Реферат: Untitled Essay Research Paper Involvement of K
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
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