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 12-13 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