BY4: Plant responses
Plants are responsive to the environment just like animals are responsive to their environment. Plants can bend towards light, leaves can orientate themselves so that they are best placed to receive maximum light from the sun for photosynthesis. The main difference is that plant responses are slow because co-ordination is achieved by chemicals, similar to hormones in animals. Plants do not have a nervous system, instead growth is co-ordinated by plant growth substances. One response to light in plants is called photo-periodism. This is the response of a plant to relative lengths of daylight and darkness. Flowering is influenced by day length (although it is actually the night length that is most important: below or above a critical length of darkness depending on whether it is a long or short day plant).
Phytochrome
The photoreceptor responsible for absorbing light has been identified as phytochrome, a blue-green pigment found in very minute quantities in plants within the leaves. Phytochrome exists in two forms that are inter-convertible:
Phytochrome 660 (Pr) which has an absorption peak at 660 nm and absorbs red light (part of visible spectrum)
Phytochrome 730 (Pfr) which has an absorption peak of 730 nm and absorbs far-red light (outside of the visible spectrum)
On absorbing light of a particular wavelength, each form of phytochrome is converted to the other form.
Sunlight contains more light of wavelength 660 nm (red light) than 730 nm. Therefore during daylight Pr absorbs this 660nm red light and is converted to Pfr, which accumulates throughout the day. Pfr is the active form of phytochrome which can either activate florigen production when it accumulates (long day plants) or inhibit florigen production when it accumulates (short day plants). Pfr therefore has opposite effects in long and short day plants (see below for more details).
Pfr is unstable and during the hours of darkness slowly reverts back to Pr which then accumulates. The plant measures day length (or more precisely the length of darkness) by the amount of phytochrome existing in each of the two forms. In daylight Pfr is the main form.
The photoperiodic stimulus is detected by the leaves of a plant. This can be demonstrated using a single plant and exposing one leaf to light whilst covering up the remainder of the leaves. The stimulus (in the form of the hormone florigen which is made in the leaves under the correct conditions) must be transmitted through the plant through the phloem to the buds, which then develop flowers. Flowering in plants is thought to be initiated by the hormone ‘florigen’. This florigen hormone will stimulate flowering at the buds irrespective of the type of plant (i.e. it is the same hormone in all species of flowering plants and has the same effect in each different species, which is to start flowering).
Plants are responsive to the environment just like animals are responsive to their environment. Plants can bend towards light, leaves can orientate themselves so that they are best placed to receive maximum light from the sun for photosynthesis. The main difference is that plant responses are slow because co-ordination is achieved by chemicals, similar to hormones in animals. Plants do not have a nervous system, instead growth is co-ordinated by plant growth substances. One response to light in plants is called photo-periodism. This is the response of a plant to relative lengths of daylight and darkness. Flowering is influenced by day length (although it is actually the night length that is most important: below or above a critical length of darkness depending on whether it is a long or short day plant).
Phytochrome
The photoreceptor responsible for absorbing light has been identified as phytochrome, a blue-green pigment found in very minute quantities in plants within the leaves. Phytochrome exists in two forms that are inter-convertible:
Phytochrome 660 (Pr) which has an absorption peak at 660 nm and absorbs red light (part of visible spectrum)
Phytochrome 730 (Pfr) which has an absorption peak of 730 nm and absorbs far-red light (outside of the visible spectrum)
On absorbing light of a particular wavelength, each form of phytochrome is converted to the other form.
Sunlight contains more light of wavelength 660 nm (red light) than 730 nm. Therefore during daylight Pr absorbs this 660nm red light and is converted to Pfr, which accumulates throughout the day. Pfr is the active form of phytochrome which can either activate florigen production when it accumulates (long day plants) or inhibit florigen production when it accumulates (short day plants). Pfr therefore has opposite effects in long and short day plants (see below for more details).
Pfr is unstable and during the hours of darkness slowly reverts back to Pr which then accumulates. The plant measures day length (or more precisely the length of darkness) by the amount of phytochrome existing in each of the two forms. In daylight Pfr is the main form.
The photoperiodic stimulus is detected by the leaves of a plant. This can be demonstrated using a single plant and exposing one leaf to light whilst covering up the remainder of the leaves. The stimulus (in the form of the hormone florigen which is made in the leaves under the correct conditions) must be transmitted through the plant through the phloem to the buds, which then develop flowers. Flowering in plants is thought to be initiated by the hormone ‘florigen’. This florigen hormone will stimulate flowering at the buds irrespective of the type of plant (i.e. it is the same hormone in all species of flowering plants and has the same effect in each different species, which is to start flowering).
Photo-periodism is the term used to describe the influence of relative periods of light and darkness on flowering. (Historically, plants are categorised as short-day or long-day. This is unfortunate as it is the length of the dark period that is crucial.)
Flowering plants can be divided into three groups according to their photoperiodic requirements prior to the production of flowers:
• Day neutral plants: flowering does not seem to be affected by the day-length e.g. tomato, cotton, cucumber.
• Long-day plants: otherwise known as SHORT NIGHT PLANTS: flowering is induced by exposure to dark periods (number of hours) shorter than a critical length e.g. cabbage, petunia.
• Short-day plants: otherwise known as LONG NIGHT PLANTS: flowering is induced by exposure to dark periods longer than a critical length e.g. chrysanthemum, tobacco, poinsettia.
In short-day plants flowering is inhibited by exposure to red light, and exposure to far red light will bring about flowering. It seems that these plants will flower only if the level of Pfr is low enough, but the situation in the long-day plants is reversed and flowering is triggered by high levels of Pfr. The length of the photoperiod is less critical than the length of the dark period and if the photoperiod is interrupted with a short period of darkness flowering still follows.
If the dark period is interrupted by as little as one minute’s exposure to light flowering is prevented. Red light is most effective in this respect yet the effect of red light treatment can be overcome if the plant is immediately exposed to infra-red light.
Flowering plants can be divided into three groups according to their photoperiodic requirements prior to the production of flowers:
• Day neutral plants: flowering does not seem to be affected by the day-length e.g. tomato, cotton, cucumber.
• Long-day plants: otherwise known as SHORT NIGHT PLANTS: flowering is induced by exposure to dark periods (number of hours) shorter than a critical length e.g. cabbage, petunia.
• Short-day plants: otherwise known as LONG NIGHT PLANTS: flowering is induced by exposure to dark periods longer than a critical length e.g. chrysanthemum, tobacco, poinsettia.
In short-day plants flowering is inhibited by exposure to red light, and exposure to far red light will bring about flowering. It seems that these plants will flower only if the level of Pfr is low enough, but the situation in the long-day plants is reversed and flowering is triggered by high levels of Pfr. The length of the photoperiod is less critical than the length of the dark period and if the photoperiod is interrupted with a short period of darkness flowering still follows.
If the dark period is interrupted by as little as one minute’s exposure to light flowering is prevented. Red light is most effective in this respect yet the effect of red light treatment can be overcome if the plant is immediately exposed to infra-red light.