Photoperiodism#Plants

{{Short description|Responses of organisms to the relative lengths of light and dark periods}}

Photoperiod is the change of day length around the seasons. The rotation of the earth around its axis produces 24 hour changes in light (day) and dark (night) cycles on earth. The length of the light and dark in each phase varies across the seasons due to the tilt of the earth around its axis. The photoperiod defines the length of the light, for example a summer day the length of light could be 16 hours while the dark is 8 hours, whereas a winter day the length of day could be 8 hours, whereas the dark is 16 hours. Importantly, the seasons are different in the northern hemisphere than the southern hemisphere.

Photoperiodism is the physiological reaction of organisms to the length of light or a dark period. It occurs in plants and animals. Plant photoperiodism can also be defined as the developmental responses of plants to the relative lengths of light and dark periods. They are classified under three groups according to the photoperiods: short-day plants, long-day plants, and day-neutral plants.

In animals photoperiodism (sometimes called seasonality) is the suite of physiological changes that occur in response to changes in day length. This allows animals to respond to a temporally changing environment associated with changing seasons as the earth orbits the sun.

== Plants ==

File:Photoperiodism in plants.jpg

In 1920, W. W. Garner and H. A. Allard published their discoveries on photoperiodism and felt it was the length of daylight that was critical,{{cite book |title=Botany for Gardeners |vauthors=Capon B |publisher=Timber Publishing |year=2005 |isbn=978-0-88192-655-2 |edition=2nd |location=Portland, OR |pages=148–51}} but it was later discovered that the length of the night was the controlling factor.{{cite journal |vauthors=Hamner KC, Bonner J |year=1938 |title=Photoperiodism in relation to hormones as factors in floral initiation and development |url=https://authors.library.caltech.edu/102478/1/2471641.pdf |journal=Botanical Gazette |volume=100 |issue=2 |pages=388–431 |doi=10.1086/334793 |jstor=2471641 |s2cid=84084837}}{{cite journal |vauthors=Hamner KC |year=1940 |title=Interrelation of light and darkness in photoperiodic induction |journal=Botanical Gazette |volume=101 |issue=3 |pages=658–87 |doi=10.1086/334903 |jstor=2472399 |s2cid=83682483}} Photoperiodic flowering plants are classified as long-day plants or short-day plants even though night is the critical factor because of the initial misunderstanding about daylight being the controlling factor. Along with long-day plants and short-day plants, there are plants that fall into a "dual-day length category". These plants are either long-short-day plants (LSDP) or short-long-day plants (SLDP). LSDPs flower after a series of long days followed by short days whereas SLDPs flower after a series of short days followed by long days.{{cite book |title=Plant Physiology and Development |vauthors=Taiz L, Zeiger E, Møller I, Murphy A |date=2015 |publisher=Sinauer Associates, Inc. |isbn=978-1-60535-353-1 |edition=Sixth |location=Sunderland, MA}} Each plant has a different length critical photoperiod, or critical night length.

Many flowering plants (angiosperms) use a circadian rhythm together with photoreceptor protein, such as phytochrome or cryptochrome,{{cite book| vauthors = Mauseth JD |title=Botany : An Introduction to Plant Biology |edition=3rd|year=2003|isbn=978-0-7637-2134-3|publisher= Jones and Bartlett Learning|location=Sudbury, MA|pages=422–27}} to sense seasonal changes in night length, or photoperiod, which they take as signals to flower. In a further subdivision, obligate photoperiodic plants absolutely require a long or short enough night before flowering, whereas facultative photoperiodic plants are more likely to flower under one condition.

Phytochrome comes in two forms: P_r and P_fr. Red light (which is present during the day) converts phytochrome to its active form (P_fr) which then stimulates various processes such as germination, flowering or branching. In comparison, plants receive more far-red in the shade, and this converts phytochrome from P_fr to its inactive form, P_r, inhibiting germination. This system of P_fr to P_r conversion allows the plant to sense when it is night and when it is day.{{cite journal | vauthors = Fankhauser C | title = The phytochromes, a family of red/far-red absorbing photoreceptors | journal = The Journal of Biological Chemistry | volume = 276 | issue = 15 | pages = 11453–11456 | date = April 2001 | pmid = 11279228 | doi = 10.1074/jbc.R100006200 | doi-access = free }} P_fr can also be converted back to P_r by a process known as dark reversion, where long periods of darkness trigger the conversion of P_fr.{{cite journal | vauthors = Casal JJ, Candia AN, Sellaro R | title = Light perception and signalling by phytochrome A | journal = Journal of Experimental Botany | volume = 65 | issue = 11 | pages = 2835–2845 | date = June 2014 | pmid = 24220656 | doi = 10.1093/jxb/ert379 | doi-access = free | hdl = 11336/4338 | hdl-access = free }} This is important in regards to plant flowering. Experiments by Halliday et al. showed that manipulations of the red-to far-red ratio in Arabidopsis can alter flowering. They discovered that plants tend to flower later when exposed to more red light, proving that red light is inhibitory to flowering.{{cite journal | vauthors = Lin C | title = Photoreceptors and regulation of flowering time | journal = Plant Physiology | volume = 123 | issue = 1 | pages = 39–50 | date = May 2000 | pmid = 10806223 | pmc = 1539253 | doi = 10.1104/pp.123.1.39 }} Other experiments have proven this by exposing plants to extra red-light in the middle of the night. A short-day plant will not flower if light is turned on for a few minutes in the middle of the night and a long-day plant can flower if exposed to more red-light in the middle of the night.{{cite book |title=What A Plant Knows| vauthors = Chamovitz D |publisher=Scientific American |year=2013 |isbn=978-0-374-28873-0 |pages=17–18}}

