By a plants very physiological attributes of been sessile, they have a limited way to respond to external stimuli. Due to they sessile nature they are unable to move away from a negative stimuli or towards a positive one. Therefore plants must respond to stimuli be changing there growth and development factors (Campell et al., 2006).
As most plants are photoautotrophs, they are able to synthesis sugars from carbon dioxide and water using the lights energy via the process of photosynthesis (Knox et al., 2005). As this is the primary way that plants are able to obtain chemical energy, light, carbon dioxide and water are curtail in the plants existence.
Light has evolved to play a major signalling role in plant growth (Briggs and Olney, 2001). The relative fitness of a plant many in part be determined by its ability to maximise its photosynthetic activity. One of the major factors in this is the plants ability to determine the abundance and direction of light and respond appropriately (Briggs and Olney, 2001). As plants are generally sessile, they have developed a number of physiological processes known as photomorphogenesis, which enables them to react to different light conditions to maximise photosynthesis levels (Briggs and Olney, 2001).
Plants are able to detect the time of day via photoreceptors. One of the most important photoreceptors of plants is the phytochromes. The phytochromes are a group of small light sensitive proteins that are able to absorb red light when the sun is out (Quail et al., 1995). The absorption of the red light allows the conversion of one form of the protein (Pr) to another (Pfr) (Quail et al.,1995). However when the sun light is not falling on the plant the Pfr protein is converted back into the Pr version (Quail et al., 1995). This ability of the plant to sense how much light is available is crucial in many of the plants other physiological responses in order to maximise growth.
As seeds have a limited amount of energy available to them once they enter the soil (Knox et al., 2005), it is important that they do not waste their energy on unnecessary growth. Through the use of photoreceptors in the seed tip, the seed can be determine where the light is coming from and germinate in that direction (Hollend et al., 2009). This process is known as phototropism.
The ciradian clocks interact with phytochromes to set up circadian rhythms and Photoperiodism (Wang and Tobin, 1998). Circadian rhythms allow the plant to set it biological clock, to detect light and dark transitions (Wang and Tobin, 1998), while photoperiodism uses the phytochrome proteins to detect the length of light and dark periods (Wang and Tobin, 1998). As the length of the light and dark periods vary throughout the year, this allows the plant to deduce what time of the year it is, and therefore allowing it to effectively maximise its reproduction at the right time of the year (Wang and Tobin, 1998).
Another important light controlled mechanism is the heliotropism movements or solar tracking. This is the ability of the plants leaves and flowers, to track the progress of the sun across the sky, to maximise exposure to the sun (Eheringer and Forseth, 1980). The leaf uses osmotic pressures to move ions (Cl, K) across their cellular membranes, to control the movement of the petiole and the pulvinus to change the orientation of the leaf (Satter and Galston, 1981). This is known as nytinastic leaf movements.
When pants are grown in darkness many structural and morphological changes maybe observed (Priestley 1925). The plant is said to display a number of observed characteristics known as etiolation (Priestley 1925). When a plant is grown under poor light conditions it is observed to have smaller pale leafs and long weak steams (Priestley 1925). This set of physiological changes is known as etiolation.
This experiment is designed to...