The Effect of Reduced Light on Vigna Radiata Growth Rate

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).

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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 test the effect of light on growth in Vigna radiata (Heimann, 2010). By placing a number of V. radiata seeding in a light poor environment, while also growing more V. radiata seedling under normal light conditions, it is possible to observe the effect of the reduced light conditions on the growing seedling. Our hypothesis states that Vigna radiata grown under poor light will enter a state of etiolation, therefore expressing a mean height significantly different to the Vigna radiata grown in normal lights conditions.

Our null hypothesis would therefore state that there will be no significant difference in mean height between Vigna radiata grown in the diffenent leight conditions. Materials and Methods The experiment was conducted at James Cook University (Longitude: 146. 75766E Latitude: 19. 32675S) in Townsville in tropical north Queensland. Vigna radiata seeds were soaked in water for 24hrs to assist in germination. The seeds were then sprayed with a small amount of liquid fungicide. The growing medium was then placed in a open lid container, on top of an absorbent piece of paper covering the drainage holes to prevent the medium from falling out.

A small amount of water was added to the medium to allow it to settle. The Vigna radiata seeds were evenly distributed over the medium. Extra growing medium was then added in order to cover the seeds. The seeds were planted on 19/02/2010 in late summer, where they were watered and left in lowlight conditions for 48hrs to allow them to germinate. After 48hrs the seedling were placed in a temperature controlled room set at 25C, half the seeds were placed on shelves covered in black plastic, as no light could reach them, and the other half were placed under white fluorescent tubes.

The seedling were left for 7 days upon which they were harvested for height analysis (Heimann, 2010). Upon harvesting each seedling was measured, in millimetres using a standard 30cm ruler from the base of the roots to the apical bud (Heimann, 2010). This height was recorded, and the data was then entered onto a Microsoft Excel spread sheet for the calculations. Discussion Living things regulate and associate themselves with their surroundings to grow, this is especially factual for plants which require sunlight, water and a soil rich in nutrients.

Plants unlike animals can adapt to certain changes by the processes called photosynthesis. Photosynthesis relies on sunlight from the sun to function; a limited amount of sunlight can hinder the growth and appearance of plants. This theory is tested with the beans of Vigna radiata which were grown in Sunlight and complete darkness. The beans grown in sunlight exhibited an appearance of green shoots and leafs which determined a customary growth and apperance, this contrasted to beans grown in no light which were much taller,exhibited yellow leaves ,smaller steams and leafs were much smaller which was a clear display of phototropism.

Results of the two different variables measured were the height and internode length which desribed the growth of the been seedlings. The results displayed the internode and height to be longer in beans grown in no light condition compared to beens grown in sunlight, there were certian anomilaies where the differnence was just 3mm, than the average differnence being 25mm. The two differnent variables each play an essential role in the growth of the bean; the Internode length separates the primary leaf from the Cotyledon displaying maturity.

The beans grown in no light had a longer internode length which corresponded to the total length that showed that they were taller and slimmer. An average geriminated bean seedling will have an average height of 182. 653191 which is calculated from the mean average of the relative frequency of beans grown in sunlight. The beans grown in no sunlight however present much longer relative frequency which determines the lack of growth and minerals required for a healthy bean plant.

The internode length data also corresponds to the colour and size of the leafs which show brown,yellow leafs in plants grown in no sunlight contrasting to green, big leafs displayed in sunlight grown beans. The characteristics of leafs grown in the shade have chloraplasts that move within cells to absorbe the maximum light without shading other chloroplasts below them, this contrasts to leafs grown in sunlight which take turns absorbing the bright light and sheltering in the shade of other chloroplasts making use of light previously bsorbed. The height informs various data into growth of the two irriadance conditions, as explained earlier the heights between the beans show significant differnence’s, distinct to the internode length, the height measures the entire length of the bean seed which may display different trends of data compared to the internode length. The height variation in the data also shows various differnences such as the leaf size and colour, length, width of stem and length of the root system.

These are essential features of the growth and healthyness of gernminated bean seeds. Leaf size and the root system could have been used to measure the impact of irradiance as the leaf size could display the amount of area required for photosynthesis,a larger leaf surface area would mean more surface area for photosythesis equalling are larger,healthy bean plant. A smaller leaf area would limit the growth of a bean plant which would eventually eradicate the plant.

The root sytem is an important part of a plant as it provides the foundation of the growth of beans, supplys water, and inorganic nutrients to the rest of the plant and stores food and nutrients for future usage. Light intensity does affect the growth of Vigna radiata which can be clearly observed from the two irradiance conditions grown. There is a corelation between light and plants which explain that as light intensity deacreses, plant function increases and vice versa.

Sunlight affects all Autotroph’s such as Vigna radiata to commence the process of photosynthesis which converts carbon dioxide into organic compounds such as sugar. This happens in the chloroplast which are located on the beans leafs, the chlorophyl stores the energy in the form of ATP (Adenoside Triphosphate) and uses the remaining energy to remove electrons from substances, the electrons are then used to convert carbon dioxide into organic compounds, this process is also called the Calvin Cycle.

Seddlings grown under limiting light tend to be tall as it attempts to obtain sunlight, this is affected by the process called Phototropism which plants grow towards the sun, the seedling tends to grow faster than the required time to grow as there is a shortage of sunlight, the leafs change colour from green to yellow has chlorophyl slowly deacreses, the stem of the seedlings are thinner as more enrgy is consumed to height moreover then width.

When there is sufficent sunlight the photosyhtneisis process occurs at a normal rate providing energy for cellular energy. The process of Photomorgenesis happens in light grown conditions, this process is vital in the survival and growth of the bean seed, germinating seeds dont resort to sunlight as all the nutrition is strored in the bean, they require sunlight on the second or third day of germinating. The data from the histogram and T-test suggest the hypothesis is correct which states that plants grown in no sunlight are taller than plants grown sunlight.

Table 1 which displays the Distribution of bean height within specific height groups illustrate the numbers increasing in observed frequency intervals of low light plants, while plants grown in light conditions discontinue growth at 239mm. Similar data is displayed in the histogram which illustrates Vigna Radiata seedlings grown in low light conditions to be longer in Size classes than seedlings grown in normal light, this brings the histogram further to the right for Fig. 2. The T-test proves similarly with the mean contrasting between the seedlings.

There are hight variances between the two groups of seedlings as shown in the T-test, this proves the differnences in height, colour, width of stem, internode length and height. This means there is more variance in the height of normal grown seedings but not at a distinct level compared to no light seedlings which have less varaiation exhibiting no variations betwen each seedling. The results illustrated from the histogram and T-test show that the null hypothesis was correct as it suggestet that Vigna raidata seedlings grown in less to no light frequencies to be longer than ones grown in sunlight while seedlings grown.

The alternative hypothesis was incorrect has it suggested that vigna radiata bean seeds grown in little to no sunlight would exhibit same features has vigna radiata seeds grown in sunlight. Futher studies should be done by analysing the roots and stems as they are important functions which contribute to the growth of been sidlings, this would coinside with differnent varaibles tested such as the temperature and amount of water. The results obtained would confidently be undertaken by large orgainization’s to sufficently grow and combat world hunger in third world countries.

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The Effect of Reduced Light on Vigna Radiata Growth Rate. (2016, Nov 09). Retrieved from