Biological Rhythms

Endogenous: internal body clockExogenous: controlled by environmental stimuli Most rhythms are endogenous but use

environmental stimuli to “re-set” or entrain the clock.

The environmental cue is termed the zeitgeber (time-giver) e.g. daylength, temperature.

Biological Rhythms

Free-running periods:

circadian = rhythm with a period of about 24 hours

circannual = rhythm with a period of about a year

circatidal = rhythm with a period of about 12-13 hours

circalunar = rhythm with a period of about 29 days,

circasemilunar = rhythm with a period of about 15 days

 

Biological Rhythms

Daily Rhythms – linked to the day-night cycle nocturnal = night-active diurnal = day-active crepuscular = active in twilight (dawn and dusk)

 

Annual Rhythms – linked to Earth rotation (axial tilt & seasons) e.g. leaf fall in deciduous trees

 

Tidal Rhythms – linked to the tides (gravitational pull of the Moon and the Earth’s rotation)

 

Lunar Rhythms – (much less common) linked to the rotation of the Moon around the Earth

 

Photoperiod ResponsesDifferent plants respond to photoperiod in

different ways.

For many there is a critical day length (CDL).

Long-day plants e.g. most grasses, flower when day length is longer than their CDL;

Short-day plants e.g. strawberry, flower when day length is shorter than their CDL.

Day-neutral plants e.g. tomato, dandelion, are insensitive to photoperiod.

 

Phytochrome pigment systemSeasonal changes in photoperiod (day-length)

Phytochrome may be: inactive (Pr) or active (Pfr)

Sun / red light: Pr Pfr

More Pfr during day light hours

Dark / far-red light: Pfr Pr

So there is more Pr at night.

Plant measures night length by the proportion of both forms.

Phytochrome pigment systemGermination

Red light stimulates germinationFar-red light inhibits germination

FloweringLong day plants need an accumulation of Pfr

Short day plants need an accumulation of Pr

Other responsesLeaf abscission, stomatal opening and closing, leaf sleep, pigment formation in ripening fruit