Cryptochromes are another type of photoreceptor that is important in photoperiodism. Cryptochromes absorb blue light and UV-A. Cryptochromes entrain the circadian clock to light.{{cite journal | vauthors = Lin C, Todo T | title = The cryptochromes | journal = Genome Biology | volume = 6 | issue = 5 | pages = 220 | date = 2005 | pmid = 15892880 | pmc = 1175950 | doi = 10.1186/gb-2005-6-5-220 | doi-access = free }} It has been found that both cryptochrome and phytochrome abundance relies on light and the amount of cryptochrome can change depending on day-length. This shows how important both of the photoreceptors are in regards to determining day-length.{{cite journal | vauthors = Mockler T, Yang H, Yu X, Parikh D, Cheng YC, Dolan S, Lin C | title = Regulation of photoperiodic flowering by Arabidopsis photoreceptors | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 4 | pages = 2140–2145 | date = February 2003 | pmid = 12578985 | pmc = 149972 | doi = 10.1073/pnas.0437826100 | doi-access = free | bibcode = 2003PNAS..100.2140M }}

Modern biologists believe{{cite journal | vauthors = Andrés F, Galbraith DW, Talón M, Domingo C | title = Analysis of PHOTOPERIOD SENSITIVITY5 sheds light on the role of phytochromes in photoperiodic flowering in rice | journal = Plant Physiology | volume = 151 | issue = 2 | pages = 681–690 | date = October 2009 | pmid = 19675157 | pmc = 2754645 | doi = 10.1104/pp.109.139097 }} that it is the coincidence of the active forms of phytochrome or cryptochrome, created by light during the daytime, with the rhythms of the circadian clock that allows plants to measure the length of the night. Other than flowering, photoperiodism in plants includes the growth of stems or roots during certain seasons and the loss of leaves. Artificial lighting can be used to induce extra-long days.

= Long-day plants =

File:Long Day Plants.jpg

Long-day plants flower when the night length falls below their critical photoperiod.{{cite book|page=517|title=Plant Structure and Function| vauthors = Starr C, Taggart R, Evers C, Starr L |volume=4|edition=13th|publisher=Brooks/Cole|isbn=978-1-111-58068-1|year=2013}} These plants typically flower during late spring or early summer as days are getting longer. In the northern hemisphere, the longest day of the year (summer solstice) is on or about 21 June.{{cite book| vauthors = Gooley T |author-link=Tristan Gooley|title=The Natural Navigator|publisher=Random House|isbn=978-0-7535-2311-7|date=2010-03-30}} After that date, days grow shorter (i.e. nights grow longer) until 21 December (the winter solstice). This situation is reversed in the southern hemisphere (i.e., longest day is 21 December and shortest day is 21 June).

Some long-day obligate plants are:

Carnation (Dianthus)
Henbane (Hyoscyamus)
Oat (Avena)

Some long-day facultative plants are:

Pea (''Pisum sativum)
Barley (Hordeum vulgare)
Lettuce (Lactuca sativa)
Wheat (Triticum aestivum)

= Short-day plants =

File:Short Day Plants.jpg

Short-day (also called long-night) plants flower when the night lengths exceed their critical photoperiod.{{cite book|title=BSCS Biology |publisher=BSCS |edition=9 |page=519 |isbn=978-0-7872-9008-5 |year=2002}} They cannot flower under short nights or if a pulse of artificial light is shone on the plant for several minutes during the night; they require a continuous period of darkness before floral development can begin. Natural nighttime light, such as moonlight or lightning, is not of sufficient brightness or duration to interrupt flowering.

Short-day plants flower as days grow shorter (and nights grow longer) after September 21st in the northern hemisphere, which is during summer or fall. The length of the dark period required to induce flowering differs among species and varieties of a species.

Photoperiodism affects flowering by inducing the shoot to produce floral buds instead of leaves and lateral buds.

