Reproduction in plants Arthur and Kelvin

Understandings -> Flowering involves a change in gene expression in the shoot apex. -> The switch to flowering is a response to the length of light and dark periods in many plants. -> Most flowering plants use mutualistic relationship with pollinators in sexual reproduction -> Success in plant reproduction depends on pollination, ,fertilization and seed dispersal.

Flowering and gene expression When a seed germinates, a young plant is formed that grows root, stems and leaves and these are called vegetative structures, which indicates that it is currently in vegetative phase. Flowers can only reproduce until something stimulates the plant to change from the vegetative phase into reproductive phase and the stimulus is a change in the length of light and dark periods. Once the flower is mature, it will spread it’s pollens (Pollination) by mutualism, which will trigger fertilization.

Short day plant and Long day plant Chrysanthemums: Irises:

Differences Short day plant flowers when the night is longer and these plants ONLY flower in fall or winter, since days are getting shorter and shorter. Short day plants will not flower if some artificial light is shined onto the plant for a few minutes during night as they require a consolidated period of darkness before floral development can begin. Moonlight or lightning does is not enough to interrupt the flowering of short day plants. Long day plants flower around spring or summer and they require more contact with light.

Photoperiods and flowering The switch in plants respond to the length of light in darkness periods which affects the flowering of plants Long-day plants flower bloom in summer when the nights are short enough and short-day plants flower bloom in autumn when the nights have become long enough Summer (shorter) and Winter (longer) have different night periods Plants contain phytochromes to measure the length of dark periods

Phytochrome Phytochrome is a pigment in leaves that plants are used to detect the length of dark periods at night Photoreceptors (light sensitive) There are two forms of phytochrome Pr and Pfr Pr and Pfr switches forms according to the light wavelength (switch is important for plants to detect night periods)

The shift from night time to day time Hit by red Light (Light wavelength of 660 nm~) Hit by far-red Light (Light wavelength of 730 nm~) Active Form of phytochrome The shift of day time to night time Sun light contains more 660 nm than 730 nm wave length so in normal sunlight phytochrome is rapidly converted from Pr to Pfr Pr is more stable than Pfr, so in darkness, Pfr very gradually changes back into Pr Receptor proteins are presented in the cytoplasm to which Pfr but not Pr binds

Long-day plants Have large amount of Pfr remains at the end of the night to bind to the receptor Promotes transcription of genes needed for flowering

Short-day plants Receptor inhibits the transcription of the genes needed for flowering when Pfr binds to it Since after long night, Pfr gradually changes into Pr, there are limited Pfr left for binding with the receptor Inhibition fails and the plant flowers

Pollination Pollination is the transfer of male parts of a flower to the female part of the flower (Stigma). Each pollen has around 5000 sperm. Pollinators such as bees are attracted because they can gain food (nectar) and the pollen will be able to attach itself onto the bees and potentially land on a stigma. This double-beneficial relationship is called Mutualism.

Bee covered in pollen

Fertilization Fertilization occurs once a pollen has landed on the stigma and a reaction called the enzymatic reaction will create a tube (Pollen tube) within the stigma that allows the sperm from the pollen to swim to the ovary. The ovary is located inside a small rounded structure called an ovule.

Diagram of flower

Seed dispersal Some plants do not have pollens to spread and an example is the coconut tree. Coconut trees lives near rivers, lakes and coastlines because once the seed is mature, it will fall off the tree and down towards the water source, which will potentially wash it ashore to a new location. The seeds are ONLY released when the outer shell of the coconut decays or is digested/broken down by animals.

Picture of coconut tree

Keywords Vegetative phase - The phase before reproductive phase and can be stimulated into the reproductive phase when the stimulus is present (Light). This can be spotted when there are vegetative structures present. Pollinators - Animals that help transfer the pollen. Examples are honey bees and hummingbirds. Mutualism - Both organisms receive benefits. Example is when the honey bees or hummingbirds obtain nectar from the flower and help spread the pollen. Enzymatic Reaction - A reaction that only occurs once a pollen lands on top of the stigma and it creates a tube called the pollen tube. The pollen tube allows the sperm from the pollen to swim towards the ovary.