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 plants leaves are oriented perpendicular to the suns rays, thereby maximizing the interception of hotosynthetically 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 suns rays. Plants that exhibit paraheliotropic behavior usually do so at midday, when the suns 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 uring 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 t, 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 ortion 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. alaestinus 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. 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 amina 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 oordinating 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, nd 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 ovements, 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 on 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 the membrane potential was below -40mV and open when the membrane potential was depolarized to above -40mV. A voltage-gated K+ channel that is opened upon depolarization has been observed in every patch clamp study of the plasma membranes of higher plants, including Samanea motor cells and Mimosa pulviner cells.