Moisture stress

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Moisture stress occurs when the water in a plant's cells is reduced to less than normal levels. This can occur because of a lack of water in the plant's root zone, higher rates of transpiration than the rate of moisture uptake by the roots, for example, because of an inability to absorb water due to a high salt content in the soil water or loss of roots due to transplantation. Moisture stress is more strongly related to water potential than it is to water content. [1] [2] [3]

Moisture stress also has an effect on stomatal openings of a plant, mainly causing a closure in stomata as to reduce the amount of carbon dioxide assimilation. [4] Closing of the stomata also slows the rate of transpiration, which limits water loss and helps to prevent the wilting effects of moisture stress. [5] This closing can be trigged by the roots sensing dry soil and in response producing the hormone ABA which when transported up the xylem into the leaves will reduce stomatal conductance and wall extensibility of growing cells. This lowers the rates of transpiration, photosynthesis and leaf expansion. ABA also increases the loosening of growing root cell walls and in turn increases root growth in an effort to find water in the soil. [6]

Phenotypic response of plants to long-term water stress was measured in corn and showed that plants respond to water stress with both an increase in root growth both laterally and vertically. [7] In all Droughted conditions the corn showed decrease in plant height and yield due to the decrease in water availability. [8]

Genes induced during water-stress conditions are thought to function not only in protecting cells from water deficit by the production of important metabolic proteins but also in the regulation of genes for signal transduction in the water-stress response. There are four pathways that have been described that show the plants genetic response to moisture stress; two are ABA dependent while two are ABA independent. They all affect gene expression that increases the plants water stress tolerance. [9]

The effects of moisture stress on photosynthesis can depend as much on the velocity and degree of photosynthetic recovery, as it depends on the degree and velocity of photosynthesis decline during water depletion. Plants that are subjected to mild stress can recover in 1–2 days however, plants subjected to severe water stress will only recover 40-60% of its maximum photosynthetic rates the day after re watering and may never reach maximum photosynthetic rates. The recovery from moisture stress starts with an increase in water content in leaves reopening the stomata then the synthesis of photosynthetic proteins. [10] [11]

See also

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Stoma in plants, a variable pore between paired guard cells

In botany, a stoma, also called a stomate is a pore, found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the stomatal opening.

Plant hormone Chemical compounds that regulate plant growth and development

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Photorespiration

Photorespiration (also known as the oxidative photosynthetic carbon cycle, or C2 photosynthesis) refers to a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some of the energy produced by photosynthesis. The desired reaction is the addition of carbon dioxide to RuBP (carboxylation), a key step in the Calvin–Benson cycle, but approximately 25% of reactions by RuBisCO instead add oxygen to RuBP (oxygenation), creating a product that cannot be used within the Calvin–Benson cycle. This process reduces the efficiency of photosynthesis, potentially reducing photosynthetic output by 25% in C3 plants. Photorespiration involves a complex network of enzyme reactions that exchange metabolites between chloroplasts, leaf peroxisomes and mitochondria.

Abscisic acid plant hormone

Abscisic acid (ABA) is a plant hormone. ABA functions in many plant developmental processes, including seed and bud dormancy, the control of organ size and stomatal closure. It is especially important for plants in the response to environmental stresses, including drought, soil salinity, cold tolerance, freezing tolerance, heat stress and heavy metal ion tolerance.

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Epidermis (botany) Layer of cells that covers leaves, flowers, roots of plants

The epidermis is a single layer of cells that covers the leaves, flowers, roots and stems of plants. It forms a boundary between the plant and the external environment. The epidermis serves several functions: it protects against water loss, regulate gas exchange, secretes metabolic compounds, and absorbs water and mineral nutrients. The epidermis of most leaves shows dorsoventral anatomy: the upper (adaxial) and lower (abaxial) surfaces have somewhat different construction and may serve different functions. Woody stems and some other stem structures such as potato tubers produce a secondary covering called the periderm that replaces the epidermis as the protective covering.

Transpiration stream

In plants, the transpiration stream is the uninterrupted stream of water and solutes which is taken up by the roots and transported via the xylem to the leaves where it evaporates into the air/apoplast-interface of the substomatal cavity. It is driven by capillary action and in some plants by root pressure. The main driving factor is the difference in water potential between the soil and the substomatal cavity caused by transpiration.

Guard cell Paired cells that control the stomatal aperture

Guard cells are specialized plant cells in the epidermis of leaves, stems and other organs that are used to control gas exchange. They are produced in pairs with a gap between them that forms a stomatal pore. The stomatal pores are largest when water is freely available and the guard cells turgid, and closed when water availability is critically low and the guard cells become flaccid. Photosynthesis depends on the diffusion of carbon dioxide (CO2) from the air through the stomata into the mesophyll tissues. Oxygen (O2), produced as a byproduct of photosynthesis, exits the plant via the stomata. When the stomata are open, water is lost by evaporation and must be replaced via the transpiration stream, with water taken up by the roots. Plants must balance the amount of CO2 absorbed from the air with the water loss through the stomatal pores, and this is achieved by both active and passive control of guard cell turgor pressure and stomatal pore size.

