Leaf Energy Balance and Temperature Regulation in Plants

Leaf Energy Balance

• Basic Concepts:
  • Energy Budget: All organisms receive and dissipate energy (heat) to surroundings.
  • Net Energy Balance:
    • If Energy In > Energy Out: Temperature increase.
    • If Energy In < Energy Out: Temperature decrease.
    • If Energy In = Energy Out: No temperature change (equilibrium).

Thermal State of Substances

• Temperature:

  • A measure of mean molecular speed (energy level).
  • Heat moves in response to temperature differences.

• Heat Content:

  • Measure of total kinetic energy based on mass, temperature, and specific heat capacity.

Temperature Effects on Plants

• Affects metabolic processes (e.g. respiration).
• Temperature Coefficient (Q10):
  • Represents the rate of reaction change per 10°C increase in temperature (~2).
• Influences:
  • Growth and reproduction rates and timing (phenology).
  • Seed germination.
  • Water loss via transpiration.
  • Extremes:
  • Freezing and cold.
  • High temperatures.
• Thermal Ranges for Growth:
  • Land plants generally thrive between -5°C to 55°C, with the most growth and activity at 5°C to 40°C.

Plant Temperature Regulation

• Endotherms:
  • Use internal metabolic heat for temperature regulation (e.g. mammals, birds, some plants).
• Ectotherms:
  • Use external (environmental) factors for temperature regulation (most plants, reptiles, insects).
  • Homeotherms: Warm-blooded.
  • Poikilotherms: Cold-blooded.

Energy from Solar Radiation

• Shortwave Radiation:
  • Solar Radiation (300-3000 nm), both direct and diffuse.
• Leaf Orientation:
  • Influences the amount of incident solar radiation received (Lambert’s Cosine Law: I = Io cos θ).
  • Plants may alter leaf positions to optimize solar exposure (solar tracking, cupping).

Leaf Optical Properties

• Fate of Incident SW:

  • Absorptance (a): Fraction absorbed.
  • Reflectance (r): Fraction reflected.
  • Transmittance (t): Fraction transmitted.
  • a + r + t = 1.0 (conservation of energy).

• Optical Property Modifications:

  • Plants may adapt leaf features to modify optical properties (e.g. leaf hairs, waxes).

Longwave (Infrared) Radiation

• Longwave radiation (>3000 nm) comes from surrounding objects, including the atmosphere.
• Stephan-Boltzman Law:
  • Amount of LW emitted is proportional to the fourth power of temperature.
• Wein’s Law:
  • Indicates warm objects emit shorter wavelength radiation than cooler ones.

Energy Dissipation Processes

• The processes include:
  • Re-radiation of LW:
  • Convection: Heat transfer by air movement.
  • Transpiration: Heat loss through evaporation.
  • Conduction: Minimal heat transfer via direct contact.

Leaf Energy Balance Equation

• General Equation:
  • SW absorbed + LW absorbed = LW re-radiation + Convection + Transpiration.
• Detailed Equation:
  • aSW + εLW = 2σ(Tleaf + 273)⁴ + 2h(Tleaf - Tair) + 2LgΔw.
  • Variables:
  • a = leaf absorptance to SW.
  • ε = emissivity for LW.
  • σ = Stefan-Boltzmann constant.
  • Tleaf, Tair: Temperatures, leaf and air respectively.
  • h = convection coefficient.
  • L = latent heat of vaporization.
  • g = stomatal conductance.
  • Δw = water vapor gradient.

Case Study: Temperature Regulation by Atriplex hymenelytra

• Experiments indicate how transpiration affects leaf temperature under high air temperature (45°C).
  • Measurements on leaf transpiration rates at varying temperatures show differential cooling capacities.

Summary of Energy-Exchange Processes

• Overview of coupling factors and organism responses to energy-exchange processes.
• Key influencing factors include leaf size, shape, temperature, and environmental conditions such as wind speed and moisture content.