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Plant Transpiration and Hydrological Cycle

Photosynthesis Fundamentals

  • Inputs: Light, water (H2O), carbon dioxide (CO2)

  • Outputs: Oxygen (O_2), Sugar

  • This process links the need for (CO_2) uptake to water loss from the plant.

Field Botany: Transpiration

  • Definition: Transpiration is the process by which plants release water vapor into the atmosphere.

  • Consequence: The capture of energy through photosynthesis inherently results in water loss due to the exchange of gases.

The Hydrological Cycle (Water Cycle)

  • Overview: Describes the continuous movement of water on, above, and below the surface of the Earth.

  • Units:

    • Storage values are given in thousand cubic kilometers (10^3\text{ km}^3).

    • Fluxes (black values) are given in thousand cubic kilometers per year (10^3\text{ km}^3/yr).

  • Key Storage Components and Values:

    • Ocean: 1,335,040 \times 10^3\text{ km}^3

    • Ice: 26,350 \times 10^3\text{ km}^3

    • Groundwater: 15,300 \times 10^3\text{ km}^3

    • Soil Moisture: 122 \times 10^3\text{ km}^3

    • Lakes/Rivers: 178 \times 10^3\text{ km}^3

    • Atmosphere: 12.7 \times 10^3\text{ km}^3

    • Permafrost: 22 \times 10^3\text{ km}^3

  • Key Fluxes and Values per year:

    • Ocean Evaporation: 413 \times 10^3\text{ km}^3/yr

    • Ocean Precipitation: 373 \times 10^3\text{ km}^3/yr

    • Land Precipitation: 113 \times 10^3\text{ km}^3/yr

    • Evaporation & Transpiration (from land): 73 \times 10^3\text{ km}^3/yr

    • Water Vapor Transport (Ocean to Land): 40 \times 10^3\text{ km}^3/yr

    • Surface Flow (Land to Ocean): 40 \times 10^3\text{ km}^3/yr

    • Groundwater Flow (Land to Ocean): 40 \times 10^3\text{ km}^3/yr

  • Balance of Fluxes (Global and Regional):

    • Global Atmosphere Balance: Total evaporation (413+73 = 486) equals total precipitation (373+113 = 486).

    • Ocean Balance:

      • Losses: Evaporation (413) + Water vapor transport to land (40) = 453

      • Gains: Precipitation (373) + Runoff from land (surface flow 40 + groundwater flow 40 = 80) = 453

      • The ocean fluxes are balanced (453 = 453).

    • Land Balance:

      • Gains: Precipitation (113) + Water vapor transport from ocean (40) = 153

      • Losses: Evaporation/Transpiration (73) + Surface flow to ocean (40) + Groundwater flow to ocean (40) = 153

      • The land fluxes are balanced (153 = 153).

Plant Anatomy for Transpiration

  • Example Plant: Potentilla sp. (Rosaceae) (e.g., found in Salt Lake County, UT)

  • Leaf Structure:

    • Cuticle: A waxy, protective layer on the outer surface of the epidermis (both upper and lower). It helps prevent excessive water loss.

    • Upper Epidermis: The outermost layer of cells on the upper side of the leaf, typically lacking chloroplasts.

    • Mesophyll Cells: The internal tissues of a leaf, located between the epidermal layers. These cells are packed with chloroplasts and are the primary site of photosynthesis.

    • Vascular Bundle: Contains xylem and phloem, responsible for transporting water and nutrients throughout the plant, including to the leaf cells.

    • Lower Epidermis: The outermost layer of cells on the underside of the leaf, often containing stomata.

    • Guard Cells: Specialized epidermal cells that surround and regulate the opening and closing of a stoma.

    • Stoma (pl. Stomata): A pore, typically on the underside of the leaf, formed by two guard cells, through which gases (CO2, O2, H_2O \text{ vapor}) are exchanged with the atmosphere.

    • Trichome: Hair-like outgrowths on the epidermis, which can help reduce water loss by trapping a layer of humid air near the leaf surface.

Mechanism of Transpiration

  • Stomata Open:

    • When stomata are open, (CO_2) can enter the leaf for photosynthesis.

    • Concurrently, water vapor (H_2O) is lost through the stomata to the atmosphere. This is the primary process of transpiration.

  • Stomata Closed:

    • When stomata are closed, there is no (CO_2) uptake.

    • Consequently, there is also no significant transpiration (no (H_2O) loss through stomata). The cuticle still provides a barrier against water loss.

Factors Affecting Guard Cell Behavior and Stomatal Closure

  • Stomatal closure, and thus the rate of transpiration, is influenced by several environmental factors:

    1. Light: Generally, light stimulates stomatal opening to allow for (CO_2) uptake for photosynthesis. Lack of light (darkness) or very intense light can cause closure.

    2. Temperature: High temperatures can increase the rate of transpiration, but excessively high temperatures can also lead to stomatal closure to conserve water.

    3. Relative Humidity: Low relative humidity in the air increases the water potential gradient between the leaf and the atmosphere, promoting higher transpiration rates. High humidity reduces this gradient, leading to lower rates.

    4. Wind Speed: Increased wind speed removes the boundary layer of moist air around the leaf, increasing the water potential gradient and thus enhancing transpiration.

Measuring Transpiration

  • Activity: Consider measuring water loss via transpiration from individual plants (e.g., sprucetop grama, Bouteloua chondrosioides) grown in 4-L pots.

  • How to Measure Transpiration:

    • A common method is to measure the decrease in the mass of the pot and plant over time, assuming that any mass loss not accounted for by plant growth is due to water transpired. This could involve covering the soil surface to prevent soil evaporation.

    • Another method involves using a portable porometer to measure stomatal conductance, which can be correlated with transpiration rates.

    • Using a potometer can measure water uptake, which approximates transpiration.

  • Units of Measure for Transpiration:

    • Typically, volume of water lost per unit time (e.g., milliliters per hour (mL/hr) or cubic centimeters per hour (cm^3/hr)).

    • Alternatively, mass of water lost per unit time (e.g., grams per hour (g/hr)).

    • Could also be expressed per unit leaf area.

  • Anticipated Problems with Measurement Method:

    • Evaporation from Soil: If the soil surface is not sealed, evaporation from the soil will be included in the measurement, making it difficult to isolate transpiration.

    • Plant Stress: Manipulating the plant or its environment for measurement might cause stress, altering its natural transpiration rate.

    • Environmental Control: Maintaining constant environmental conditions (light, temperature, humidity, wind) for accurate and comparable measurements can be challenging.

    • Accuracy of Balances/Sensors: Ensuring precise measurements of small changes in mass or water volume.

    • Plant Growth: Changes in plant biomass could affect the overall mass measurement, requiring careful accounting.

    • Root Respiration: Respiration by roots could cause a minor mass change as (CO_2) is released, though this is usually negligible compared to transpiration.