Photoperiodism

The effects of photoperiods or daily duration of light hours (and dark periods) on the growth and development of plants, especially flowering, is called photoperiodism.

Photochemical receptors in the leaves are the biliproteins (pigments) located in the cell membrane and are called phytochromes.

Plants that flower usually during summer are called long-day plants.

• Definition — Effect of duration of light on flowering. Coined by Garner & Allard (1920). Leaf is the chief organ for receiving stimulus (Knoff, 1934).
• Three Types — SDP: day < critical photoperiod (Dahlia, Tobacco, Xanthium); LDP: day > critical photoperiod (Wheat, Pea, Spinach); DNP: independent of photoperiod (Tomato, Maize, Sunflower).
• Critical Dark Period — SDP = long night plants (uninterrupted dark needed; light flash stops flowering). LDP = short night plants (light flash during dark promotes flowering).
• Florigen — Hormonal chemical stimulus transported through phloem from leaves to the flowering site.
• Phytochrome — Proteinaceous pigment in leaf cell membranes (Hendricks & Borthwick, 1952). Two forms: Pr (660 nm, red) and Pfr (730 nm, far red).
• Phytochrome Action — Day: Pfr accumulates → inhibits SDP, promotes LDP. Dark: Pfr → Pr → promotes SDP, inhibits LDP.
• Photomorphogenesis — Control of plant development by light and phytochrome.