PLANT TRANSPIRATION AND COOLING

Plant temperature is one of the most important factors of its’ health. Transpiration process helps carrying the water through the plant, resulting to evaporation through tiny pores on the leaves’ surface. More importantly, the process is one of the key factors for maintaining the required temperature and humidity. This post tries to analyze the mechanics of plants’ self cooling processes, as well as discuss different adaptation techniques.

HEAT BUDGETS

The heat transfer between the plant and its’ environment occurs essentially in 3 ways :

  • reradiation of long wavelengths to nearby surfaces and sky;
  • conduction and convection of sensible heat;
    solar radiation

    Figure 1 [1]

  • latent heat exchange (transpirational cooling).

Irrespective of species or location, any organism, over an extended period, will lose exactly as much energy as it gains otherwise that organism would overheat and die. Though they have evolved with adaptive features that make them expert at balancing their energy budgets, however, plants probably never achieve short-term equilibrium with their environment. [1]

SENSIBLE HEAT EXCHANGE

An exchange of sensible heat depends on the temperature difference between the leaf and the surrounding air. The hotter the air, the higher rate of exchange. Heat transfer occurs by molecular conduction across a thin boundary layer of air which is ‘attached’ to both sides of any leaf. This layer of air is not moving. Boundary-layer resistance to heat transfer varies according to the thickness of that layer which in turn is a function of leaf geometry and imposed wind speed. The larger the boundary layer, the slower the rates of transpiration. [2]

stomata

Figure 2 : Stomatal conductance [3]

Boundary layer size depends on many features : size and geometry, a large amount of hairs (figure 3) would increase the boundary layer. Some plants possess stomata that are sunken into the leaf surface (figure 4), dramatically increasing the boundary layer and slowing transpiration. Plants from desert climates often have small leaves so that their small boundary layers would help cool the leaf with higher rates of transpiration. [4]

leaf structure

Figure 3 : A scanning electron micrograph of an uncoated and rapidly frozen piece of tobacco leaf showing a hairy lower leaf surface and cross-sectional anatomy at low magnification. [14]

Hair slightly block the wind and increase the thickness of still air layer, this way decreasing transpiration rate.

Sensible heat transfer between leaf and air can also occur by free convection. [2]

LATENT HEAT TRANSFER

Evaporation (transpiration in plants, perspiration by animals) is a cooling process. Water evaporating from wet cell walls inside leaves collects as vapour within substomatal cavities, and passes via stomatal pores and the leaf boundary layer to ambient air. (Figure 2) Despite consuming a large amount of energy, transpiration process is very effective. [6]

Heat exchange depends on boundary-layer resistance, and varies according to leaf shape, size and aerodynamic conditions. In addition, latent heat exchange will be subject to control by atmospheric humidity, and will be a more significant component of leaf heat budgets on days of low humidity. Though most well-watered plants benefit from transpirational cooling in balancing their heat budgets, desert plants have to rely on sensible heat exchange to a greater extent. They would have increased transpiration because of smaller size, also thick surface coatings would increase reflectivity and ensure protection in hot climate areas. [6]

xeromorph cactus

Figure 4 : Xeromorph cactus Rhipsalis dissimilis. The guard cells are sunken deep into the surface and the cuticle layer s very thick – classic adaptation of a xerophyte. [5]

Plants regulate the rate of transpiration by controlling the size of the stomatal apertures. (Figures 5, 6) The rate of transpiration is also influenced by boundary layer conductance, humidity, soil water supply, temperature, wind and incident sunlight. The amount of water lost by a plant also depends on its size and the amount of water absorbed at the roots.  [7]

stomata

Figures 5, 6 : Stomatal apertures [8]

Factors, that influence the rate of transpiration :

  • Light
    diagrams

    Figures 7, 8, 9 : the effects on transpiration rate of plants. [9]

Light heats the plant and stimulates the stomata, which leads to quicker transpiration in high exposure. [7]

  • Temperature

Plants transpire faster when the temperature increases. For example, a leaf can transpire, at 30 degrees Celsius, three times faster than it would at 20 degrees Celsius. (Figure 7) [7]

  • Relative humidity

As the difference in a substances concentration in the two regions increases, the rate of that substances diffusion will increase. When the air that surrounds a plant is dry, the water diffusing out of a leaf will occur more rapidly. (Figure 8) [7]

  • Air movement and wind

When no breeze is present, the air that surrounds a leaf becomes increasingly humid, so transpiration slows down. When there is a breeze, humid air is replaced by drier air because the breeze carries the humid air away. (Figure 9) [7]

  • Soil Water and Soil-Moisture Availability

If water loss occurs and this loss of water is not replaced by water in the soil, a plant will no longer be able to transpire quickly. When a plant’s roots absorption of water fails to keep up with how quickly transpiration is occurring, the stomata closes and turgor loss occurs. When this happens, the rate of transpiration is immediately reduced, as is photosynthesis. [7]

CRASSULACEAN ACID METABOLISM (CAM) PHOTO-SYNTHESIS

A further adaptation to high temperature and in particular arid conditions is crassulacean acid metabolism (CAM) photo-synthesis. CAM is essentially an adaptation to reduce water loss through stomata by reversing the diurnal rhythm of CO2 fixation. These plants close their stomata during the day, when normally, evaporative rates would be highest. Typically, CAM plants are equipped with fleshy assimilatory organs and include the majority of cacti (figure 10) and orchids as well as many salt-tolerant succulents (such as the ice plant Mesembryanthemum crystallinum) (figure 11) and pineapple. CAM plants operate more like a biochemical storage battery. [10]

During the night, a plant employing CAM has its stomata open, allowing CO2 to enter and be fixed as organic acids that are stored in vacuoles. During the day the stomata are closed (thus preventing water loss), and the carbon is released to the Calvin cycle so that photosynthesis may take place. [11]

CAM plants

Figure 10 : cactus [14] , 11 : Ice plant Mesembryanthemum crystallinum [15] 

CAM photosynthesis is more of a ‘survival option’ where slow assimilation is an acceptable trade-off for a water-retentive physiology. The spectacular success of CAM photosynthesis in arid zones and dry habitats worldwide confirms this principle. [10]

LINKS AND RESOURCES

  1. Plant heat budgets
  2. Sensible heat exchange
  3. Stomatal conductance
  4. Leaf boundary layer
  5. Stomates That Are Adapted to Different, Partly Extreme Habitats
  6. Latent heat transfer
  7. Factors Affecting Plant Transpiration
  8. Stoma shown via colorized scanning electron microscope
  9. Transpiration
  10. Crassulacean acid metabolism (CAM)
  11. Crassulacean acid metabolism
  12. “Cuticular water permeability and its physiological significance” Gerhard Kerstiens
  13. How the Plant Temperature Links to the Air Temperature in the Desert (research article)
  14. Adaptations of desert plants
  15. ice plant Mesembryanthemum crystallinum

 

 

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