09-Plant Water Use and Carbon Consequences (2)

10-Plant Water Use and Carbon Consequences

Today's Lecture

• Announcements

• We are covering Chapter 33 in your book

• GEV seminar

• Science News

• Short distance plant transport

• Water Potential

• Stomatal regulation of water movement

• Transpiration

Science News - All Things Amazon

• Key Message

  • Plants can create weather.

  • The process of transpiration generates clouds, leading to increased rainfall in the Amazon.

  • Evaporation alone does not cause this effect.

  • Droughts that prevent trees from transpiring or cause deforestation lead to increased drought conditions.

  • This impacts biodiversity and carbon storage in tropical rainforests, which are crucial carbon sinks.

  • Deforestation in the Amazon can result in runaway climate change.

  • An estimated 1 million indigenous people reside in the Amazon.

Figure 33.1: Water and Carbon Dioxide Movement

• Gas Exchange:

  • CO2: Reactant of photosynthesis

  • O2: Product of photosynthesis

  • H2O: Loss of water via transpiration

• CO2 enters the leaves, while O2 and H2O exit through stomata (see Figure 33.8).

• H2O enters the plant through roots.

Transport Mechanisms in Plants

• Short distance vs. Long distance transport:

  • Transport occurs on three scales:

  1. Individual Cells: Transport of water and solutes by root hairs.

  2. Cell-to-Cell: Short-distance transport within tissues and organs.

  3. Xylem and Phloem: Long-distance transport across the entire plant.

Short Distance Transport

• Proton Pumps:

  • Create a hydrogen ion gradient, providing potential energy for work.

  • Contribute to the electric charge difference across the cell membrane, enabling the import of cations like potassium (K+).

  • Coupling of H+ diffusion back into the cell assists in the uptake of anions such as chloride.

Water Potential

• Definition: A measurement that combines solute concentration and pressure effects.

• Equation: Ψ = ΨS + ΨP + Ψg

• Measurement Focus: During labs, pressure potential (ΨP) will be assessed to determine plant water status (lower numbers indicate higher water stress).

• Directional Flow: Water moves from regions of less negative water potential to more negative (abbreviation = Ψ, units - MPa).

Solute Water Potential

• Impact: Proportional to the number of dissolved molecules.

• Cell Behavior: A cell in a higher solute concentration environment will lose water, resulting in plasmolysis.

Plant Level Transport

• Water and minerals ascend from roots to shoots through the xylem.

• Transpiration causes substantial water loss from leaves and aerial parts.

• This loss of water must be replenished by upward transport from roots, creating a "pull" within the plant.

• Xylem sap can rise over 100 m in the tallest plants.

Stomatal Regulation of Transpiration

• Leaves exhibit broad surface areas, increasing photosynthesis but also water loss.

• Guard Cells control transpiration by adjusting turgidity (open when turgid, closed when flaccid).

• CO2 enters, while H2O exits through stomata.

Stomatal Characteristics

• Approximately 90% of water loss occurs through stomata (up to 20,000 per square cm).

• Stomatal numbers can be influenced by environmental factors (e.g., scarcity of water leads to fewer stomata).

• Plants can minimize water loss by closing stomata, and environmental signals influence this process.

CO2 and Stomatal Control

• The Earth's atmospheric CO2 concentration is partially driven by stomatal control.

• Seasonal Variation: Recorded at Mauna Loa Observatory (Parts per million).

The Transpiration-Cohesion-Tension Mechanism

• Process:

  1. Water vapor diffuses out of stomata through transpiration.

  2. Evaporated water creates tension in the xylem.

  3. Cohesion of water molecules maintains the column of water in xylem sap.

Transpirational Pull

1. Water vapor diffuses out of the stomata during transpiration.

2. Water evaporates from mesophyll cell walls.

3. Tension in the xylem pulls water from veins into the apoplast of mesophyll cells.

4. Tension moves the water column through leaf veins and upwards in the xylem of the stem and root.

5. Water enters roots by osmosis, forming a cohesive column.

Xylem Sap and Water Potential Gradient

• The movement of xylem sap is influenced by a water potential gradient from soil to the air.

Effects of Transpiration

• Significant water loss through transpiration can lead to wilting if not replaced.

• Drought conditions reduce photosynthesis and lower crop yields due to stomatal closure.

Evaporative Cooling

• Transpiration helps lower leaf temperature, protecting enzymes involved in photosynthesis and metabolic processes.