ABSTRACT
Photoperiodism is a physiological reaction of organisms to the relative duration of daily illumination. More precisely, “Photoperiodism is the ability of organisms to assess and use the day length as an anticipatory cue to time seasonal events in their life histories” (Bradshaw and Holzapfel 2007). In effect, the sun is acting as a master timer, controlling the initiation of biological developments (Figure 5.1). In the following discussion, the term “photoperiodism” is used with specific reference to induction of flowering. Although Garner and Allard (1920) are given considerable credit for clarifying the influence of photoperiod on plant development, Tournois (1912) is thought to have first discovered photoperiodism in plants (see Jarillo et al. 2008 for a review of the subject). Based on studies of hemp and its relative Japanese hop, Tournois observed that flowering was promoted by short duration of daylight over consecutive day/night intervals (giving rise to the expression “short-day plants”) and delayed by days with long periods of light. As with most other plants in which flowering is induced by a requirement for a given duration of darkness, a single exposure of a few minutes of light during the dark period can disrupt flowering and delay maturation (moonlight and lightning have no effect). “Long-night” plants would have been a better label than “short-day” plants, since the length of darkness, not the length of light, is the stimulus. In the northern hemisphere, nights (i.e., length of dark periods) increase after the summer solstice (about June 21), providing the flowering stimulus. Although not yet clarified for Cannabis, it appears that many flowering plants use a photoreceptor protein such as phytochrome or cryptochrome in the foliage to sense seasonal changes in day length and chemically transmit a signal to the plant’s buds to flower.
