BSC 314 Lecture Notes - Lecture 30: Photoperiodism, Phytochrome, Germination
27 Jun 2018
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Circadian Rhythms
Many plants exhibit a rhythmic behavior on about a 24 hour cycle, such as the flowers ‐
that open in late afternoon every day. This regular repetition of growth or activity on
approximately a 24 hour cycle is called a‐ circadian rhythm. All sorts of metabolic
processes are circadian, such as cell divisions in root tips, and protein or hormone
synthesis. Sleep movements of leaves are well known circadian rhythms, as are the ‐
opening and closing of night blooming or day blooming flowers.‐ ‐
Circadian rhythms are endogenous, meaning they are controlled by an internal timing
mechanism called the biological clock of the plant. Although circadian rhythms are not
triggered by an external stimulus, the environment does set and keep the biological
clock in harmony with external changes such as darkness and light. The resetting of the
biological clock is called entrainment. Entrainment, for example, keeps the circadian
periodicity of flowering in sequence with light and dark periods even as the day lengths
change seasonally. The biological clock keeps the plant responding appropriately for
each season by measuring the changing day lengths. The mechanism by which it does
this involves the pigment phytochrome.
Photoperiodism
Photoperiodism is a biological response to a change in the proportions of light and dark
in a 24 hour daily cycle. Plants use it to measure the seasons and to coordinate ‐
seasonal events such as flowering.
Phytochrome
Plants make such adjustments by utilizing the pigment phytochrome, which exists in two
forms: P r, which absorbs red light, and P fr, which absorbs far red light. Each can ‐
convert to the other when they absorb light. During the day, the two forms convert back
and forth (P rbecomes P fr, and vice versa), until they reach an equilibrium of 60:40 P fr:
P r in plant tissues. During the night, P fr slowly converts to P r or else disintegrates.
P r is stable in the dark.
P fr is the biologically active form, acting as the switch that turns on such plant
responses as flowering or seed germination. When the threshold concentration of P fr is
attained, the response is stimulated. Thus, it is the length of the night period, not the
day period, that determines the response. Short nights (meaning long days) favor
activities that require large amounts of P fr; conversely, if the night is long (and the day
short), more P fr is converted back to P r and responses triggered by small amounts of
P fr are favored. P r, synthesized from amino acids, is the inactive form.
Photoperiodic responses
Photoperiodism was first studied in relation to flowering. Plants can be described in
relation to their photoperiod responses as short day, long day, day neutral, ‐ ‐ ‐
and intermediate day‐ plants. Plants that flower in late summer and fall are short day; ‐
they have a critical period of light exposure of less than about 16 hours. Long day plants‐
are summer flowering and have a critical period of longer than 9 to 16 hours. Day‐
neutral plants flower in photoperiods of any length, while intermediate day plants flower ‐
only in periods neither too long nor too short for the particular plant (that amount of time
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Document Summary
Many plants exhibit a rhythmic behavior on about a 24 hour cycle, such as the flowers that open in late afternoon every day. This regular repetition of growth or activity on approximately a 24 hour cycle is called a circadian rhythm. All sorts of metabolic processes are circadian, such as cell divisions in root tips, and protein or hormone synthesis. Sleep movements of leaves are well known circadian rhythms, as are the opening and closing of night blooming or day blooming flowers. Circadian rhythms are endogenous, meaning they are controlled by an internal timing mechanism called the biological clock of the plant. Although circadian rhythms are not triggered by an external stimulus, the environment does set and keep the biological clock in harmony with external changes such as darkness and light. The resetting of the biological clock is called entrainment.
