Photosynthesis is the conversion of light energy into chemical energy. In the chloroplast, chlorophyll 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 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, water and nutrients keep up.
Daily light integral (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) or starving the plant (pre-harvest flushing) 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.
The green pigment in chloroplasts that absorbs mainly blue and red light to drive photosynthesis; chlorophyll a and b are the main forms in plants.Photosynthetic photon flux density: the number of PAR photons landing on one square metre each second, in µmol/m²/s; the standard measure of light intensity at the canopy.Vapour pressure deficit: how much more water vapour the air could hold at its temperature, in kPa; it is the main environmental driver of transpiration.Daily light integral: the total number of PAR photons a square metre of canopy receives in one day, in mol/m²/day; PPFD multiplied by hours of light.The practice of keeping plants in total darkness for 24-72 hours before cutting to boost resin; GenetixBay rates it a Myth and runs a standing challenge against it.Feeding only plain water for days or weeks before harvest to "clean" the plant; GenetixBay rates it a Myth that harms yield, unlike a root-zone salt flush.The daily length of light versus dark; photoperiod cannabis flowers when nights grow long enough, rather than at a fixed age.Microscopic pores, mostly on the underside of leaves, each opened and closed by two guard cells to control gas exchange and water loss.CO2 enrichment raises carbon dioxide in a sealed grow space from about 420 ppm to roughly 800–1,500 ppm, letting plants use higher light intensities productively.
