# Photosynthesis

> The process in chloroplasts that uses light energy to turn carbon dioxide and water into sugars and oxygen; cannabis uses the C3 pathway.

- Canonical: https://genetixbay.com/knowledge/glossary/photosynthesis.html
- Also written: photosynthetic, carbon fixation, C3 photosynthesis, light reactions
- Glossary category: Plant biology · 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"

Photosynthesis is the conversion of light energy into chemical energy. In the chloroplast, [chlorophyll](https://genetixbay.com/knowledge/glossary/chlorophyll.md) absorbs light, splits water and makes energy carriers (light reactions); the Calvin cycle then uses them to fix CO₂ into sugars. Cannabis is a C3 plant, so its rate responds strongly to CO₂ concentration as well as light.

## In practice

Every gram of dry bud is mostly fixed carbon. In cannabis, yield rises roughly in line with light up to very high canopy intensities: Rodriguez-Morrison et al. (2021) saw yield keep climbing up to about 1,800 [µmol/m²/s](https://genetixbay.com/knowledge/glossary/ppfd.md) with ambient CO₂. Growers usually target about 600-900 µmol/m²/s in flower without added CO₂ and up to 1,200-1,500 with enrichment, provided temperature, [VPD](https://genetixbay.com/knowledge/glossary/vpd.md), water and nutrients keep up.

[Daily light integral](https://genetixbay.com/knowledge/glossary/dli.md) (DLI, mol/m²/day) is the useful total: intensity times hours.

House position: the last weeks of flower are when the plant packs on most of its final weight and resin. That's why cutting light ([darkness before harvest](https://genetixbay.com/knowledge/glossary/darkness-before-harvest.md)) or starving the plant (pre-harvest [flushing](https://genetixbay.com/knowledge/glossary/flushing.md)) works against you.

## Don't confuse with

- **Respiration**: burning sugars for energy, day and night.
- **Photoperiod**: day-length signalling, not energy capture.
- **Photomorphogenesis**: light shaping growth via photoreceptors.

## Related terms

[Chlorophyll](https://genetixbay.com/knowledge/glossary/chlorophyll.md), [Stomata](https://genetixbay.com/knowledge/glossary/stomata.md), [PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md), [DLI](https://genetixbay.com/knowledge/glossary/dli.md), [CO2 enrichment](https://genetixbay.com/knowledge/glossary/co2-enrichment.md)

## Read more

- [PPFD and DLI by stage, with the evidence](https://genetixbay.com/knowledge/environment/ppfd-dli-by-stage.md)
- [CO₂ enrichment: when it pays, how much, and how to stay safe](https://genetixbay.com/knowledge/environment/co2-enrichment.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 14:299-306. https://doi.org/10.1007/s12298-008-0027-x
3. Taiz L, Zeiger E, Møller IM, Murphy A. 2015. Plant Physiology and Development, 6th edition. Sinauer Associates.