Involvement Of K+ In Leaf Movements During Suntracking

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Term Paper Title	Involvement Of K+ In Leaf Movements During Suntracking
# of Words	2174
# of Pages (250 words per page double spaced)	8.7

Involvement of K+ in Leaf Movements During Suntracking

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 percepti...

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