Photoperiodism and vernalisation
How plants time flowering by day length and winter cold: short-day and long-day plants, phytochrome, vernalisation, and what it means for varieties.
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A crop that flowers at the wrong time fails, however well it is fed and watered. Flowering too early leaves a small plant with a small yield; too late, and frost, drought or the end of the rains catches the grain unfilled. Plants time flowering partly by reading two seasonal signals: the length of the day (photoperiodism) and a spell of winter cold (vernalisation).
These responses decide where a variety can be grown and when it must be sown, which is why they are core topics in plant physiology and crop production, and why insensitivity to day length was one of the keys to the Green Revolution.
Photoperiodism
In 1920 W. W. Garner and H. A. Allard, working in the United States, found that a large tobacco variety, Maryland Mammoth, flowered only when days became short, and that soybean varieties sown on different dates flowered at almost the same time of year. They called the response photoperiodism: the control of development by the relative length of day and night.
The response groups
| Group | Flowers when | Examples |
|---|---|---|
| Short-day plants (SDP) | days are shorter than a critical length (nights longer than a critical night) | many rice varieties, soybean, chrysanthemum, poinsettia, sugarcane, many tropical maize and sorghum types, cotton (wild types) |
| Long-day plants (LDP) | days are longer than a critical length | wheat, barley, oats, rye, spinach, lettuce, radish, sugar beet (bolting) |
| Day-neutral plants | largely regardless of day length, after reaching a certain size or age | tomato, cucumber, many modern varieties of beans, maize and rice |
Responses can be obligate (qualitative), where the plant will not flower at all under the wrong day length, or facultative (quantitative), where the right day length only speeds flowering.
It is really the night
Experiments in the 1930s and 1940s showed that plants measure the length of the dark period. A short-day plant kept under long nights will not flower if the night is interrupted by a brief flash of light (a night break); the same flash does not matter if given during the day. "Short-day plants" are therefore better described as long-night plants, and the critical value is a critical night length.
Phytochrome
The light signal is detected by phytochrome, a pigment that exists in two interconvertible forms:
- Pr, which absorbs red light (about 660 nm) and converts to Pfr;
- Pfr, which absorbs far-red light (about 730 nm) and converts back to Pr, and also slowly reverts to Pr in darkness.
In classic experiments by H. A. Borthwick, S. B. Hendricks and colleagues in the 1940s and 1950s, on flowering and on lettuce seed germination, red light had one effect and far-red light immediately afterwards reversed it: a red night break blocked flowering of short-day plants, and far-red cancelled the break. The last flash given decided the outcome. Plants combine phytochrome signals with their internal circadian clock to measure night length; blue-light receptors (cryptochromes) also contribute.
Where the signal goes
Day length is perceived by the leaves, but flowers form at the shoot tip. Grafting experiments showed that a transmissible signal, long called florigen, travels from induced leaves to the apex. In the 2000s it was identified as a protein, FLOWERING LOCUS T (FT), carried in the phloem.
Day length depends on latitude and season
How long is the longest day?
Day length (from sunrise to sunset, ignoring twilight) can be calculated from latitude φ and the sun's declination δ, which is about +23.44° at the June solstice:
cos H = − tan φ × tan δ, and day length = 2H ⁄ 15 hours (H in degrees).
- At 30° N on the June solstice: tan 30° × tan 23.44° = 0.577 × 0.434 = 0.250; H = arccos(−0.250) = 104.5°; day length = 2 × 104.5 ⁄ 15 ≈ 13.9 hours.
- At 50° N: 1.192 × 0.434 = 0.517; H = arccos(−0.517) ≈ 121.1°; day length ≈ 16.2 hours.
- At the equator: tan 0° = 0, H = 90°, day length = 12 hours all year.
This is why onion varieties are classed by the day length they need to form bulbs: short-day onions bulb at about 11 to 12 hours and suit low latitudes; long-day onions need about 14 hours or more and suit high latitudes. Grown at the wrong latitude, a short-day onion bulbs too early and small, and a long-day onion may never bulb at all.
Uses in agriculture
- Variety adaptation: photoperiod-sensitive rice varieties of the monsoon tropics flower as the days shorten at the end of the rains, whatever their sowing date, which protects them from flowering in floods or drought; photoperiod-insensitive varieties can be grown in any season with irrigation. The semi-dwarf rices and wheats of the Green Revolution were bred to be largely insensitive to day length, so one variety could be grown across many latitudes and seasons.
