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

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A.R. Reddy and A.S. Raghavendra
biophysical and molecular responses in plant cells under photooxidative stress. Special
emphasis is given on chloroplast processes under excess light regimes.
Plants are under stress when the available light is either in excess or limiting.
The present article deals only with the response of plants to excess light. Readers
interested in the topic of low light stress or phenomena such as sun-flecks may refer to
relevant reviews (Pearcy, 1998; Noctor et al., 2002). The phenomenon of photooxidative
stress develops not only under supra-optimal light but also at normal light when the
biochemical reactions are limited by sub-optimal levels of temperature, water or nutrition.
There are also several excellent reviews on the topic of photoinhibition and oxidative
stress (Foyer et al., 1994; Ort, 2001;Oquist and Huner, 2003) besides few books
(Pearcy, 1999; Das, 2004; Demmig-Adams et al., 2005)
2. LIGHT USE BY PLANTS
Light is the ultimate energy source for photosynthesis and it is also one of the most
deleterious environmental factors causing photooxidative stress in plants (Asada, 1999).
Less than 1% of the 1.3kWm -2 solar energy reaching the earth is absorbed by plant
tissues and is used in the synthesis of energy-rich biomolecules (Salisbury and Ross,
1992). It is estimated that 3 x 10 8 kJ of chemical energy derived from sunlight per year are
fixed globally in the form of 2x10 11 tons of fixed carbon. Photosynthesis is the only basic
energy-supplying process on the earth. Leaf photosynthetic capacity (rate of photosynthesis
per unit leaf area) differs greatly for species living in diverse habitats in both
tropical and temperate climates. Many crops have photosynthetic efficiencies ranging
from 0.1% to 3%, since only 0.83 kWm -2 (64%) of the total 1.3 kWm -2 radiant energy
reaching the earth is in the PAR region (McDonald, 2003). Plants are known to adapt to
a wide range of light environments ranging from deep shade of rain forests to high
radiation environments of deserts and mountain tops. The leaves of shade plants exhibit
morphological and anatomical features which differ from plants growing in sunlight.
Shade plants have more chloroplasts than sun leaves while the latter become
thicker than the shade leaves due to longer and for additional palisade cells (Bjorkman
and Powels, 1981). Mohr and Schopfer (1995) reported that tomato plants have hundred
stomata per mm 2 on the lower epidermis in low light. However, when the plants were
transferred to high light the leaves developed more number of stomata within three
days of a change in the light condition.
Plants have evolved mechanisms protecting against photodamage which include
chloroplast movements that reduce light exposure for the organelle and photosynthetic
complexes (Haupt, 1990) and leaf movement or paraheliotropism to avoid
light and heat (Ludlow and Bjorkman, 1984; Pastenes et al., 2004). Paraheliotropism is
known to result from an osmotic change at the pulvinus and this phenomenon confers
protection against photoinhibition and maintains leaf temperature well below air temperatures
(Assman, 1993). Light absorption can also be regulated at the tissue and
organelle level and accordingly, isobilateral and dorsiventral leaves are known based