Measuring Day Length
Plants measure NIGHT length โ not day length
Photoperiodism is the response of plants (and other organisms) to the relative lengths of light and dark periods. It was first described by Garner and Allard in 1920 working with Maryland Mammoth tobacco (a giant variety that only flowered in winter despite growing all summer) and soybeans.
Critical insight: plants actually measure the length of the DARK period (night), not the light period (day). This was proved by interrupting the night period with a brief flash of light โ a night break that was too short to be a real day but was enough to reset the dark measurement. Night breaks fool short-day plants into 'thinking' the night is shorter than it actually is โ preventing their flowering even on otherwise appropriate photoperiods.
๐ก Florigen โ The Flower-Promoting Signal
The question of how photoperiod information (detected in leaves) travels to the shoot apex (where flowers form) puzzled botanists for decades. The signal was hypothesized as a transmissible 'florigen' by Chailakhyan in 1937 โ but its identity remained unknown for 70 years.
In 2005โ2007, the molecular identity of florigen was revealed: FLOWERING LOCUS T (FT) protein in Arabidopsis. The photoperiod signal (detected by phytochrome and cryptochrome in leaves) activates transcription of the FT gene in leaf phloem companion cells โ FT protein is loaded into phloem โ moves from leaves to the shoot apex โ at the apex, FT interacts with another transcription factor (FD) โ activates floral identity genes (APETALA1, LEAFY) โ shoot apical meristem transitions from vegetative to floral identity โ flowers form.
Graft experiments confirm florigen: a grafted SDP induced to flower (by long nights) can promote flowering in an attached LDP kept in non-inductive conditions โ showing that a transmissible protein signal (FT) crosses the graft union. This was predicted by Chailakhyan's original hypothesis 70 years before the molecular proof.
SD
Short-day plants โ flower when nights are long
Short-day plants (SDPs) flower when the night length EXCEEDS their critical night length โ in practice, this means they flower in late summer, fall, and winter (when nights are long). Despite the name, what actually matters is the long night โ a short-day plant is really a 'long-night plant.'
Examples: chrysanthemums (flower in fall), poinsettias (red bracts appear in response to long nights โ commercially induced by keeping plants in darkness for 14+ hours), soybeans (day-length sensitive cultivars flower in response to shortening days of late summer), cocklebur (Xanthium โ used in classic photoperiodism experiments, extremely sensitive: one long night is sufficient to induce flowering), strawberries (flower in short days of spring in some varieties).
If you interrupt a long night with even a brief flash of red light (night break), SDPs revert to vegetive growth โ the dark measurement is reset. This is used commercially to prevent poinsettias from flowering prematurely.
Memory trick: Short-Day Plants = Long Nights needed. Chrysanthemums, poinsettias, soybeans, cocklebur. Flower in fall/winter. Night break (brief red light) PREVENTS flowering. 'Short day = short day, but really = LONG NIGHT.'
LD
Long-day plants โ flower when nights are short
Long-day plants (LDPs) flower when the night length is SHORTER than their critical night length โ in practice, they flower in late spring and summer when days are long and nights are short. Again, what matters is the short night.
Examples: spinach (bolts and flowers in long days of summer), wheat and barley (many varieties are long-day plants and flower in early summer), iris, henbane (Hyoscyamus niger โ classic long-day plant used in research), Arabidopsis thaliana (the model plant for molecular biology โ flowers in long days).
Night breaks PROMOTE LDP flowering โ because a brief light interruption makes the dark period seem shorter, and LDPs flower when nights are short. This is the opposite of what a night break does to SDPs.
LDPs in greenhouses can be induced to flower year-round by providing supplemental night-break lighting โ extending the effective light period to keep nights below the critical length.
Memory trick: Long-Day Plants = Short Nights needed. Spinach, wheat, Arabidopsis. Flower in summer. Night break PROMOTES flowering (makes night seem shorter). Bolt = rapid stem elongation before flowering in LDPs.
DN
Day-neutral plants and phytochrome
Day-neutral plants (DNPs) do not respond to photoperiod for flowering โ they flower based on developmental age or other factors. Examples: tomatoes, cucumbers, roses, dandelions, many tropical plants. These plants flower year-round regardless of day length.
