Week 2: Crop, Plant, and Organ Temperature - Wim van Ieperen & Tijmen Kirstens (11/3)

Absorbed Radiation

Total absorbed solar (and thermal) radiation entering the leaf.

LR

Longwave radiation (net thermal radiation emitted by the leaf)

S

Sensible heat flux (convective heat transfer between the leaf surface and surrounding air).

λE

Latent heat flux (heat loss due to transpiration/evaporation of water).

The leaf heat balance equation

R_net + S + λE = 0

R_net - Net radiation (short and longwave)

S - Sensible heat

λE - Latent heat

Leaf warming/cooling rate equation

Stefan-Boltzmann Law for emitted long-wave radiation

Double-sided emission rule for leaves

A leaf emits long-wave radiation from both upper and lower sides

2*Boltzmann

PAR share of absorbed solar heat in a sunlit leaf

~70% of all absorbed short wave radiation heat comes from the PAR band (400-700nm)

Net long-wave radiation equation (LR_net)

Absorbed - emitted LR

Why is photosynthesis neglected in leaf heat balance?

Photosynthetic energy storage is only about 3% of absorbed solar radiation, making it negligible

Wavelength ranges for SR vs. LR radiation

SR: 0.3-3 µm

LR: 3-100 µm

What is the primary driver of plant development speed?

The temperature of the shoot apex, NOT air temperature

Effect of stomatal closure on leaf energy balance

Transpiration stops (λE —> 0), removing evaporative cooling and causing leaf temperature (T_leaf) to rise

ε

Emissivity, which measures how effectively an object emits thermal radiation compared to an ideal "black body" at the exact same temperature