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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. 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

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