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Biological Rhythms Endogenous: Endogenous: internal body clock Exogenous: Exogenous: controlled by environmental stimuli Most rhythms are endogenous but.

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Presentation on theme: "Biological Rhythms Endogenous: Endogenous: internal body clock Exogenous: Exogenous: controlled by environmental stimuli Most rhythms are endogenous but."— Presentation transcript:

1 Biological Rhythms Endogenous: Endogenous: internal body clock Exogenous: Exogenous: controlled by environmental stimuli Most rhythms are endogenous but use environmental stimuli to “re-set” or entrain the clock. zeitgeber The environmental cue is termed the zeitgeber (time-giver) e.g. daylength, temperature.

2 Biological Rhythms Free-running periods: circadian circadian = rhythm with a period of about 24 hours circannual circannual = rhythm with a period of about a year circatidal circatidal = rhythm with a period of about hours circalunar circalunar = rhythm with a period of about 29 days, circasemilunar circasemilunar = rhythm with a period of about 15 days

3 Biological Rhythms Daily Rhythms Daily Rhythms – linked to the day-night cycle nocturnal = night-active diurnal = day-active crepuscular = active in twilight (dawn and dusk) Annual Rhythms Annual Rhythms – linked to Earth rotation (axial tilt & seasons) e.g. leaf fall in deciduous trees Tidal Rhythms Tidal Rhythms – linked to the tides (gravitational pull of the Moon and the Earth’s rotation) Lunar Rhythms Lunar Rhythms – (much less common) linked to the rotation of the Moon around the Earth

4 Photoperiod Responses Different plants respond to photoperiod in different ways. critical day length For many there is a critical day length (CDL). Long-day plants Long-day plants e.g. most grasses, flower when day length is longer than their CDL; Short-day plants Short-day plants e.g. strawberry, flower when day length is shorter than their CDL. Day-neutral plants Day-neutral plants e.g. tomato, dandelion, are insensitive to photoperiod.

5 Phytochrome pigment system Seasonal changes in photoperiod (day-length) inactive (Pr) active (Pfr) Phytochrome may be: inactive (Pr) or active (Pfr) Sun / red light: P r  P fr More P fr during day light hours Dark / far-red light: P fr  P r So there is more P r at night. Plant measures night length by the proportion of both forms.

6 Phytochrome pigment systemGermination Red light stimulates germination Far-red light inhibits germinationFlowering Long day plants need an accumulation of P fr Short day plants need an accumulation of P r Other responses Leaf abscission, stomatal opening and closing, leaf sleep, pigment formation in ripening fruit


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