# Light stress

> Light stress is damage when leaves or buds absorb more light than they can use or dissipate, showing as bleached white or yellow tops nearest the fixture.

- Canonical: https://genetixbay.com/knowledge/glossary/light-stress.html
- Also written: light burn, light bleaching, bleaching, photobleaching, white tops, light damage
- Glossary category: Environment and light · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Light stress (light burn, bleaching) happens when the light a tissue absorbs outpaces its capacity to use the energy in [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md) or shed it safely as heat. Excess energy forms reactive oxygen species that destroy chlorophyll and other pigments. The classic sign is bleaching: top leaves and bud [bracts](https://genetixbay.com/knowledge/glossary/bract.md) closest to the light turn pale yellow, then white, while lower growth stays green.

## In practice

Bleaching shows up most often under high-output [LEDs](https://genetixbay.com/knowledge/glossary/led.md) hung too close, because LEDs emit less radiant heat and growers do not feel the warning on their hand the way they would under HPS. It is more likely when [PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md) is high (above about 1,200 µmol/m²/s at the tops), CO2 is ambient, [leaf temperature](https://genetixbay.com/knowledge/glossary/leaf-temperature.md) is wrong or [VPD](https://genetixbay.com/knowledge/glossary/vpd.md) is very high, and late in flower when leaves are senescing.

Signs in order: upward cupping (tacoing) of top leaves, yellowing between veins at the top only, then bleached white bracts. Fix by raising or dimming the fixture to bring top PPFD back to 900–1,100 µmol/m²/s, and check leaf temperature and VPD. Bleached buds do not regreen; they are usually lower in potency and aroma.

## Don't confuse with

- **Nutrient deficiency**: yellowing that follows mobility patterns (old vs new leaves), not distance from the light.
- **Heat stress**: driven by temperature; often occurs alongside light stress.
- **Foxtailing**: a growth form change, which can be a response to light or [heat stress](https://genetixbay.com/knowledge/glossary/heat-stress.md).

## Related terms

[PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md), [Heat stress](https://genetixbay.com/knowledge/glossary/heat-stress.md), [Light saturation point](https://genetixbay.com/knowledge/glossary/light-saturation-point.md), [Foxtailing](https://genetixbay.com/knowledge/glossary/foxtailing.md)

## Read more

- [Heat and light stress: taco leaves, bleaching and foxtails](https://genetixbay.com/knowledge/environment/heat-light-stress.md)

## Sources

1. Rodriguez-Morrison V, Llewellyn D, Zheng Y, 2021. Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science 12:646020. https://doi.org/10.3389/fpls.2021.646020
2. Chandra S, Lata H, Khan IA, ElSohly MA, 2008. Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiology and Molecular Biology of Plants 14(4):299–306. https://doi.org/10.1007/s12298-008-0027-x