plant_physiology → solar_radiation
Solar radiation is the total electromagnetic energy emitted by the sun and received at the Earth’s surface.
It includes: - Photosynthetically active radiation (PAR) - Infrared radiation (heat) - Small amounts of ultraviolet radiation
Plants respond to both the quantity and timing of this energy.
Irradiance describes the instantaneous power of incoming solar radiation.
High irradiance means high short-term energy input.
Radiation sum describes the total accumulated solar energy over time.
Common units: - MJ/m² per day - kWh/m² per day
Radiation sum integrates irradiance over time:
- High irradiance for a short time
- Moderate irradiance for a long time
Both can produce the same radiation sum.
Radiation sum is closely linked to: - Biomass accumulation - Yield potential - Sugar production - Crop development rate
Plants respond more strongly to total received energy than to brief peaks.
Solar radiation → PAR → photosynthesis → carbohydrates
Growth responds to energy accumulated over time, not just brightness at noon.
Solar radiation is the total electromagnetic energy emitted by the sun and received at the Earth’s surface.
It includes: - Photosynthetically active radiation (PAR) - Infrared radiation (heat) - Small amounts of ultraviolet radiation
Plants respond not just to how bright it is at any moment, but to how much energy they receive over time.
Irradiance describes the instantaneous power of incoming solar radiation.
High irradiance means a strong energy input at that moment, but does not describe total daily energy.
Practical meaning:
“How strong is the sun right now?”
Radiation sum describes the total accumulated solar energy over a defined period, usually a day.
Common units: - MJ/m² per day - kWh/m² per day
Radiation sum integrates irradiance over time. For example:
- Short periods of very high irradiance
- Long periods of moderate irradiance
Both can produce the same radiation sum.
Practical meaning:
“How much solar energy did we receive today?”
Plant growth correlates much more closely with total received energy than with brief peaks in brightness.
Radiation sum strongly influences: - Photosynthesis over the day - Carbohydrate production - Biomass accumulation - Yield potential
Short spikes of strong sunlight cannot compensate for low total daily energy.
Only a portion of solar radiation is usable by plants.
The chain is: Solar radiation → PAR → photosynthesis → carbohydrates
Radiation sum describes energy availability, while DLI describes biologically usable light.
W/m² (Irradiance)
“How strong is the sun right now?”
MJ/m²/day (Radiation sum)
“How much solar energy did we receive today?”
PAR (µmol m⁻² s⁻¹)
“How much of that energy can plants use right now?”
DLI (mol m⁻² day⁻¹)
“How much usable light did plants get today?”
Plants respond more reliably to daily totals than to instantaneous values.
Plant growth depends on both energy supply and temperature-driven metabolism.
Growth is most efficient when: - Radiation is high - Temperature is within the crop’s optimal range
High temperature without sufficient radiation increases respiration losses.
High radiation with extreme temperatures reduces efficiency.
Radiation determines how much growth is possible.
Thermal time determines how fast development progresses.
This explains many real-world yield and growth patterns.
As a result: - Bright, cool conditions build sugars and yield - Warm, overcast conditions increase stress and reduce performance
This is why: - Crops can stall during cloudy heat - Bright northern summers can outperform expectations - Yield drops often follow warm, dull periods
In protected cropping, not all solar radiation reaches the crop.
Common transmission losses include: - Glass or plastic coverings (10–30%) - Condensation on surfaces - Structural shading (frames, gutters) - Dirt, algae, and dust accumulation - Ageing or degraded plastics
These losses are cumulative and often increase over time.
Key point:
External radiation measurements do not represent the energy available to plants inside the structure.
Plants grow on accumulated energy, not momentary brightness.