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PHOTOPERIODISM IN MARIJUANA
The term short-day is something of a misnomer; what marijuana needs is a sufficiently long night.
Marijuana flowers only if it has been kept into he dark for at least 9-11.5 hours depending on variety, this critical light period occurs for 5-7 days consecutively.
Interruption of an otherwise long night by red light (666 nm) prevents flowering unless it is followed by irradiation with far-red (730 nm) light.
An intense exposure to far-red light at the start of the night reduces the dark re-requirement by two hours. These responses are mediated by phytochrome.
Phytochrome exists in two inter-convertible forms-
Pr because it absorbs red (R; 660 nm) light;
Pfr because it absorbs far-red (FR; 730 nm) light
These are the relationships:
Absorption of red light by Pr converts it into Pfr
Absorption of far-red light by Pfr converts it into Pr
In the dark, Pfr spontaneously converts back to Pr over a two hour time period.
Sunlight is richer in red (660 nm) than far-red (730 nm) light, so at sundown all the phytochrome is Pfr.
During the night, the Pfr converts back to Pr.
The Pr form is needed for the release of the flowering signal.
Therefore marijuana needs 9-11.5 hours of darkness for several days of this regimen in which it converts the Pfr present at sundown into Pr to carry out the supplemental reactions leading to the release of the flowering signal.
If this process is interrupted by a flash of 660 nm light, the Pr is immediately reconverted to Pfr and the night's work is undone.
A subsequent exposure to far-red (730 nm) light converts the pigment back to Pr and the steps leading to the release of "florigen" can be completed.
Exposure to intense far-red light at the beginning of the night sets the clock ahead about two hours or so by eliminating the need for the spontaneous conversion of Pfr.
It requires 10 days to switch to the flowering cycle and 10 days to switch back to vegetative.