Photoperiodism

Higher plants reproduce sexually by producing flowers, and the shift from vegetative to reproductive growth -- flowering -- is, like vegetative growth itself, controlled by environmental and nutritional factors. Among environmental factors, light and temperature have the most profound effect on flowering: the influence of the duration of light on flowering is called photoperiodism, while the influence of temperature is called vernalization (Section 7.11).

Light affects seed germination, vegetative growth and photosynthesis through three distinct properties -- its quality, its intensity and its duration -- but it is specifically the duration of light exposure that has the greatest effect on flowering. The term photoperiodism was coined by Garner and Allard in 1920, who were studying the flowering behaviour of soybean and the Maryland Mammoth variety of tobacco. They found that soybean flowered in late summer and this tobacco variety flowered in winter, regardless of when the plants had germinated or how long they had been growing; testing different temperatures, nutrition and soil moisture failed to explain this pattern. What they eventually found, using artificial illumination to vary day length experimentally, was that the relative length of the day was the crucial factor controlling flowering -- and they coined the term photoperiodism for this response. On the basis of their photoperiodic response, plants are classified into three categories: Short Day Plants (SDP), Long Day Plants (LDP) and Day Neutral Plants (DNP).