EFB530 Plant Physiology

Phytochrome, daylength, and circadian rhythms

Many plant activities vary in a cyclic fashion, often over a 24 hr period, to be entrained with the light:dark cycle=circadian rhythm

• circadian rhythms are not simply responses to light and dark, since the cycle of variation in activity continues when light is switched to continuous dark (or sometimes continuous light) and the regulation can 'anticipate' the onset of the light period
• activities that vary in a circadian rhythm include:

stomatal opening, photosynthesis gene expression (especially light harvesting complex (LHC) genes, nyctinasty leaf movements, petal movements

The circadian rhythm is maintained by an internal clock (also called the oscillator)

• in Arabidopsis, the genes TOC1, LHY, and CCA1 are key to oscillator function

Phytochrome can reset circadian rhythms=entraining the oscillator

• entrainment assures that the rhythm is in tune with the light-dark cycle
• a pulse of red light can reset some diurnal cycles (acts as a zeitgeber)
• there is different sensitivities to light at different periods of the cycle

1) Maturity- Many plants must make a switch from juvenile to mature phase before they are competent to flower

2) Photoperiodism- Daylength is a major determinant of floral induction in some plants

• gives plants to synchronize their development/reproduction with the season
• flowering is a switch from vegetative shoot meristem to a floral meristem

Measurement of "daylength" is actually a measurement of nightlength

• some plants require short days to flower (actually long nights), some require long days (actually short nights), others are day neutral
• the length of night necessary to induce flowering is the critical night length
• the appropriate daylength (actually nightlength) conditions are said to be inductive
• some plants are induced by a single treatment, others require many treatments
• different plants have more or less rigid critical periods
• detection of daylength can occur in a single leaf
• measurement of daylength = photoperiodism

Phytochrome is involved in measuring the length of the night

• night can be interrupted by a brief pulse of R light = night break
• this night break is reversed by FR light
• therefore in general, Pfr inhibits flowering in SDP; Pfr promotes flowering in LDP

The time when the R light pulse is given is important

• the sensitivity of plants to the pulse is overlayed on a circadian rhythm
• plants are most sensitive in the middle of the night period, even over an extended continuous dark

3) Temperature

• many plants require vernalization before flowering
• some duration of exposure to cold (1 - 10C), either of imbibed seeds or of the whole plant

4) GA

• gibberellins can also induce flowering in many plants
• application of GA to Long-day, rosette plants (i.e. cabbage, spinach) can induce bolting even in short days
• application of GA to some conifers induces flowering (premature transition to mature phase)

Flowering can require proper integration of many regulators: photoperiod, temp, and GA

There appears to be a mobile floral inducer=florigen

• uninduced plants can be grafted onto induced plants, they are induced to flower
• this presumed mobile signal was named florigen and was studied for many years by Jan Zeevaart and Anton Lang, but still remains to be identified and isolated

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