Untitled Essay Research Paper Involvement of K
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Untitled Essay, Research Paper Involvement of K+ in Leaf Movements During SuntrackingIntroduction Many plants orient their leaves in response to directional light signals. Heliotropic movements, or movements that are affected by the sun, are common among plants belonging to the families Malvaceae, Fabaceae, Nyctaginaceae, and Oxalidaceae. The leaves of many plants, including Crotalaria pallida, exhibit diaheliotropic movement. C. pallida is a woody shrub native to South Africa. Its trifoliate leaves are connected to the petiole by 3-4 mm long pulvinules (Schmalstig). In diaheliotropic movement, the plant’s leaves are oriented perpendicular to the sun’s rays, thereby maximizing the interception of photosynthetically active radiation (PAR). In some plants, but not all, his response occurs particularly during the morning and late afternoon, when the light is coming at more of an angle and the water stress is not as severe (Donahue and Vogelmann). Under these conditions the lamina of the leaf is within less than 15? from the normal to the sun. Many plants that exhibit diaheliotropic movements also show paraheliotropic response as well. Paraheliotropism minimizes water loss by reducing the amount of light absorbed by the leaves; the leaves orient themselves parallel to the sun’s rays. Plants that exhibit paraheliotropic behavior usually do so at midday, when the sun’s rays are perpendicular to the ground. This reorientation takes place only in leaves of plants that are capable of nastic light-driven movements, such as the trifoliate leaf of Erythrina spp. (Herbert 1984). However, this phenomenon has been observed in other legume species that exhibit diaheliotropic leaf movement as well. Their movement is temporarily transformed from diaheliotropic to paraheliotropic. In doing so, the interception of solar radiation is maximized during the morning and late afternoon, and minimized during midday. The leaves of Crotalaria pallida also exhibit nyctinastic, or sleep, movements, in which the leaves fold down at night. The solar tracking may also provide a competitive advantage during early growth, since there is little shading, and also by intercepting more radiant heat in the early morning, thus raising leaf temperature nearer the optimum for photosynthesis. Integral to understanding the heliotropic movements of a plant is determining how the leaf detects the angle at which the light is incident upon it, how this perception is transduced to the pulvinus, and finally, how this signal can effect a physiological response (Donahue and Vogelmann). In the species Crotalaria pallida, blue light seems to be the wavelength that stimulates these leaf movements (Scmalstig). It has been implicated in the photonastic unfolding of leaves and in the diaheliotropic response in Mactroptilium atropurpureum and Lupinus succulentus (Schwartz, Gilboa, and Koller 1987). However, the light receptor involved can not be determined from the data. The site of light perception for Crotalaria pallida is the proximal portion of the lamina. No leaflet movement occurs when the lamina is shaded and only the pulvinule is exposed to light. However, in many other plant species, including Phaseolus vulgaris and Glycine max, the site of light perception is the pulvinule, although these plants are not true suntracking plants. The compound lamina of Lupinus succulentus does not respond to a directional light signal if its pulvini are shaded, but it does respond if only the pulvini was exposed. That the pulvinus is the site for light perception was the accepted theory for many years. However, experiments with L. palaestinus showed that the proximal 3-4 mm of the lamina needed to be exposed for a diaheliotropic response to occur. If the light is detected by photoreceptors in the laminae, somehow this light signal must be transmitted to the cells of the pulvinus. There are three possible ways this may be done.
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