Photoperiodism class 12 | photoperiodism class 11

Aslam o alikum I am Hassam ur Rahman and I am teaching Fsc biology since 2014 .I am always trying to improve myself and provide best lecture to students.i am taking content for the lecture from authentic and relevant sources but human errors are possible . you are requested to please highlight the mistakes.My lectures are equally reliable for Fsc and mdcat students #visiblescience #mdcatbiology #alevelbiology #neetbiology #fscbiology #photoperiodism This lecture is about Photoperiodism Apart from photosynthesis and phototropic responses, another very important way in which light exerts its influence on living organisms is through variations in day length called photoperiod. In plants, photoperiod and temperature affect flowering, fruit and seed production, bud and seed dormancy, leaf fall and germination. Photoperiod affects flowering, when shoot meristems start producing floral buds instead of leaves and lateral buds. Effect of photoperiodism was first studied in 1920 by Garner and Allard. They studied that tobacco plant flowers only after exposure to a series of short days. Tobacco plant naturally flowers under same conditions, in autumn, but flowering could be induced by conditions artificially to short days exposing. With further studies they were able to classify flowering plants into long-day plants, which require long days for flowering and day-neutral plants flower without being influenced by photoperiod. Later on, further studies indicated that it is really the length of the dark period which is critical. Thus short-day plants are really long-night plants. If they are grown in short days, but the long night is interrupted by a short light period, flowering is prevented. Long-day plants will flower in short days if the long night period is interrupted Further experimentation also revealed that quantity of light is also influenced by the quality of light. Cocklebur, a short day plant, will not flower if its long night is interrupted but experiments revealed that red light was effective in preventing flowering and far-red light reversed the effect of red light. It was also demonstrated that the last light treatment always determines the response. This response to light intensity and quality led to the discovery of blue pigment that is red light sensitive protein, the phytochromes. Phytochrome exists in two forms i.e. P 660 and P 730. P 660 a quiscent form absorbs red light at a wave length of 660 nm and is converted to active P 730, P 730 absorbs far red light at 730 nm and is converted to P 660. In nature, the P 660 to P 730 conversion takes place in day light and P 730 to P 660 conversion occurs in the dark. Thus during the day a plant has P 730 phytochromes while during the night it contains more phytochromes in the form of P 660. The presence of either form provides the plants with a means of detecting whether it is in a light or dark environment. The rate at which P 730 is converted to P 660 provides the plant with a “clock” for measuring the duration of darkness. It has been found that red light inhibits flowering the short day plants but promotes flowering in long day plants, under conditions during which flowering normally takes place. This observation led to hypothesize that the P730-P660 interconversion might be the lant time - regulator for flowering. According to this hhypothesis, p 730, converted from P 660 by the absorption of red light, would inhibit flowering in short day plants but promote flowering in long day plants. Because P 730 accumulates in the day and diminishes at night, short day plants coud flower only if the night were long enough, during which a great amount of P730 would not be completely inactivated, so that enough P 730 would remain at the end of night to promote flowering. But now it is generally agreed that the time measuring phenomenon of flowering is not totally controlled by the interconversion of P 660 to P 730. Other factors, like presence or absence of light and length of dark, or light period also play an important role in flowering. Phytochromes seems to be responsible for the detection of either light or darkness. The biological clock once stimulated causes production of florigen hormone in leaves, which travels through phloem to the floral buds, initiating flowering.