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Biological Clocks

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Biological Clocks Biological clocks - Chronobiology Chronos = the time , It was first discovered in plants that some developments are controlled by the changing lengths of the days (Erwin Bunning 1967). The plant has an endogenous rhythm that is about 24 h (= circadian) & which can be used to compare with what is going on outside. Bean leaf position reflects physiological clock at night (“sleep”) & at daytime Light-Dark coordinated leaf movements continue also without “zeitgeber” or trigger stimuli.Although, now they are slightly changed & circadian! Emergence of insects from their pupae, hibernation, gonade growth & wakening in hamsters, luminescense of unicellular alga Gonyaulax, photosynthesis, abscission, flowering Erwin Bunning (1967) The physiological clock. Springer New York Photoperiodism and Thermoperiodism In nature animals and plants are well aware of circadian changes in illumination (light-dark Î light intensity, light quality Î color or wavelength) & temperature (relative day to night temperatures required, in a to induce growth, reproduction, or flowering of plants or animals). However the ratio of duration of the phases as day : night & warm : cold increases from winter to summer and then declines again. The duration of the dark or cold periods is easily measured by comparison of an internal rhythmic process – a biological clock Maintaining the biological clock and adapting it to the prevailing photo- and thermo-periods of the season depends both on red light (phytochrome)and blue light (cryptochrome) Plants & Time: Waiting for the Spring Thermoperiodism In some organisms thermoderiods play an important role in the phase setting of circadian rhythmicity. Plants; such as, chrysanthemum and tomato respond to alternating periods of low and high temperatures.& will flower earlier if subjected to low night and high day temperatures. Diurnal temperature differences influence internode length, plant height, leaf orientation, shoot orientation, chlorophyll content, lateral branching and petiole and flower stalk elongation (Moe et. al. 1995). Hyanzinth bulbs grown at home or in the green house do not stretch the peduncle but flower close to the bulb. They require cold to grow like this Î Thermoperiodism ÍÎ Photoperiodism Vernalization is a process in which cold stimulates ... It is often required before the photoperiodic system is activated. This makes sense both increases in day temps and day length suggest that spring is coming Henbane Hyoscyamus niger needs thermal stimulus (cold) before responding to longer days to flower The opening of Flowers Î Flower Clock Linnaeus' flower clock was a garden plan hypothesized by Carolus Linnaeus that would show plants that open or close their flowers at particular times of the day to accurately predict the time. He called it specifically the Horologium Florae (lit. "flower clock"), and proposed the concept in the 1751 publication Philosophia 3 a.m. Tragopogon pratensis Goat's-Beard Botanica. 4-5 a.m. Cichorium intybus L. Chicory 5 a.m. - 12 p.m. Taraxacum officinale Dandelion He may never have planted such 8-9 a.m. Goat's beard Convolvulus. Morning Glory 12 AM Lactuca sativa L. Garden Lettuce a garden, but the idea was 1-3 p.m. Nymphaea alba L. White Waterlily attempted by several botanical 3 p.m. Calendula officinalis L.Pot marigold gardens in the early 19th 3-4 p.m. Alyssum alyssoides L. Papaver century, with mixed success nudicaule L. Iceland poppy Flower Opening Opening of flowers is interesting 7 gets lots of interest . However it is not related to photoperiodism – the induction of flowers by changing day lengths. Flower Induction The Shoot Apical Meristem can change into stage 3 (sexual maturity) under the influence of internal (food, age, stress) and external influences (photoperiod, cold, warmth, thermoperiodism) Flower Induction The switching of the SAM form making vegetative shoots to flowers must be really complicated, right? =Î wrong it needs only 3 genes to switch on the production of sepals, pistils and stamens Flower Induction by day-length changes? Garner & Allard (1920) found a new mutant of tobacco called “Maryland Mammoth” that would not flower in Maryland even when grown in the greenhouse see mutant right and normal tobacco plant on the left Î Later attempts with cuttings showed the MM could flower in December in a warm greenhouse Î MM is a “Short Day Plant” ??? Other SDPs were found in chrysanthemum and Poinsettia Î concept of photoperiodism developed for MM applied to other plants & later adapted to animals like hamsters & humans Depending on their geographic origin, plants flower either (1) When days get longer = spring in Northern hemisphere Î Long-Day Plants or (2) When days get shorter = in some warmer and dryer