural practice, to increase the heat tolerance of plants, foliar application of a 0.05% zinc salt solution is used.

Plant Adaptations to drought.

Plants growing in arid habitats—xerophytes—have developed adaptations that allow them to endure periods of drought.

Plants use three Main Methods of protection:

1) preventing excessive water loss by cells (avoidance of desiccation);

2) tolerance of desiccation;

3) avoidance of the drought period.

The most general adaptation is for water retention in cells.

The group of xerophytes is very heterogeneous. According to their ability to withstand drought conditions, they are divided into the following types (according to P. A. Genkel):

1. Succulents (according to N. A. Maksimov—false xerophytes)—plants that store moisture (cacti, aloe, houseleek, spurge). Water is concentrated in leaves or stems covered with a thick cuticle and hairs. Transpiration, photosynthesis, and growth proceed slowly. They tolerate dehydration poorly. The Root System spreads wide rather than deep.

2. Non-succulent species. Based on their transpiration rate, they are divided into several groups:

a) true xerophytes (euphellophytes/euxerophytes—wormwood, speedwell, mullein, etc.). Plants with small leaves, often pubescent, heat-tolerant, with low transpiration, capable of withstanding severe dehydration, with high osmotic pressure in cells. The root system is highly branched but located at a shallow depth;

b) hemixerophytes (sage, wild licorice, camel thorn, etc.). They possess intensive transpiration supported by the activity of a deep root system, which usually reaches groundwater. They tolerate dehydration and atmospheric drought poorly. Cytoplasmic viscosity is low;

c) steppoxerophytes—steppe grasses (feather grass, etc.). Adapted to overheating, they rapidly utilize summer rainfall moisture, but withstand only short-term water shortages in the soil;

d) poikiloxerophytes (blue-green Algae, Lichens, etc.) cannot regulate their water regime and enter a dormant state (anabiosis) upon significant dehydration. They are capable of withstanding desiccation;

3. Ephemerals—plants with a short growing season coinciding with the rainy period (a method of drought avoidance in arid habitats). Studying the physiological nature of xerophyte drought resistance, N. A. Maksimov (1953) proved that these plants are not drought-loving: sufficient soil moisture promotes their intensive growth. Drought resistance consists of their ability to withstand water loss.

Mesophytic plants can also adapt to drought. The Study of leaf adaptation methods to unfavorable water supply conditions has shown that the Anatomical Structure of leaves from different tiers on the same plant depends on the level of water supply, illumination, etc. The higher up the stem the leaves are located, the smaller their cells, the more Stomata per unit area, the smaller their size, the denser the network of vascular bundles, the more developed the palisade parenchyma, and so on. These regularities in leaf apparatus variation are reflected in Zalensky's law. Since such structural features are characteristic of a number of xerophytes, this leaf structure came to be called xeromorphic.

Thus, The Emergence of a xeromorphic leaf structure is one of the types of anatomical adaptations to water scarcity, alongside the sinking of stomata into leaf tissues, pubescence, thick cuticle, leaf reduction, etc.

Biochemical defense mechanisms protect cells against dehydration, ensure the detoxification of Metabolic waste products, and facilitate the Repair of Damaged cytoplasmic structures. The high water-retention capacity of the cytoplasm under drought conditions is driven by the accumulation of low-molecular-weight hydrophilic proteins that bind a significant volume of water within their Hydration shells. This process is further facilitated by the interaction of proteins with Proline—whose concentration increases markedly under water stress—as well as by an elevated level of monosaccharides in the cytoplasm.

An intriguing adaptation that minimizes water loss through stomata is characteristic of succulents. Due to the peculiarities of their photosynthetic pathway (CAM metabolism), their stomata remain closed during hot, dry daytime hours in the desert, as CO2 fixation occurs at night.

Drought induces significant shifts in the plant hormonal system: the levels of growth-promoting Hormones—such as Auxins, Cytokinins, Gibberellins, and phenolic growth stimulants—decrease, while the concentrations of Abscisic acid (ABA) and Ethylene rise. Under drought conditions, the rate at which growth processes are halted generally dictates plant survival. Furthermore, during the Cytology/cytology/16.html">Early stages of drought, the rapid surge in growth inhibitors plays a primary role, since even under balanced cellular water supply, rapid stomatal closure is triggered by an accelerated accumulation of ABA. In addition, ABA promotes cellular water retention by activating proline synthesis. ABA also inhibits RNA and Protein Synthesis AND, upon accumulating in the roots, suppresses cytokinin production. Thus, elevated ABA levels during water deficit reduce stomatal water loss, enhance protein-bound hydration water storage, and shift cellular metabolism into a 'dormant' state.

Water stress triggers a substantial release of ethylene. For instance, in wheat leaves experiencing a mere 9% decrease in water content, ethylene production increases 30-fold within four hours. In many plants, atmospheric and soil droughts also induce the accumulation of phenolic growth inhibitors (such as chlorogenic acid, Flavonoids, and phenolic carboxylic acids). The aforementioned changes in growth-inhibiting phytohormone levels are typically observed in mesophytic plants subjected to drought. In poikiloxerophytes, which enter an anabiotic state upon the onset of drought, growth arrest is not associated with the accumulation of growth inhibitors.

The drought tolerance of agricultural crops can be enhanced through presowing hardening. Adaptation to dehydration occurs in seeds that, prior to sowing, are soaked once and then re-dried. Plants grown from such seeds exhibit morphological traits of xeromorphism that correlate with enhanced drought resistance.