Photosynthesis
Photosynthesis captures light energy and converts CO₂ + water into sugars (assimilates), releasing O₂ as a by-product. Those assimilates power growth, yield, root activity, and stress resilience.
What it needs
| Factor |
Why it matters |
Typical limiter |
| Light (PAR / DLI) |
Drives energy capture |
Low DLI (winter/overcast), self-shading in dense canopies |
| CO₂ |
Carbon source for sugar |
Low canopy CO₂, poor distribution under glass |
| Water status |
Keeps stomata open + transport |
Dry-downs, high VPD spikes, salinity reducing uptake |
| Temperature |
Controls enzyme activity |
Cold slows rates; heat increases stress + reduces efficiency |
What limits photosynthesis in practice
Light
- Low light → less sugar supply → slower growth and weaker fruit fill
- Very high light can outpace CO₂/water/nutrition supply → stress and reduced efficiency
CO₂ availability (glasshouse especially)
- CO₂ is often the “quiet limiter” in closed canopies
- Enrichment only pays when light and temperature allow the plant to use it
- Poor air movement can leave the canopy CO₂-limited even if the greenhouse reading looks fine
Water status and VPD
- Water stress closes stomata → CO₂ entry drops quickly
- High VPD commonly triggers midday closure
- High substrate EC can “physiologically dry” plants even when media looks wet
Temperature
- Too cold → slow biochemistry
- Too hot → protective responses + higher photo-damage risk → reduced assimilation
Scouting clues
- Growth stalls despite “enough feed”
- Smaller leaves, weaker roots, poor fruit size/fill
- Midday wilt on bright high-VPD days
- Chlorosis from limitations (often N, Mg, Fe) reducing chlorophyll performance
Practical levers
- Canopy: manage density to reduce self-shading and improve airflow
- Irrigation: align strategy with radiation/VPD; avoid repeated midday stress
- CO₂: enrich only when light/temperature allow; ensure distribution into canopy
- Nutrition: keep N, Mg, Fe adequate; avoid salinity stress (EC/Na/Cl)