Phytochrome โ the molecular photoreceptor for photoperiodism: Phytochrome is a plant photoreceptor protein that exists in two interconvertible forms: Pr (red-absorbing, inactive, ฮปmax ~660 nm) and Pfr (far-red-absorbing, active, ฮปmax ~730 nm). Sunlight (which contains more red than far-red) converts Pr โ Pfr during the day. In darkness, Pfr slowly reverts to Pr (dark reversion). The ratio of Pfr:Pr is the indicator of day vs night: high Pfr:Pr = day; low Pfr:Pr (after long dark period) = night.
Night break mechanism: brief red light during the night โ Pr converted to Pfr โ plant 'resets' to daytime measurement โ short-day plants cannot accumulate the required Pfr:Pr drop โ no flowering signal sent. Brief far-red light after the night break โ Pfr converted back to Pr โ reverses the night break โ SDP flowering can proceed.
Memory trick: Phytochrome: Pr + Red light โ Pfr (active). Pfr + Far-red (or darkness/time) โ Pr. Day = Pfr high. Long night = Pfr falls to Pr. Short-day plants flower when Pfr is LOW (long night). Night break = restore Pfr = reset the clock.
๐ฌ Applied Scenario โ Photoperiodism in Commercial Horticulture
Controlling photoperiod allows year-round production of seasonally flowering plants:
A
Poinsettias for Christmas. Poinsettias (Euphorbia pulcherrima) are short-day plants โ they develop their colorful red bracts (modified leaves surrounding the small yellow true flowers) in response to long nights (>12 hours). Commercial growers begin applying long-night treatments in late September/early October โ covering plants with black cloth from 5 PM to 8 AM โ to ensure red coloration by late November for holiday sales. Any light leakage during the dark period will prevent bract coloration (night break effect).
B
Chrysanthemums year-round. Chrysanthemums are short-day plants that naturally flower in fall. Commercial greenhouses produce chrysanthemums year-round by: (1) Preventing unwanted summer flowering using night-break lighting (brief light interruption during the night to prevent the continuous long dark period SDPs require) and (2) Inducing flowering at any time by providing artificially long nights (black cloth treatments) regardless of season. This allows year-round supply of a traditionally fall flower.
C
Supplemental lighting for long-day crops. Long-day vegetables and flowers (spinach, lettuce, carnations, petunia) are encouraged to flower and bolt in winter greenhouses by providing night-break lighting โ even brief flashes of low-intensity light during the night period prevent the dark period from exceeding the critical night length, keeping the plants in a 'long-day' state and promoting bolting and flowering year-round.
D
Soybean breeding and photoperiod sensitivity. Soybean varieties (Glycine max) differ dramatically in their critical photoperiod โ this determines where in the world each variety can be grown. Varieties adapted to northern latitudes (long days of summer โ short nights) flower appropriately in summer. The same variety grown in the tropics (where nights are longer year-round) would flower prematurely as a tiny plant. Plant breeders develop photoperiod-insensitive (day-neutral) soybean varieties for tropical regions where photoperiod-sensitive varieties fail.
๐ Exam Application
1. Short-day plants: Flower when NIGHTS exceed critical night length. Long nights = late summer/fall/winter. Chrysanthemum, poinsettia, soybean, cocklebur. Night break PREVENTS flowering (red light resets dark measurement).
2. Long-day plants: Flower when NIGHTS are shorter than critical night length. Short nights = spring/summer. Spinach, wheat, Arabidopsis, iris. Night break PROMOTES flowering.
3. Day-neutral: Photoperiod doesn't control flowering. Tomato, cucumber, rose, dandelion. Flower based on developmental age.
4. Phytochrome: Pr (red light โ Pfr, active). Pfr (dark/far-red โ Pr). Day = high Pfr. Long night = Pfr converts to Pr. SDPs measure how long Pfr stays low.
5. Florigen = FT protein: Made in leaves โ travels in phloem to shoot apex โ triggers flowering. Confirmed by graft experiments.
