DEVELOPMENT OF THE INTERNODE LENGTH OF VIGNA RADIATA WHEN GROWN UNDER DIFFERENT LIGHT INTENSITIES
Throughout their growth plants exhibit varying ways to adapt and change depending on their circumstances, with changes in the amount of available light being a major factor in plant growth. The aim of the study was to determine whether there are significant differences in the growth patterns of Vigna radiata when grown under varying light intensities. Plants were grown in two different light intensities, normal light and no light, for a period of 10 days to determine two different phototrophic effects. The results demonstrated that there was a significant difference between the internode lengths of V. radiata grown under the different light intensities, with the plants grown in normal light having shorter internode lengths than the plants grown in no light. It is clear that the rate of development of structural characteristics is significantly greater for plants grown in normal light intensities then those grown in no light as their features are underdeveloped.
Unlike most animals once a plant has established its root system in a specific location in cannot move significantly. Plants require sunlight in order for photosynthesis to occur and thus allow the plant to produce its own glucose. In order to maximise their ability to photosynthesis plants can alter their growth, either positively or negatively in response to external stimuli. These responses often cause a directional movement or elongation in growth and are called tropisms (Raven et al., 2005). If this growth exhibits a positive response to light then it is called a phototropism and will result in the plant growing or bending toward the light source (Campbell et al., 2006). Photomorphogenic responses are light induced growth patterns which occur when a plant seedling is grown in sufficient light conditions. These responses initiate the uncurling of seedling stems and increase leaf formation thus maximising the plants photosynthetic ability. At high light intensities retardation of the stem will also occur. These responses occur regardless of photosynthetic ability (Mohr, 1962; Raven et al., 2005). Phytochrome is the name given to photoreceptive pigments within the cells of plants that enables them to detect the presence and direction of light (Neuhaus et al., 1993). This allows some plants to perform heliotropism or solar tracking. Heliotropism is the diurnal nature of plants to follow the direction of the sun usually from east to west throughout the day (Raven et al., 2005). Similar to heliotropism, nyctinastic movements relate to the movement of leaves in response to light and dark cycles of the day. However these responses will occur even in the absence of sunlight. For example in legumes the leaves are orientated vertically at night and horizontally during the day for optimal photosynthesis (Satter and Galston, 1981). Despite light intensities the internal functions, both biochemically and physically, of plants are regulated within a 24 hour period. These are known as circadian rhythms and continue despite any external stimulus. Circadian rhythm is regulated by an internal, biological clock. Although the biological clock maintains a rhythm period of 21-27 hours in idealised conditions, the clock will undergo entrainment and may advance or delay the circadian rhythm in order to become synchronized with the light-dark external cycles of its environment (Nagoshi et al., 2004). Photoperiodism is the response of a change in ratio of light and dark within a 24 hr period and has a significant impact on induction of flowering throughout the year (Raven et al.,2005). In contrast, etiolation is the term used to describe the growth patterns of a seedling grown in a darkened environment. Etiolation maximises elongation of the stem and retards the development of cotyledons and leaves, giving the plant the maximum opportunity to reach sunlight...
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