The response of plant to the relative length of light and dark period is called photoperiodism. I here are number of plants responses controlled FA photoperiods. Some of these are:

I. Flowering in short day and long din Plants

  1. formation of tubers in potato and bulbs in onion is also promoted by short days.
  2. Short days promote enlargement of tap root and storage of food in radish and carrot.
  3. The leases are shed. the growth of plant stop and it becomes dormant during the winter.
  4. Photoperiod determines the ratio of male and female flower in some plants like hemp and maize.

Historical back ground of photoperiodism

following plant physiologist played their role in the discovery of photoperiodism:

  1. Julien Tournois

He performed experiments on hemp (cannabis satixusflower. He planted it early in the spring. It produces flowers.It     remained in a vegetative stage when he planted it in later spring or summer.He concluded that flowering in hemp is controlled by duration of light. Tournois found  that short  photoperiods of 6 hours durations is required
for floweringin hemp. It remained in a vegetative phase if this duration is more than 6 hours.

  1. George Kleb (1918)

He was working with house leek plants. it is a long day plant. He found that the normal time of flowering for this plant is June. He introduce flowering in it in midwinter in a greenhouse by artificial light. It was believed at that time that the role of huht in the life of plants is nutritional. Light produce more carbohydrates. It causes flowering in it. George exposed hop and hemp to low light intensities of light for longer periods. The plants did not flower. It confirmed that photosynthesis did not play any role. Therefore. the photoperiod (day length) is more important than intensity of light.

  1. Garner and Allard

They give first hypothesis about photoperiodism in 1920. They worked on two plants:

(a) Biloxi soybean: They germinated these plants in may, June, and August. They all flowers in September and October. They observed that there are variations of 59 days in germination date from May and June. But there was difference of only 11 days for flowering  in these plants. They suggested that there is seasonal timing mechanism for flowering in SOy bean.



xaxa(B)Tobacco (Nicotiana tobaccum): They also studied the flowering in tobacco plants in maryland state U.S.A. They germinated seeds of tobacco plant during Feb March in- beds. They transplanted them in fields during April-May. the day length is  hours at that time. The plants. flower during late summer. Al. that time. the day length as about 12 hours. Garner .and At lard observed a mutant strain in the fields. It was taller. It had much larger leaves. It did not flower throughout the season. They named it Maryland Mammoth. They. transferred it into green house at the end of the season. The plant flowered in winter and produced seeds. The days are shorter in winter.

Next Gamer and Allard grew plants during summer. They exposed them to day length equal to winter day (shorter duration or light). Then they placed the plants in dark chamber. They liquid that plant remains vegetative when the light period was increased during winter. Garner and Allard concluded that the length of light and dark periods within a 24- hour cycle promotes flowering. .Garner and Allard called Maryland Mammoth a short-day plant (SDP). the also studied many other short day plants.

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