Chapter 3: Coordination and Control in Plants
Phytochrome is found the leaves. Acts as a photoreceptor.
Long day plants: Flower if length of day exceeds a minimum amount
Short day plants: Flower if length of darkness exceeds a minimum length
Red light
P₆₃ₒ P₇₃ₒ P₆₆ₒ is rapidly converted to P₇₃ₒ during the day as daylight
Far red light contains more red than far red light.
Darkness P₇₃ₒ is the physiologically active form.
Duration Plant Response
Long day LDP Flowering as duration of day/time of light is long enough
for enough P660 to be converted to P730
Short day LDP No flowering, darkness is long enough for P730 to be
converted back t0 P660. P730 can’t reach a critical level
Long day SDP No flowering, inhibitory effect of P730. Daylight caused
P660 to convert to P770, not enough darkness to remove
inhibitory effect by converting back to P660
Short day SDP Flowering, dark period is long enough to remove the
inhibitory effect of P730 that had been built up during
the day.
Substance Site of production Function
Auxins Tip, move down the stem Increased cell elongation in the
zone of elongation
Cytokinins Meristematic tissues Increased cell division in the
apical meristems
Gibberellins Leaves Increased cell elongation in the
internodes
Important notes
1. It is the length of the dark period that is critical in determining whether flowering occurs in
both LDPs and SDPs, as it determines how much P730 can be converted back to P660.
2. In SDPs the critical period of darkness must be continuous/uninterrupted. This is because a
short flash of light will rapidly convert P660 to P730 but the conversion of P730 to P660 in
darkness is slow.
3. The plant growth substances interact and seldom act in isolation. For example cytokinins
promote cell division only in the presence of auxins.
Phytochrome is found the leaves. Acts as a photoreceptor.
Long day plants: Flower if length of day exceeds a minimum amount
Short day plants: Flower if length of darkness exceeds a minimum length
Red light
P₆₃ₒ P₇₃ₒ P₆₆ₒ is rapidly converted to P₇₃ₒ during the day as daylight
Far red light contains more red than far red light.
Darkness P₇₃ₒ is the physiologically active form.
Duration Plant Response
Long day LDP Flowering as duration of day/time of light is long enough
for enough P660 to be converted to P730
Short day LDP No flowering, darkness is long enough for P730 to be
converted back t0 P660. P730 can’t reach a critical level
Long day SDP No flowering, inhibitory effect of P730. Daylight caused
P660 to convert to P770, not enough darkness to remove
inhibitory effect by converting back to P660
Short day SDP Flowering, dark period is long enough to remove the
inhibitory effect of P730 that had been built up during
the day.
Substance Site of production Function
Auxins Tip, move down the stem Increased cell elongation in the
zone of elongation
Cytokinins Meristematic tissues Increased cell division in the
apical meristems
Gibberellins Leaves Increased cell elongation in the
internodes
Important notes
1. It is the length of the dark period that is critical in determining whether flowering occurs in
both LDPs and SDPs, as it determines how much P730 can be converted back to P660.
2. In SDPs the critical period of darkness must be continuous/uninterrupted. This is because a
short flash of light will rapidly convert P660 to P730 but the conversion of P730 to P660 in
darkness is slow.
3. The plant growth substances interact and seldom act in isolation. For example cytokinins
promote cell division only in the presence of auxins.
