Recap – Water Relations in Plants
Properties of Water & Importance in Plants
- Water is a polar molecule, giving it partial charges.
- Forms hydrogen bonds, resulting in:
- Cohesion (water molecules stick to each other).
- Adhesion (water molecules stick to surfaces, e.g., xylem walls).
- Cohesion + adhesion enable capillary action and the continuous water column in xylem.
Transpiration
- Defined as the loss of water vapor through stomata.
- Drives the transpiration stream: water uptake from roots → movement through xylem → evaporation at leaf surfaces.
- Provides:
- Water for photosynthesis.
- Cooling effect (via evaporative cooling).
- Nutrient transport.
Plant Adaptations to Water Availability
- Desert plants (xerophytes):
- Thick cuticle, sunken stomata, trichomes (hairs), reduced leaf area, CAM photosynthesis.
- Aquatic plants (hydrophytes):
- Large air spaces (aerenchyma), stomata only on upper surface, thin cuticle.
- Mesophytes: intermediate adaptations for moderate environments.
Photosynthetic Pathways & Water Use
- C3 plants:
- First stable product is a 3-carbon compound (3-PGA).
- More common in cooler, wetter climates.
- Can photosynthesize at lower temperatures.
- Less water use efficiency (WUE) because of photorespiration.
- C4 plants:
- First stable product is a 4-carbon compound (oxaloacetate).
- Use a spatial separation of initial CO₂ fixation (mesophyll) and Calvin cycle (bundle sheath cells).
- Minimize photorespiration, higher WUE.
- Better adapted to hot, dry climates.
- CAM plants:
- Use temporal separation (stomata open at night, CO₂ fixed into malic acid; Calvin cycle occurs during the day).
- Extremely high WUE.
- Common in succulents, cacti, bromeliads.
Water Availability & Plant Productivity
- The spatial and temporal patterns of available water determine:
- How much water is extractable by plants.
- The length of the growing season.
- Plant survival and ecosystem distribution.
- Water availability is a major limiting factor for terrestrial productivity.