Some short-day facultative plants are:{{cite book |vauthors=Jones HG |title=Plants and Microclimate: A Quantitative Approach to Environmental Plant Physiology |page=[https://archive.org/details/plantsmicroclima0000jone/page/225 225] |isbn=978-0-521-42524-7 |publisher=Cambridge University Press |year=1992 |url=https://archive.org/details/plantsmicroclima0000jone/page/225}}

Kenaf ( Hibiscus cannabinus)
Marijuana (Cannabis)
Cotton (Gossypium)
Rice (Oryza)
Sorghum (Sorghum bicolor)
Green gram (Mung bean, Vigna radiata)
Soybeans{{cite book |vauthors=Purcell LC, Salmeron M, Ashlock L |title=Arkansas Soybean Production Handbook - MP197 |date=2014 |chapter=Chapter 2 |chapter-url=http://www.uaex.edu/publications/pdf/mp197/chapter2.pdf |publisher=University of Arkansas Cooperative Extension Service |location=Little Rock, Arkansas |pages=5–7 |url=http://www.uaex.edu/publications/mp-197.aspx |access-date=21 February 2016}} (Glycine max)

= Day-neutral plants =

Day-neutral plants, such as cucumbers, roses, tomatoes, and Ruderalis (autoflowering cannabis) do not initiate flowering based on photoperiodism.{{cite book| vauthors = Meneely P |title=Genetic Analysis: Genes, Genomes, and Networks in Eukaryotes|page=373|edition=2nd |publisher=Oxford University Press|year=2014|isbn=978-0-19-968126-6}} Instead, 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).

Animals

Daylength, and thus knowledge of the season of the year, is vital to many animals. A number of biological and behavioural changes are dependent on this knowledge. Together with temperature changes, photoperiod provokes changes in the color of fur and feathers, migration, entry into hibernation, sexual behaviour, and even the resizing of organs.

In insects, sensitivity to photoperiod has been proven to be initiated by photoreceptors located in the brain.{{Cite journal | vauthors = Claret J |date=1966 |title=Recherche du centre photorecepteur lors de l'induction de la diapause chez Pieris brassicae L. |journal=Comptes Rendus de l'Académie des Sciences |volume=262 |pages=553–556}}{{cite journal | vauthors = Bowen MF, Saunders DS, Bollenbacher WE, Gilbert LI | title = In vitro reprogramming of the photoperiodic clock in an insect brain-retrocerebral complex | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 81 | issue = 18 | pages = 5881–4 | date = September 1984 | pmid = 6592591 | pmc = 391816 | doi = 10.1073/pnas.81.18.5881 | bibcode = 1984PNAS...81.5881B | doi-access = free }} Photoperiod can affect insects at different life stages, serving as an environmental cue for physiological processes such as diapause induction and termination, and seasonal morphs.{{Cite journal | vauthors = Saunders DS |date=2012 |title=Insect photoperiodism: seeing the light |journal=Physiological Entomology |language=en |volume=37 |issue=3 |pages=207–218 |doi=10.1111/j.1365-3032.2012.00837.x|s2cid=85249708 |doi-access=free }} In the water strider Aquarius paludum, for instance, photoperiod conditions during nymphal development have been shown to trigger seasonal changes in wing frequency and also induce diapause, although the threshold critical day lengths for the determination of both traits diverged by about an hour.{{Cite journal | vauthors = Harada T, Numata H |date=1993 |title=Two Critical Day Lengths for the Determination of Wing Forms and the Induction of Adult Diapause in the Water Strider, Aquarius paludum |journal=Naturwissenschaften |volume=80 |issue=9 |pages=430–432|doi=10.1007/BF01168342 |bibcode=1993NW.....80..430H |s2cid=39616943 }} In Gerris buenoi, another water strider species, photoperiod has also been shown to be the cause of wing polyphenism,{{cite journal | vauthors = Gudmunds E, Narayanan S, Lachivier E, Duchemin M, Khila A, Husby A | title = Photoperiod controls wing polyphenism in a water strider independently of insulin receptor signalling | journal = Proceedings. Biological Sciences | volume = 289 | issue = 1973 | pages = 20212764 | date = April 2022 | pmid = 35473377 | pmc = 9043737 | doi = 10.1098/rspb.2021.2764 }} although the specific daylengths changed between species, suggesting that phenotypic plasticity in response to photoperiod has evolved even between relatively closely related species.

