Untitled Essay Research Paper Involvement of K — страница 2

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One is that the light is channeled to the pulvinus from the lamina. However, this is unlikely since an experiment with oblique light on the lamina and vertical light on the pulvinus resulted in the lamina responding to the oblique light. Otherwise, the light coming from the lamina would be drowned out by the light shining on the pulvinus. Another possibility is that some electrical signal is transmitted from the lamina to the pulvinus as in Mimosa. It is also possible that some chemical is transported from the lamina to the pulvinus via the phloem. These chemicals can be defined as naturally occuring molecules that affect some physiological process of the plant. They may be active in concentrations as low as 10-5 to 10-7 M solution. Whatchemical, if any, is used by C. pallida to

transmit the light signal from the lamina of the leaflet to its pulvinule is unknown. Periodic leaf movement factor 1 (PLMF 1) has been isolated from Acacia karroo, a plant with pinnate leaves that exhibits nychinastic sleep movements, as well as other species of Acacia, Oxalis, and Samanea. PLNF 1 has also been isolated from Mimosa pudica, as has the molecule M-LMF 5 (Schildknecht). The movement of the leaflets is effected by the swelling and shrinking of cells on opposite sides of the pulvinus (Kim, et al.) In nyctinastic plants, cells that take up water when a leaf rises and lose water when the leaf lowers are called extensor cells. The opposite occurs in the flexor cells (Satter and Galston). When the extensor cells on one side of the pulvinus take up water and swell, the

flexor cells on the other side release water and shrink. The opposite of this movement can also occur. However, the terms extensor and flexor are not rigidly defined. Rather, the regions are defined according to function, not position. Basically, the pulvini cells that are on the adaxial (facing the light) side of the pulvinus are the flexor cells, and the cells on the abaxial side are the extensor cells. Therefore, the terms can mean different cells in the same pulvinus at varying times of the day. By coordinating these swellings and shrinkings, the leaves are able to orient themselves perpendicular to the sunlight in diaheliotropic plants. Leaf movements are the result of changes in turgor pressure in the pulvinus. The pulvinus is a small group of cells at the base of the

lamina of each leaflet. The reversible axial expansion and contraction of the extensor and flexor cells take place by reversible changes in the volume of their motor cells. These result from massive fluxes of osmotically active solutes across the cell membrane. K+ is the ion that is usually implicated in this process, and is balanced by the co-transport of Cl- and other organic and inorganic anions. While the mechanisms of diaheliotropic leaf movements have not been studied extensively, much data exists detailing nyctinastic movements. Several ions are believed to be involved in leaf movment. These include K+, H+, Cl-, malate, and other small organic anions. K+ is the most abundant ion in pulvini cells. Evidence suggests that electrogenic ion secretion is responsible for K+

uptake in nyctinastic plants. The transition from light to darkness activates the H+/ATPase in the flexor cells of the pulvinus. This leads to the release of bound K+ from the apoplast and movement of the K+ into the cells by way of an ion channel. This increase in K+ in the cell decreases the osmotic potential of the cells, and water than influxes into the flexor cells, increasing their volume. In Samanea, K+ levels changed four-fold in flexor cells during the transition from light to darkness. In a similar experiment, during hour four of a photoperiod, the extensor apoplast of Samanea had 14mM and the flexor apoplast had 23 mM of K+. After the lights were turned off, inducing nyctinastic movements, the K+ level in the apoplast rose to 72 mM in the extensor cells and declined to

10mM in the flexor cells. Therefore, it appears that swelling cells take up K+ from the apoplast and shrinking cells release K+ into the apoplast. In the pulvinus of Samanea saman, depolarization of the plasma membrane opens K+ channels (Kim et al). The driving force for the transport of K+ across the cell membranes is apparently derived from activity of an electrogenic proton pump. This creates an electrochemical gradient that allows for K+ movement. From concentration measurements in pulvini, K+ seems to be the most important ion involved in the volume changes of these cells. How then, is K+ allowed to be at higher concentrations inside a cell than out of it? Studies indicate that the K+ channels are not always open. In protoplasts of Samanea saman, K+ channels were closed when