โ ๏ธ Most Common Photoperiodism Mistakes
Plants measure NIGHT length โ not day length. Despite being called 'short-day' and 'long-day' plants, what is actually measured is the length of the uninterrupted dark period (night). A short-day plant is really a 'long-night plant' โ it needs LONG nights. A long-day plant needs SHORT nights. The definitive proof: interrupting the night with a brief flash of light (night break) can promote LDP flowering and prevent SDP flowering โ manipulating night length, not day length, is what controls the response.
Night break prevents SDP flowering and promotes LDP flowering โ not vice versa. A brief light flash during the night makes the night seem 'shorter' to the plant. Shorter night = bad for SDPs (they need long nights) = prevents their flowering. Shorter night = good for LDPs (they need short nights) = promotes their flowering. Students frequently reverse which group is helped and which is prevented by a night break.
Pr is the INACTIVE form; Pfr is the ACTIVE form of phytochrome. Red light converts Pr (inactive) โ Pfr (active). Far-red light or prolonged darkness converts Pfr back to Pr (inactive). The active form (Pfr) suppresses SDP flowering and promotes LDP flowering. During a long night, Pfr slowly reverts to Pr in darkness โ when Pfr levels fall below a threshold, the SDP receives the 'long night' signal and can proceed to flower.
โ Quick Self-Test
1. What is the difference between short-day and long-day plants in terms of what they actually measure?
2. How does a night break affect short-day plants vs long-day plants?
3. What are the two forms of phytochrome and how do they interconvert?
4. What is florigen and how was its molecular identity discovered?
5. Give two commercial applications of photoperiodism control in horticulture.
Answers:
1. Both short-day plants (SDPs) and long-day plants (LDPs) actually measure the length of the uninterrupted dark period (night), not the light period. SDPs flower when the night length EXCEEDS their critical night length (they need long nights โ they are 'long-night plants' despite the name). LDPs flower when the night length is SHORTER than their critical night length (they need short nights). The historical naming based on 'day' length is misleading โ the night length is what both types measure.
2. A night break (brief flash of light interrupting the middle of the night) effectively makes the plant perceive the night as shorter than it actually is. For short-day plants (which need long nights): a night break PREVENTS flowering โ the required long dark period is interrupted, so the plant does not receive the long-night signal needed for flowering. For long-day plants (which need short nights): a night break PROMOTES flowering โ interrupting the night makes it seem shorter, satisfying the LDP's requirement for a night below its critical length.
3. Phytochrome exists in two interconvertible forms: Pr (the red-light-absorbing, inactive form, ฮปmax ~660 nm) and Pfr (the far-red-light-absorbing, active form, ฮปmax ~730 nm). Interconversion: Pr + red light โ Pfr (active). Pfr + far-red light โ Pr (inactive). Pfr also slowly reverts to Pr in darkness (dark reversion). During the day, sunlight (which contains more red than far-red) drives Pr โ Pfr, keeping Pfr levels high. During the night, Pfr gradually reverts to Pr โ the longer the night, the more completely Pfr is converted to Pr.
4. Florigen is the transmissible flower-promoting signal produced in leaves in response to inductive photoperiods and transported to the shoot apex to trigger flowering. Hypothesized by Chailakhyan in 1937 based on grafting experiments. Molecular identity revealed 2005โ2007: FLOWERING LOCUS T (FT) protein in Arabidopsis. Under inductive photoperiod, phytochrome and cryptochrome signals activate FT gene expression in leaf phloem companion cells โ FT protein enters phloem โ travels to shoot apex โ interacts with FD transcription factor โ activates floral meristem identity genes (LEAFY, APETALA1) โ vegetative meristem converts to floral meristem โ flowers form.
5. Two commercial applications: (1) Poinsettia production for Christmas โ SDPs treated with 14+ hours of darkness per day starting in early October to ensure red bract development by late November; black cloth applied from 5 PM to 8 AM daily. (2) Year-round chrysanthemum production โ SDPs prevented from summer flowering by night-break lighting; induced to flower any month by providing artificial long-night (black cloth) treatments. Also acceptable: supplemental lighting for LDP spinach/lettuce in winter greenhouses.