regions this is when rains stop and it is the best time to survive the dry season Î Short-Day Plants Flower Induction by day-length changes? SDPs and LDPs are coming from different areas of the world and their demands are almost opposite. While LDPs demand a short night or one that is interrupted by light, SDPs hate this to the point of being sterile Flower Induction long-day plants typically flower in the northern hemisphere during late spring or early summer as days are getting longer (longest day of the year is 21 June (solstice). After that days grow shorter (i.e. nights grow longer) until 21 December (solstice). This situation is reversed in the southern hemisphere (i.e. longest day is 21 December and shortest day is 21 June). Obligate LD plants are: •Carnation (Dianthus) •Henbane (Hyoscyamus) •Oat (Avena) •Ryegrass (Lolium) •Clover (Trifolium) •Bellflower (Campanula carpatica) •Oat, Ryegrass •Spinach •Dill, clover Some long-day facultative plants are: •Pea (Pisum sativum) LDP are de facto •Barley (Hordeum vulgare) short night plants Î •Arabidopsis thaliana (model organism) do not like a long dark period Flower Induction Short-day plants flower when the night is longer than a critical length. They require a consolidated period of darkness before floral development can begin. short-day plants typically flower in the Southern hemisphere during late summer or fall as days are getting shorter (shortest day down under is 21 June, longest day is 21 December). Secondly, they originate form regions where cold is not crucial but drought and rain are Short-day plants •Chrysanthemum •Coffee •Poinsettia •Strawberry •Tobacco, var. Maryland Mammouth •Common duckweed, (Lemna minor) •Cocklebur (Xanthium) •Cosmos •Violet Some short-day facultative plants are: •Hemp (Cannabis) •Cotton (Gossypium) •Rice •Sugar cane Flower Induction Day-neutral plants do not initiate flowering based on photoperiodism at all; they flower regardless of the night length. They may initiate flowering after attaining a certain overall developmental stage or age, or in response to alternative environmental stimuli, such as vernalisation (a period of low temperature), thermoperiodism, age rather than to photoperiod Day-neutral plants Cucumbers roses tomatoes, Blue grass Potatoes Viburnum Azalea Flower types Where does the signal for flowering come from? Experiments show clearly that the signal for photoperiodic stimulation comes from the leaves and transfers from there to the shoot apical meristem. What is the photoreceptor? Where does the signal for flowering come from? The photoreceptor for the production of the flower stimulus in the leaves is a pigment absorbing red light between 600 & 700 nm Î phytochrome What is the nature of the flower signal? Grafting experiments by Chailakhyan (1936) showed clearly that the signal was a chemical substance that he called Î florigen = anthesin + gibberellin. This substance was active & hence identical in LDP, SDP & day-neutral plants Flower Induction Flowering is the third stage of maturation of the shot apical meristem. Flower-promoting substance or signal is moving from one plant Hyanzinth bulbs grown at home to to the draft partner. or in the green house do not stretch the peduncle but flower In addition to This substance/signal is close to the bulb. They require photoperiodism (1), transported by the cold to grow like this Î thermoperiodism there phloem from the leaves is also the effects of of the flower-inducted vernalization (2, cold) plant to the one that is and Giberellic acid (3) not. that stimulate flowering Flower Induction Florigen is coming of age after 70 years Jan Zeevart in the Plant Cell 18: 1783-1789 (2006) Julius Sachs (1865) developed concept of a “flower hormone” produced by … …….. … illuminated leaves of Tropaeolum majus going to dark-kept SAM Garner & Allard 1920 Flowering depends on day length Î Photoperiodism Knott 1934 Day length is perceived by leaves while flower formation occurs in . the stem implies long-distance transport of hormone Chailakhyan 1936 introduced “florigen” = Anthesin + Gibberellin after grafting . between LDP and SDP Î florigen is universal in plants 1980ies so-called molecular genetics approach of studying mutant plants . .. with deficiencies in flowering ignorant of “florigen” concept 2000s network of 4 pathways controlling flowering in Arabidopsis . involve photoperiod, gibberellins, Vernalization & internal stimuli An et al 2004 speculated that the Flowering Locus T gene or protein (FT) might . be a mobile signal Huang et al. 2005 FT mRNA is the long-sought florigen was considered .. .. … #3 breakthrough of 1905 by journal Science Bohlenius et al 2006 Constans CO protein is mediating between leaf-perceived ……… . shortening of day length and stem-located induction of bud dormancy Other Photoperiodic Effects
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