Involvement of K+ in Leaf Movements During Suntracking
Introduction
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
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 ...