PHYSICS

tri phosphate (ATP). A detailed descri ption of photosynthesis is
not within the scope of this book. It is important to note that the
absorbed energy can be released as heat or radiation such as
fluorescence. Chlorophyll a fluorescence emission represents only a
very small part (-2 - 5%) of the absorbed energy. The yield of
chlorophyll fluorescence depends on number of complex factors.
The relationshi p between in vivo chlorophyll fluorescence and
photosynthetic activity in plants can be used to study the effect of
various natural (e.g., high-intensity light, ultraviolet irradiation,
heat, water shortage, mineral deficiency) and anthropogenic
(e.g., agrochemical treatments, air pollutants, ozone, acid rains,
heavy metals) stresses on plants.
23.3.2. Phototropism
Phototropism is a light mediated alteration in the pattern of
plant growth due to a directional light stimulus. Growth toward a
source of light is called positive phototropism; growth away from
the source is termed negative phototropism. The tips of shoots
normally exhibit a positive phototropic response while roots are
negatively phototropic. There is considerable evidence that the
change in growth rate causing the alteration in direction is
controlled by a light induced differential in the concentration of
the phytohormone auxin (indole-3-acetic acid) across the shoot.
I.
plants are controlled or influenced by photoperiod (e.g., the onset
and breaking of bud dormancy in woody perennials, leaf abscission,
rooting of cuttings, seed germination, bulb and tuber formation,
development of frost resistance). Adults of the fruit fly Drosophila
emerge from the pupa only during the hours near dawn. In humans,
mental acuity varies with the time of day, as does body temperature,
sleep, wakefulness, and hormone levels.
23.3.4. Photomorphogenesis
Photomorphogenesis is a non-directional growth, development,
or differentiation process that is regulated by non-directional and
non-periodic light stimuli. These include effects on seed germination,
stem elongation, leaf expansion, chloroplast development,
and the synthesis of chlorophyll. Typically, plants are
responsive to wavelengths of light in the blue, red and far-red
regions of the spectrum and are controlled through the action of
several different photosensory systems. The photoreceptors for red
and far-red wavelengths are known as phytochrome (from the
Greek words for plant and color). Most plants have multiple phytochromes
encoded by different genes. Cryptochromes are blue
light receptors, two of which have been identified in plants.
23.3.5. Photomovement
23.3.3. Photoperiodism
Photoperiodism is a mechanism by which certain developmental
processes respond to a non-directional but periodic light stimulus.
Most seasonable events are triggered by the duration or pattern of
light/dark cycles. Maxima in various rate measurements continue to
recur roughly every 24 hours under constant illumination. Since the
period of the cycle was about (but not exactly) 24 hours, they are
called circadian ("about a day") rhythms. For example, the marine
dinoflagellate Gonyaulax polyedra emits bright flashes of light at
night, while during the day it emits much less light. Photoperiod is
used to signal seasonal changes in broad cross-section of plant
species at which time the growing apices is transformed from a
vegetative to a floral state. Various other developmental processes in
Photomovement is any motile response or its alteration induced
by light. The light-dependent behavior of freely motile organisms
such as algae and protozoa includes various light-induced movements
in space. The motility of organisms can be described using
well-known physical parameters: velocity (V), direction (r) and
trajectory (I). These parameters can be further subdivided to more
precisely characterize types of movements by organisms. For
example, velocity may be linear and angular. Both linear and
angular velocities may be constant or vary as a function of time.
Certain classes of linear or angular velocity may be divided
according to specific velocity limits inherent to a particular organism.
The light stimulus, in turn, is characterized by parameters
such as intensity (I), direction (s), the spectral composition
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