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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