Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

Photooxidative Stress
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on the distribution of the photosynthetic cells, as well as density and location of
chloroplasts within the leaves for optimized capture of light energy (McDonald, 2003).
High light stress can induce photoinhibiton, photoactivation, photodamage and degradation
of photosynthetic proteins in plant cells (Deming-Adams and Adams, 1992;
Long and Humphries, 1994; Jiao et al., 2004). Such excess light conditions might arise
from high irradiance and in concert with other stressful conditions such as drought and
high or low temperatures.
3. PHOTOOXIDATIVE STRESS
The light dependent generation of active oxygen species is termed as photooxidative
stress (Foyer et al., 1994). During the life cycle of plants, they are exposed to varying
light environments and plants develop several acclimation responses. Evolution has
refined the photosynthetic apparatus for high photosynthetic efficiency in limiting
light with regulatory features to ensure that light intensities can be endured without
photodamage (Ort and Baker, 2002). Miyake and Vokata (2000) indicated that high growth
irradiance enhances the electron partitioning to O 2
at PSI. Golden variety of tropical fig,
Ficus microcarpa showed hypersensitivity to strong light as it lacks heat stable
dehydroascorbate reductase (DHAR), suggesting the crucial role of ascorbic acid (AsA)
regeneration system for the tolerance against high irradiance (Yamasaki et al., 1999).
Photorespiration also supplies electron acceptors to PSI and has a photoprotective role
against the damage due to strong illumination (Kozaki and Takeba, 1996). Thus, the
regulation of photosynthesis has been viewed as a dynamic balancing act in which
photoprotection is reversibly traded for photosynthetic efficiency (Ort, 2001). The ability
of plants to changing light environment allows them to achieve greater evolutionary
success by growing under high irradiance intensity. Such variations in light environment
may range from few seconds or minutes up to few or many days.
4.1. Singlet Oxygen ( 1 O 2
) Generation
4. REACTIVE OXYGEN SPECIES
Under optimal growth conditions, light energy absorbed by the leaves is primarily used
for carbon assimilation. However, when plants absorb more energy than is used in
photosynthesis, they are subjected to photooxidative stress (Foyer, 2002; Krieger-
Liszkay, 2005). Under such conditions, the light absorbed by the leaf can not be efficiently
used for photosynthesis and becomes potentially damaged because the excess
electrons react with the abundantly present oxygen. The relatively stable ground state
of oxygen in a triplet state with the unpaired electrons is not directly a problem. However,
under high light conditions, highly reactive singlet oxygen ( 1 O 2
) can be produced
by a triplet chlorophyll formation in the photosystem II (PSII) reaction center and in the
antennae systems. Thus, the chlorophylls, in addition to use light energy in photosyn-