The singing frequency of birds such as the canary depends on the photoperiod. In the spring, when the photoperiod increases (more daylight), the male canary's testes grow. As the testes grow, more androgens are secreted and song frequency increases. During autumn, when the photoperiod decreases (less daylight), the male canary's testes regress and androgen levels drop dramatically, resulting in decreased singing frequency. Not only is singing frequency dependent on the photoperiod but the song repertoire is also. The long photoperiod of spring results in a greater song repertoire. Autumn's shorter photoperiod results in a reduction in song repertoire. These behavioral photoperiod changes in male canaries are caused by changes in the song center of the brain. As the photoperiod increases, the high vocal center (HVC) and the robust nucleus of the archistriatum (RA) increase in size. When the photoperiod decreases, these areas of the brain regress.{{cite book | vauthors = Nelson RJ | date = 2005 | title = An Introduction to Behavioral Endocrinology | page = 189 | location = Sunderland, MA | publisher = Sinauer Associates }}

Mammals

In mammals, daylength is registered in the suprachiasmatic nucleus (SCN), which is informed by retinal light-sensitive ganglion cells, which are not involved in vision. The information travels through the retinohypothalamic tract (RHT). In most species the hormone melatonin is produced by the pineal gland only during the hours of darkness, influenced by the light input through the RHT and by innate circadian rhythms. This hormonal signal, combined with outputs from the SCN inform the rest of the body about the time of day, and the length of time that melatonin is secreted is how the time of year is perceived.

Many mammals, particularly those inhabiting temperate and polar regions, exhibit a remarkable degree of seasonality in response to changes in daylight hours(photoperiod). This seasonality manifests in a broad spectrum of behaviors and physiology, including hibernation, seasonal migrations, and coat color changes. A prime example of the adaptation to photoperiods is the seasonal coat color (SCC) species.{{Cite journal |last1=Zimova |first1=Marketa |last2=Hackländer |first2=Klaus |last3=Good |first3=Jeffrey M. |last4=Melo-Ferreira |first4=José |last5=Alves |first5=Paulo Célio |last6=Mills |first6=L. Scott |date=August 2018 |title=Function and underlying mechanisms of seasonal colour moulting in mammals and birds: what keeps them changing in a warming world? |url=https://onlinelibrary.wiley.com/doi/10.1111/brv.12405 |journal=Biological Reviews |language=en |volume=93 |issue=3 |pages=1478–1498 |doi=10.1111/brv.12405 |pmid=29504224 |issn=1464-7931|hdl=10216/118423 |hdl-access=free }} These animals undergo molting, transforming from dark summer fur to white coat in winter, that provides crucial camouflage in snowy environments.

= Humans =

The view has been expressed that humans' seasonality is largely believed to be evolutionary baggage.{{cite web |url= http://www.abc.net.au/rn/scienceshow/stories/2009/2762261.htm |title=Extra-retinal photo receptors |access-date=2010-05-28 | vauthors = Foster R, Williams R |date=5 December 2009 |format=Interview |work=Science Show |publisher=ABC Radio National |quote=...we have the evolutionary baggage of showing seasonality but we're not entirely sure what the mechanism is.}}{{relevance inline|date=April 2018}}. Human birth rate varies throughout the year, and the peak month of births appears to vary by latitude.{{Cite journal |last1=Martinez-Bakker |first1=Micaela |last2=Bakker |first2=Kevin M. |last3=King |first3=Aaron A. |last4=Rohani |first4=Pejman |date=2014-05-22 |title=Human birth seasonality: latitudinal gradient and interplay with childhood disease dynamics |journal=Proceedings of the Royal Society B: Biological Sciences |language=en |volume=281 |issue=1783 |pages=20132438 |doi=10.1098/rspb.2013.2438 |issn=0962-8452 |pmc=3996592 |pmid=24695423}} Seasonality in human birth rate appears to have largely decreased since the industrial revolution.{{Cite journal |last=Wehr |first=Thomas A. |date=August 2001 |title=Photoperiodism in Humans and Other Primates: Evidence and Implications |url=http://journals.sagepub.com/doi/10.1177/074873001129002060 |journal=Journal of Biological Rhythms |language=en |volume=16 |issue=4 |pages=348–364 |doi=10.1177/074873001129002060 |pmid=11506380 |s2cid=25886221 |issn=0748-7304|url-access=subscription }}

Other organisms

Photoperiodism has also been demonstrated in other organisms besides plants and animals. The fungus Neurospora crassa as well as the dinoflagellate Lingulodinium polyedra and the unicellular green alga Chlamydomonas reinhardtii have been shown to display photoperiodic responses.Tan Y, Merrow M, Roenneberg T. Photoperiodism in Neurospora crassa. J Biol Rhythms. 2004 Apr;19(2):135-43. https://doi.org/10.1177/0748730404263015. PMID 15038853.

Suzuki, L., Johnson, C. Photoperiodic control of germination in the unicell Chlamydomonas. Naturwissenschaften 89, 214–220 (2002). https://doi.org/10.1007/s00114-002-0302-6

Ivonne Balzer, Rüdiger Hardeland ,Photoperiodism and Effects of Indoleamines in a Unicellular Alga, Gonyaulax polyedra.Science253,795-797(1991).https://doi.org/10.1126/science.1876838