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Transpiration Process of water moving through a plant

Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth and metabolism. The remaining 97–99.5% is lost by transpiration and guttation. Leaf surfaces are dotted with pores called stomata, and in most plants they are more numerous on the undersides of the foliage. The stomata are bordered by guard cells and their stomatal accessory cells that open and close the pore. Transpiration occurs through the stomatal apertures, and can be thought of as a necessary "cost" associated with the opening of the stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis. Transpiration also cools plants, changes osmotic pressure of cells, and enables mass flow of mineral nutrients and water from roots to shoots. Two major factors influence the rate of water flow from the soil to the roots: the hydraulic conductivity of the soil and the magnitude of the pressure gradient through the soil. Both of these factors influence the rate of bulk flow of water moving from the roots to the stomatal pores in the leaves via the xylem.

Photosynthesis system

Photosynthesis systems are electronic scientific instruments designed for non-destructive measurement of photosynthetic rates in the field. Photosynthesis systems are commonly used in agronomic and environmental research, as well as studies of the global carbon cycle.

By definition, stomatal conductance, usually measured in mmol m-2 s-1, conditions the net molar flux of carbon dioxide (CO2) entering or water vapor exiting through the stomata of a leaf, for a given concentration difference of CO2 or water vapor between the atmosphere and the sub-stomatal cavity. The so conditioned molar fluxes are for CO2 the net CO2 assimilation rate and for water vapour the transpiration rate.

Breeding for drought resistance is the process of breeding plants with the goal of reducing the impact of dehydration on plant growth.

9-cis-epoxycarotenoid dioxygenase (EC 1.13.11.51, nine-cis-epoxycarotenoid dioxygenase, NCED, AtNCED3, PvNCED1, VP14) is an enzyme with systematic name 9-cis-epoxycarotenoid 11,12-dioxygenase. This enzyme catalyses the following chemical reaction

Hydraulic signals in plants are detected as changes in the organism's water potential that are caused by environmental stress like drought or wounding. The cohesion and tension properties of water allow for these water potential changes to be transmitted throughout the plant.

Alarm photosynthesis

Alarm photosynthesis is a variation of photosynthesis where calcium oxalate crystals function as dynamic carbon pools, supplying carbon dioxide (CO2) to photosynthetic cells when stomata are partially or totally closed. This biochemical appendance of the photosynthetic machinery is a means to alleviate the perpetual plant dilemma of using atmospheric CO2 for photosynthesis and losing water vapor, or saving water and reducing photosynthesis. The function of alarm photosynthesis seems to be rather auxiliary to the overall photosynthetic performance. It supports a low photosynthetic rate, aiming at the maintenance and photoprotection of the photosynthetic apparatus rather than a substantial carbon gain.

References

  1. ↑ Waring, R. H.; Cleary, B. D. (1967). "Plant Moisture Stress: Evaluation by Pressure Bomb". Science. 155 (3767): 1248–54. Bibcode:1967Sci...155.1248W. doi:10.1126/science.155.3767.1248. PMID   17847540. S2CID   2516520.
  2. ↑ Chappelka, Arthur H.; Freer-Smith, Peter H. (1995). "Predisposition of trees by air pollutants to low temperatures and moisture stress". Environmental Pollution. 87 (1): 105–117. doi:10.1016/S0269-7491(99)80013-X. PMID   15091613.
  3. ↑ Lee, J. A.; Stewart, G. R. (1 January 1971). "Desiccation Injury in Mosses. I. Intra-Specific Differences in the Effect of Moisture Stress on Photosynthesis". The New Phytologist. 70 (6): 1061–1068. doi: 10.1111/j.1469-8137.1971.tb04588.x . JSTOR   2431023.
  4. ↑ Hand, JM; Young, E; Vasconcelos, AC (1982). "Leaf water potential, stomatal resistance, and photosynthetic response to water stress in peach seedlings". Plant Physiol. 69 (5): 1051–4. doi:10.1104/pp.69.5.1051. PMC   426357 . PMID   16662343.
  5. ↑ Freeman, Scott (2014). Biological Science: Fifth Edition. USA: Pearson Education, Inc. p. 765. ISBN   978-0-321-74367-1.
  6. ↑ Lambers, Hans; Chapin II, F. Stuart; Pons, Thijs L. (2008). Plant physiological Ecology. New York, NY: Springer. p. 349. ISBN   978-0-387-78340-6.
  7. ↑ Singh, Chandrakant; Wang-Erlandsson, Lan; Fetzer, Ingo; Rockström, Johan; van der Ent, Ruud (2020-12-05). "Rootzone storage capacity reveals drought coping strategies along rainforest-savanna transitions". Environmental Research Letters. 15 (12): 124021. Bibcode:2020ERL....15l4021S. doi: 10.1088/1748-9326/abc377 . ISSN   1748-9326.
  8. ↑ Weaver, J.E. (1926) Root Development of Field Crops. McGraw Hill, New York.
  9. ↑ Shinozaki, K.; Yamaguchi-Shinozaki, K. (1997). "Gene Expression and Signal Transduction in Water-Stress Response". Plant Physiology. 115 (2): 327–334. doi:10.1104/pp.115.2.327. PMC   158490 . PMID   12223810.
  10. ↑ Chaves, M. M.; Flexas, J.; Pinheiro, C. (2009-02-01). "Photosynthesis under drought and salt stress: regulation mechanisms from whole plant to cell". Annals of Botany. 103 (4): 551–560. doi:10.1093/aob/mcn125. ISSN   0305-7364. PMC   2707345 . PMID   18662937.
  11. ↑ Kirschbaum, M. U. F. (1988). "Recovery of photosynthesis from water stress in Eucalyptus pauciflora-a process in two stages". Plant, Cell and Environment. 11 (8): 685–694. doi:10.1111/j.1365-3040.1988.tb01151.x.
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