Biological clocks - Chronobiology

Erwin Bunning (1967) The physiological clock. Springer New York

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 reflectsphysiological 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, luminescenseof unicellular alga Gonyaulax, photosynthesis, abscission, flowering

Photoperiodism and Thermoperiodism

In nature animals and plants are well aware of circadian changes in illumination (light-dark light intensity, light quality 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

ThermoperiodismIn 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

Thermoperiodism PhotoperiodismVernalization 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 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 Botanica.

He may never have planted such a garden, but the idea was attempted by several botanical gardens in the early 19th century, with mixed success

3 a.m. Tragopogon pratensis Goat's-Beard 4-5 a.m. Cichorium intybus L. Chicory 5 a.m. - 12 p.m. Taraxacum officinale Dandelion 8-9 a.m. Goat's beard Convolvulus. Morning Glory 12 AM Lactuca sativa L. Garden Lettuce 1-3 p.m. Nymphaea alba L. White Waterlily3 p.m. Calendula officinalis L.Pot marigold 3-4 p.m. Alyssum alyssoides L. Papaver

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 InductionThe 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 InductionThe switching of the SAM form making vegetative shoots to flowers must be

really complicated, right? = wrong it needs only 3 genes to switch on the production of sepals, pistils and stamens

Flower Induction by day-length changes?

Depending on their geographic origin, plants flower either

(1) When days get longer = spring in Northern hemisphere Long-Day Plantsor

(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 Short-Day Plants

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

Later attempts with cuttings showed the MM could flower in December in a warm greenhouse

MM is a “Short Day Plant” ??? Other SDPs were found in chrysanthemum and Poinsettia

concept of photoperiodism developed for MM applied to other plants & later adapted to animals like hamsters & humans

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

Obligate LD plants are:•Carnation (Dianthus) •Henbane (Hyoscyamus) •Oat (Avena) •Ryegrass (Lolium) •Clover (Trifolium) •Bellflower (Campanula carpatica) •Oat, Ryegrass•Spinach•Dill, cloverSome long-day facultative plants are:•Pea (Pisum sativum) •Barley (Hordeum vulgare) •Arabidopsis thaliana (model organism)

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).

LDP are de facto short night plantsdo not like a long dark period

Flower Induction

Short-day plants•Chrysanthemum•Coffee•Poinsettia•Strawberry•Tobacco, var. Maryland Mammouth•Common duckweed, (Lemna minor) •Cocklebur (Xanthium) • Cosmos•VioletSome short-day facultative plants are:•Hemp (Cannabis) •Cotton (Gossypium) •Rice•Sugar cane

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 plantstypically 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

Flower Induction

Day-neutral plantsCucumbersroses

tomatoes, Blue grassPotatoesViburnumAzalea

Day-neutral plantsdo 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

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 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 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 to to the draft partner.

This substance/signal is transported by the phloem from the leaves of the flower-inducted plant to the one that is not.

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

In addition to photoperiodism (1), thermoperiodism there is also the effects of vernalization (2, cold) and Giberellic acid (3) 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 PhotoperiodismKnott 1934 Day length is perceived by leaves while flower formation occurs in . . . the stem implies long-distance transport of hormoneChailakhyan 1936 introduced “florigen” = Anthesin + Gibberellin after grafting . . . . between LDP and SDP florigen is universal in plants1980ies so-called molecular genetics approach of studying mutant plants . .. . with deficiencies in flowering ignorant of “florigen” concept2000s 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 signalHuang 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

In addition to photoperiodism (1), thermoperiodism there is also the effects of vernalization (2, cold) and Giberellic acid (3) that stimulate flowering

Photoperiodism of Pseudotsuga menziesii

Plants us photoreceptors as phytochrome or cryptochrometo sense seasonal changes in night length (photoperiod) to flower, grow, or drop their leaves & go dormant.

The coincidence of the active forms ofphytochrome or cryptochrome, created by light during the daytime, with the rhythm of the circadian clock allows plants to measure the length of the night.

Other than flowering,photoperiodism in plants includes

(1) the growth of stems or roots (2) the loss of leaves (leaf abscission) (3) dormancy of buds see

Other Photoperiodic Effects

Other Photoperiodic effects not associated with flowering is the leaf abscission occurring in the fall due to shortening of day length.

Leaf abscission is therefore a SD effect.

Other Photoperiodic Effects - animals

In addition to photoperiodism (1), thermoperiodism there is also the effects of vernalization (2, cold) and Giberellic acid (3) that stimulate flowering

Photoperiodism

At a latitude of 0 (i.e. equator) or the arctic you should not expect plants with PP. At a latitude of 40 N (Chicago) you should. Note that the altering day lengths is a particularity of our moderate climate zone

Basics of Photoperiods

LDPs requiring > 16 h day are at the limit because the onset of the cold would prevent successful fruiting & seeding. SDPs requiring > 12 h darkness are not adopted to the Northern climate since they flower later than September and that is no good.

Is flowering unhealthy for plants? In annuals and biennials as well as some perennials like agaves and bamboo it is flowering rather than age that causes sudden senescence and death of the individual plant. These plants cannot

Perennial shrubs & trees

Spinach is a dioecious plant meaning what? …………

It shows that the male plants with pistillate flowers and no fruit senesce and die as fast as unfertilized staminate plants

Comparison of flowering and fruiting plants shows that both undergo senescence and death so…..

Flowers provide the death signal no matter what. What chemical is it ?

The death of spinach Spinacia oleracea(amaranthaceae)