Chapter 3 - Coordination and Control in Plants Flashcards
As complex living organisms, what stimuli are plants able to respond to in their environment?
As complex living organisms, plants are able to respond to many stimuli in the environment including gravity, water, chemicals and light.
As complex living organisms, plants are able to respond to many stimuli in the environment including gravity, water, chemicals and light. Describe how one of these factors is studied.
The effect of light on plant growth and development is easily demonstrated, and historically, much studied. One example of the effect of light on plant growth and development involves the phytochrome system.
What is phytochrome?
- Phytochrome is a pigment system found in the leaves of flowering plants.
- It is primarily involved in the timing of flowering in many species.
What triggers the flowering process in plants?
While in some species, the flowering process is initiated when average temperatures reach a certain level, for example, tomatoes, for most British species the trigger is the duration of light or day length (the photoperiod).
For species sensitive to day length what are the two categories of plants?
- Long-day plants (LDPs), for example, cabbage and petunia. These species flower only if the day length exceeds a certain minimum length.
- Short-day plants (SDPs), for example, chrysanthemum and strawberry. These species only flower if the days are shorter than a critical value (or the nights exceed a minimum length).
What is the function of phytochrome?
- It is primarily involved in the timing of flowering in many species.
- Phytochrome pigments act as the photoreceptor.
Phytochrome pigments act as the photoreceptor. Why are they suitable for this role?
Phytochrome is sensitive to light and its suitability for this role is linked to its ability to exist in two inter-convertible forms:
- P660 (PR) absorbs red light (light from the red part of the spectrum) with an absorption peak of 660 nm.
- P730 (PFR) absorbs far red light with an absorption peak of 730 nm.
Why are the two forms of phytochrome known as P660 and P730?
Due to the difference in absorption peaks
What will happen to P660 if it is subjected to red light?
If subjected to red light the P660 is rapidly converted to the P730 form.
What will happen to P730 if it is subjected to far red light?
If subjected to far red light the P730 is rapidly converted to the P660 form.
What will happen to P730 during darkness?
In darkness (as P730 is much less stable than P660), P730 will slowly convert to the P660 form.
Draw a flow diagram illustrating the conversion between the two forms of phytochrome
Textbook page 47
State two key points about phytochrome and flowering in plants
• As daylight contains more red than far red light, P660 is converted to P730 during the day.
- Consequently, as day length gets longer in spring/early summer the proportion of phytochrome that exists in the P730 form increases.
- In LDPs, as day length increases, the amount of P739 eventually reaches a critical level that initiates flowering.
- The intensity of light is also important, as the conversion is quicker in high light intensities.
• Furthermore, it is the P730 form that is the physiologically active form, whether it is that sufficient P730 has accumulated to promote flowering in LDPs or that there is too much P730 to allow flowering to occur in SDPs.
- In SDPs the inhibitory effect of high levels of P730 must be removed before flowering can take place.
Describe the phytochrome response and effect of a short day - long night photoperiod on a long day plant
Phytochrome response:
P660 converted to P730 during the day but the night (dark period) is long enough for sufficient P730 to be slowly converted back to P660 to prevent the P730 reaching the critical level needed for flowering.
Effect: No flowering
Describe the phytochrome response and effect of a long day - short night photoperiod on a long day plant
Phytochrome response:
Long day length allows P660 to be converted to P730 in high concentrations. The night (darkness) is too short for enough P730 to be converted back to P660. P730 builds up to critical level.
Effect: Flowering
Describe the phytochrome response and effect of a short day - long night photoperiod on a short day plant
Phytochrome response:
P660 is converted to P730 during the day. The dark period is long enough for a sufficient level of P730 to be converted to P660 to remove the inhibitory effect of P730.
Effect: Flowering
Describe the phytochrome response and effect of a long day - short night photoperiod on a short day plant
Phytochrome response:
P660 is converted to P730 during the day but the dark period is not long enough for a sufficient level of P730 to be converted to P660 to remove the inhibitory effect of P730.
Effect: No flowering
What is the critical factor in determining whether flowering occurs in both LDPs and SDPs?
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.
Why do commercial plant growers manipulate the photoperiod?
- Commercial plant growers need to have flowering plants available at the times of year when sales are likely to be highest, for example, roses for St Valentine’s Day and a wide range of flowers for Christmas.
- Most species will not naturally flower at these times of year but manipulation of the photoperiod (the light regime) can ensure that supply and demand are matched.
