Phytochromes are light-sensing proteins that switch between two forms. Red light (about 660 nm) converts the inactive form (Pr) to the active form (Pfr); far-red light (about 730 nm) and darkness convert it back. The balance tells the plant about day length and shading.
In practice
Phytochrome is how cannabis measures the night. During a long dark period, Pfr drops; if the dark lasts long enough, flowering is triggered in photoperiod cultivars. A light leak or a brief light break during the night can reset the clock. That is why light-tight flower rooms matter, and why green work lights are used: phytochrome responds weakly to green.
Far-red affects shape: a high far-red-to-red ratio (as under a canopy) signals shade and drives stretching. Some LED fixtures add far-red to speed growth and canopy expansion; brief far-red at the end of the day is used by some growers to nudge flowering. Results vary by cultivar.
Don't confuse with
- Chlorophyll: captures light energy; phytochrome only senses it.
- Cryptochrome: blue-light receptor shaping stem length and leaf form.
- Photoperiod: the day-length response phytochrome helps measure.
Far-red is light from about 700 to 750 nm, just beyond visible red; it adds to canopy photosynthesis when mixed with other light and triggers shade responses like stretch.The daily length of light versus dark; photoperiod cannabis flowers when nights grow long enough, rather than at a fixed age.A light leak is unwanted light entering the grow space during the dark period; in flower it can delay or disrupt flowering in photoperiod cannabis.An LED (light-emitting diode) grow light uses solid-state diodes to produce light; modern fixtures are the most efficient grow lights, at roughly 2.5–3 µmol/J.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.Sinsemilla (sensi) is seedless cannabis flower from unpollinated female plants; the word is Spanish for "without seed".A plant that flowers when the night exceeds a critical length (days are short enough); photoperiod cannabis is a short-day plant.The length of stem between two adjacent nodes; short internodes mean compact plants and tight bud stacking, long ones mean stretch.The section of seedling stem between the root and the cotyledons; it elongates to lift the seed leaves above the medium.