- Soybean maturity groups describe adaptation to latitude: groups suited to long summer days at high latitudes, and others to shorter days nearer the equator.
- Flower production: chrysanthemums and poinsettias are forced to flower for markets out of season by blacking out greenhouses to lengthen nights, or kept vegetative by night-break lighting.
- Potato tuber formation is promoted by short days in many traditional varieties.
Vernalisation
Many temperate plants flower only after experiencing a prolonged period of cold. Vernalisation (from Latin vernus, of spring) is this acquisition of the ability to flower through exposure to low temperature, typically about 0 to 10 °C for several weeks.
- Winter annuals, such as winter wheat, winter barley and winter rye, are sown in autumn, vernalised by winter, and flower in late spring. Sown in spring, an unvernalised winter variety stays vegetative or heads very late. Spring varieties have little or no requirement.
- Biennials, such as cabbage, carrot, beet, onion and celery, grow vegetatively in the first year, are vernalised over winter, and flower (bolt) in the second. Vegetable seed production depends on providing that cold.
- Vernalisation is perceived by the shoot apex, not the leaves, and only in actively growing tissue.
- It can be reversed by high temperature shortly afterwards (devernalisation).
- Many vernalised plants still need long days to flower, so the two responses work together.
In the model plant Arabidopsis, prolonged cold progressively silences the flowering repressor gene FLC through stable chromatin changes, which is why the plant "remembers" winter. In cereals, VRN genes control the requirement and are used in breeding spring and winter types.
Unwanted vernalisation causes premature bolting: onions, sugar beet, celery or cabbage planted too early and exposed to cold may flower in their first season instead of forming a crop.
Key points
- Photoperiodism: flowering controlled by day and night length (Garner and Allard, 1920).
- Short-day plants flower when nights exceed a critical length; long-day plants when days exceed a critical length; day-neutral plants largely ignore day length.
- Plants measure the night: a night break of red light prevents short-day flowering; far-red reverses it (phytochrome Pr ⇄ Pfr).
- Leaves perceive day length; the FT protein (florigen) moves to the shoot tip.
- Vernalisation: weeks of cold (about 0–10 °C) enable flowering in winter annuals and biennials; perceived by the shoot apex; can be reversed by heat.
- Applications: variety adaptation to latitude and season, photoperiod-insensitive varieties, flower forcing, onion types, and avoiding premature bolting.
What the exams ask
- School-leaving and entrance papers: examples of short-day, long-day and day-neutral plants, and the meaning of vernalisation with winter wheat as the example.
- Degree and research-fellowship entrances: critical night length, night-break and red–far-red experiments, phytochrome, florigen, the site of perception, devernalisation, and genes such as FLC and VRN.
- Crop production and horticulture examinations: choosing varieties for a latitude or season, photoperiod insensitivity, forcing ornamentals, onion bulbing, and bolting in vegetable crops.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why does a rice variety flower at the same time even when sown on different dates?
Photoperiod-sensitive varieties flower when the nights lengthen past their critical value, so sowing earlier mainly lengthens the vegetative period rather than moving flowering. Insensitive varieties flower after a fixed number of days instead.
Is vernalisation the same as seed dormancy being broken by cold?
No. Cold stratification breaks dormancy so that seeds germinate. Vernalisation acts on growing plants and makes them able to flower.
Why did Green Revolution varieties need to be day-length neutral?
So that a single variety could be grown in different seasons and at different latitudes, including in the dry season under irrigation, which made double and triple cropping possible.
Sources
- Garner, W. W. and Allard, H. A. "Effect of the relative length of day and night and other factors of the environment on growth and reproduction in plants." Journal of Agricultural Research 18, 553–606 (1920).
- Taiz, L., Zeiger, E., Møller, I. M. and Murphy, A. Plant Physiology and Development, 6th edition. Sinauer, 2015. Chapter on the control of flowering.
- Borthwick, H. A., Hendricks, S. B., Parker, M. W., Toole, E. H. and Toole, V. K. "A reversible photoreaction controlling seed germination." PNAS 38, 662–666 (1952).
- Corbesier, L. et al. "FT protein movement contributes to long-distance signaling in floral induction of Arabidopsis." Science 316, 1030–1033 (2007).
- Kim, D.-H., Doyle, M. R., Sung, S. and Amasino, R. M. "Vernalization: winter and the timing of flowering in plants." Annual Review of Cell and Developmental Biology 25, 277–299 (2009).
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