Cannabis gas exchange, photosynthesis, transpiration, stomatal conductance, PPFD, temperature, and carbon-dioxide response under measured conditions.
Abiotic disorders
Excessive Light & Photoinhibition
High photon exposure can exceed the capacity of photosynthetic processes, especially when combined with heat, water limitation, or abrupt increases in light. Bleaching and upper-canopy injury should be mapped against measured light rather than diagnosed from appearance alone.
What to look for
- Bleaching or paling concentrated at the most exposed canopy surfaces
- Upward leaf curl or reduced leaf expansion near the highest PPFD zones
- A clear timeline after fixture power, height, or photoperiod intensity changed
- Less injury on shaded or lower leaves of the same plant
Important look-alikes
- Heat stress
- Chemical or spray injury
- New-growth chlorosis from root-zone chemistry
- Genetic variegation
How to confirm
- Map PPFD across affected and unaffected canopy positions
- Measure leaf temperature at the same points
- Review the rate and timing of recent lighting changes
- Compare symptoms with shaded tissue and neighboring plants
Management principles
- Reduce excessive exposure and correct interacting heat or water-delivery stress
- Change intensity progressively when acclimation is needed
- Do not use canopy bleaching as proof of a mineral deficiency
- Monitor new tissue for recovery because bleached tissue may not regain normal appearance
Prevention
- Commission fixtures with a PPFD map
- Increase light progressively after transplant or major canopy changes
- Coordinate light intensity with temperature, humidity, CO2 strategy where applicable, and root water supply
- Check canopy uniformity after plant height changes
Visual study guide
- PPFD heat map
- Bleaching exposure gradient
- Light-versus-heat differential
- Acclimation timeline
Evidence & further reading