PHYSICS

adiation arise by photochemical change in the epidermis due to
vasodilation of blood vessels seen as abnormal redness of the skin
or erythema.
Melanogenesis. When exposed to UV radiation, melanocytes in
the germinative layer of the skin produce melanin, which gets
absorbed into the surrounding cells. This creates a protective
barrier from UV radiation.
Antirickets action. Ultraviolet radiation takes part in the
conversion of 7-dehydrocholesterol to previtamin D3 in the skin,
which is thermally isomerized to vitamin D]" The vitamin affects
calcium absorption and the calcification of bones; insufficient
solar UV- B exposure causes rickets in children.
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Germicidal irradiation. This technology involves the application
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of ultraviolet radiation for the sterilization of microorganisms
(viruses and bacteria). Death is caused by breakage of bonds and
damage to the organism's DNA.
23.2.2. Terrestrial Plants
A number of physiological and developmental processes in
plants are adversely affected by UV-B radiation. The response to
UV- B radiation varies among species and cultivars of the same
species. As a consequence, it is necessary to use cultivars with
high UV-B tolerance and facilitate the breeding new ones.
Indirect affects induced by UV- B radiation include changes in
plant form, dry matter allocation within the plant, the timing of
developmental phases, and secondary metabolism, each of which
in certain situations can be more important than direct damage
caused by UV-B.
23.2.3. Aquatic Ecosystems
Exposure to solar UV- B radiation results in a significant
reduction in phytoplankton productivity and damage during the
early developmental stages of fish, shrimp, crab, amphibians and
other aquatic animals. The high levels of exposure to solar UV-B
radiation may play a role in phytoplankton distributions; even
small increases in UV-B exposure could result in significant
reduction in the size of the population of consumer organisms.
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23.3. EFFECT OF PHOTOSYNTHTICALLY ACTIVE
RADIATION ON LIVING ORGANISMS
Light is one of the most important environmental factors
affecting plants. Photosynthetically active radiation (PAR) is radiation
between 400 and 700 nm that is used by plants for
photosynthesis. In this region, leaves have weak reflectance
(15% maximum) and very low transmittance. Most of the radiation
is absorbed by the foliar pigments, primarily chlorophyll a
and b and to a lesser extent by carotenoids. The absorption maxima
of chlorophyll and the carotenoids overlap, such that the presence
of the carotenoids is masked in healthy green leaves.
However, if the chlorophyll concentration diminishes more rapidly
than that of the carotenoids due to stress or senescence,
the leaves become yellow. The primary photobiological reactions
in plants are photosynthesis, phototropism, photoperiodism, and
photomorphogenesis.
23.3.1. Photosynthesis
Photosynthesis is a complex series of light and dark reactions in
which light energy is converted into a stable form of chemical
energy. It involves the absorption of light by a pigment, energy
transfer, energy trapping or stabilization by reaction centres, and
initiation of chemical reactions from donor to acceptor molecules.
Two princi pal photochemical reactions are operating in series,
and the electron acceptor of one of them (reaction 2) is reoxidized
by the other reaction (reaction I), through a chain of electron
carriers. These two photochemical reactions are catalyzed by
two different reaction centres, each one collecting excitation
energy on its antenna consisting of ca. 300 chlorophyll molecules,
organized in several chlorophyll-protein complexes. The functional
unit consisting of a reaction centre and its antenna is defined
aphotosystem (PS). Photosystems I and II are the principal
functional units in the light reactions in plants and algae. The
transfer of the absorbed light energy from the photosystem reaction
centre is accomplished through a series of oxidation-reduction
reactions in which lead to the formation of reduced nicotinamide-adenine
dinucleotide phosphate (NADPH) and adenosine
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