Multiple inductive pathways control the timing of flowering Long-day photoperiod Gibberellins (GA) Vernalization Autonomous pathway
Induction of flowering Multiple cues
Photoperiodism Duration of the Light Period As a result of seasonal changes in daylength, plants have evolved systems to ensure viability of seeds: - protection before winter - coincide with the rainy/dry seasons Photoperiodism - plant ability to detect and respond to day length
Photoperiodic Response Short Day Plant (SDP) - flower when the day length is less than the Critical Day Length Long Day Plant (LDP)- flower when the day length is greater than the Critical Day Length Day Neutral- flower without respect to day length
PHOTOPERIODIC CONTROL OF FLOWERING A short-day plant will not flower until it s exposed to a continuous dark period exceeding a critical length. For a long-day plant, flowering occurs when the night length is shorter than the critical length.
Photoperiodic Response
Photoperiodic Regulation Plants actually measuring NIGHT length That means that during short day periods of the year by interrupting or splitting a long night with a relatively short photoperiod the plant perceives a short night and a long day even though the natural day length has not changed
Classes of Photoperiodic Plants Obligate - plant that must absolutely meet the day length requirement to flower Facultative - plant that will flower under most photoperiods but will flower most readily when the photoperiodic requirement is met
PHYTOCHROME IS A LIGHT DETECTOR THAT MAY HELP SET THE BIOLOGICAL CLOCK How does a plant actually measure photoperiod? Photoreceptive pigments phytochromes are proteins with a light-absorbing component.
What Is Phytochrome? Phytochrome is a pigment found in some plant cells that has been proven to control plant development. This pigment has two forms or phases in can exist in. P-red light sensitive (Pr) and P far red light sensitive (Pfr) forms. The actual plant response is very specific to each specie, and some plants do not respond at all. R FR
La struttura del fitocromo Un dimero di 1200 aminoacidi con numerosi domini e 2 molecole di cromoforo. Cromofori 660 nm 730 nm Pr Pfr Si lega alle membrane
The chromophore group is a linear tetrapyrrole that differs in the conformation and absorption spectrum of its P R state clearly from its P FR state. It is mainly produced in darkness and exists first of all as P R (or P 660 ). The exposure to light of the wave length λ = 660 nm (red) transfers it into P FR (or P 730 ; FR = far- red). P FR is re-transferred into P R by exposure to light of the wave length λ= 730 nm. P R is the biologically inactive, P FR the biologically active state.
How Phytochrome Works?
660 nm Synthesis Pr Red Light (Fast) Far Red Light Dark Reversion Pfr 740 nm Destruction Vegetative (Non-Flowering) (Slow) Reproductive (Flowering)
Mid-Summer Sunlight 660 nm 740 nm Synthesis Pr Red Light (Fast) Far Red Light Pfr Destruction Dark Reversion Vegetative (Non-Flowering) (Slow) Reproductive (Flowering) Long-Day Plants Need Low Pr!
660 nm Synthesis Pr Red Light (Fast) Far Red Light Pfr Dark Reversion Long Night 740 nm Destruction Vegetative (Non-Flowering) (Slow) Reproductive (Flowering) Long-Day Plants Need Low Pr!
Mid- Summer Sunlight 660 nm Synthesis Reproductive (Flowering) Pr Red Light (Fast) Far Red Light Dark Reversion (Slow) Pfr 740 nm Destruction Vegetative (Non-Flowering) Short-Day Plant Need Low Pfr!
Long Night 660 nm 740 nm Synthesis Pr Red Light (Fast) Far Red Light Dark Reversion Pfr Destruction Reproductive (Flowering) (Slow) Vegetative (Non-Flowering) Short-Day Plants Need Low Pfr!
PLANTS HAVE INTERNAL CLOCKS As we have body activities that change with time of the day, plants display rhythmic behaviour too. Circadian rhythm: o Innate biological clock of about 24 hours. o Persists when organism not exposed to environmental cues. o E.g. bean plant still sleeps at the same time of day even when kept in constant light or dark. O Hence circadian rhythms occur with or without external stimuli. o These are controlled by biological clocks.