High Light Stress Response and Tolerance Mechanism in Plants
Authors: Jyoti Chauhan*, Sunita chaudhary2, Rekha Sodani, and Seema

Introduction: As a primary source of energy, light is one of the most important environmental factors for plant growth and development. The intensity and quality of light are essential for the growth, morphogenesis and other physiological responses of plants (Rajapakse et al., 1992, Li et al., 2009). Excess light (EL) is the light absorbed by plants and algae that exceeds from their photosynthetic capacity. Although light is an necessary for diverse plant processes like seedling development, chlorophyll biosynthesis, photosynthesis (Marcus RA, Sutin N. 1985) phototropism (Jorge J. Casal, 2000) , photomorphogenesis ,light mediated circadian clock and metabolic fluxes, flowering, light mediated hormonal regulation of growth and development (Di Wit et al.,2016). Absorption of EL can lead to increased production of highly reactive oxygen inter intermediates (ROS) and by-products that can potentially cause photo-oxidative damage and inhibit photosynthesis (Niyogi KK. 1999). Environmental stresses generally decrease the maximum photosynthetic capacity of plants and algae .Thus, natural conditions such as drought, high salinity, nutrient deprivation, or temperature stress can influence and exacerbate EL stress (Demmig-Adams B.1992.). Photosynthetic organisms have evolved a variety of direct and indirect mechanisms for sensing EL.

The phototropins and neochromes play important roles in chloroplast avoidance movement. Phototropin, phytochrome cryptochrome and UV recptor (Eisinger et al., 2003) has important role in stomatal movement. The rhodopsins are of major importance in controlling phototaxis and photophobic movement on the basis of studies with the green algaChlamydomonas reinhardtii. In the model plant Arabidopsis thaliana, cryptochromes have been shown to control the expression of a large number of EL-responsive genes (Adamiec et al ., 2008).

Effect of high light intensity on plant growth and development

Seed germination: Higher plants have evolved a remarkable plasticity in their developmental pathways with respect to many environmental parameters. s eeds of many weed species need light to germinate (Noronha et al ., 1997), or may acquire this requirement only after being buried in the soil for a period of time. Light intensity and wavelength composition are important factors in determining the speed of cell growth, of pigment accumulation, and of plastid differentiation. In S. wantianshuea seed 100% light and deepest shading did not facilitate the germination while medium shading accelerated the seed germination and The specific leaf area increased with increasing shading in a certain range, and peaked under 4.2% light. (Yan XF & Cao M, 2007). Yan X F( 2007) reported that the seed germination rate and germination index were the highest at 55.4% natural sunlight, declined with decreasing light intensity, and were the lowest at 0.3% natural sunlight. The germination parameters revealed that germination was higher in seeds of A. cruentus and C. Olitorus under light while the seeds of D. regia germinated more in the dark (Ologundudu et al., 2005).

Vegetative stage: Vegetative stage of the plant is important stage for synthesizing photosynthates product and fitness of the plant survivality under different varying environment condition. Effect of light intensity over the range 200, 500, 1000, 1750, ft-c in marquis wheat resulted in higher rates of leaf initiation, emergence, and 2500 and expansion, and increases in breadth and thickness, but a decrease length. The greatest area was formed at 1000–1750 day length from 8 to 24 hours increased leaf length, breadth, and area (Friend et al, 1962). Internode length and the number of cells in broad bean are gradually further reduced as the intensity is raised to 1,000 f.c. Cell length is reduced inthe first (basal) internode by 10 f.c. and above, but in the second is reduced only at 1,000 f.c ( R. D. Butler 1961). In a spring variety of wheat an increase in light intensity over the ft-c reduced the length of the lamina by reducing both  (range 200 to 5000) the number and length of epidermal cells.

Photosyntesis : Usually, the increases in net photosynthesis rate (Pn) correlates with increases in light intensity. However, high light intensity resulted in decreases of net photosynthesis rate (Bowes et al., 1971). Exposure of photosynthetic organisms to strong light results in inhibition of the activity of photosystem II (PSII) when the rate of absorption of light energy by photosynthetic pigments exceeds the rate of its consumption in chloroplasts, the absorbed light energy accelerates the process of photoinhibition (Powles, S.B. 1984). High light accelerate photoinhibition which can reduce photosynthesis process by inhibition of PSII activity, destruction of D1 protein and by generation of ROS species (oxidative stress). When photo inhibition activity is more than repairing mechanism than there is significantly affect on biomass production.

A consistent response to growth in high light is observed when compared with low light that growth in high light leads to increases in levels of PSII, cytochrome b/f complex, ATP synthase, and components of the Calvin cycle (especially ribulose-1,6-bisphosphate carboxylase/ oxygenase, Rubisco), while there are reductions in the levels of the major chlorophyll a/b-binding light-harvesting complexes associated with PSII (LHCII); these changes are reflected in increased capacities for oxygen evolution, electron transport and CO₂ consumption, and an increased ratio of chlorophyll a to chlorophyll b (Chl a/b) (Bailey et al., 2001).

Morphological changes: Low light levels may lead to increase in specific leaf area (SLA) and plant height. These adaptations maximize the capture of the available light, meeting the demand for photosynthesis (Steinger et al., 2003). Whereas, high irradiances are related to many acclimating morpho-physiological characteristics, such as reduction in specific leaf area (SLA) in order to protect the plant from high irradiance; increase in leaf thickness, due to the quantity of layers or growth of palisade tissue; deep development of spongy layer. These measures prevent or mitigate light damage caused by excessive light energy, ensuring the proceeding of photosynthesis.

Metabolism: Under photoautotrophic conditions growth rate is light dependent, suggesting that cell division is an effective sink for the excess reductant produced under HL. In response to HL exposure, there is an increase in pools of amino acids and metabolites involved in nitrogen metabolism, vital for feeding the enhanced growth rate. Ledford et al. (2004) reported for a-tocopherol (vitamin E), where accumulation was only noted 6 h following a shift into HL in Chlamydomonas. Cultures exposed to HL for 30 min excreted complex sugars into the surrounding medium, and two of the sugars were putatively identified as glycerol and sorbitol .

High light stress coping mechanism in plants:

Chloroplast movement : Plants are generally rooted in place, but they exhibit several well-known movement responses to light, such as phototropism. At a cellular level, chloroplasts move to the sides of a plant cell so that they are positioned parallel to the direction of incident light to avoid absorption of EL and thereby minimize photodamage. This response is called chloroplast avoidance movement, and it is mediated by blue light and a blue light photoreceptor, phototropin (Wada et al., 2003). In most plants chloroplast movement is a typical organellar response that is regulated by light. During photo-relocation, the chloroplasts situated along the periclinal cell walls, optimizing their potential to harvest sufficient sunlight for optimal photosynthesis under low-light conditions. Under high light, the chloroplasts move away from the periclinal walls and toward the anticlinal walls, minimizing potential. Kasahara et al., (2002) showed recently that in Arabidopsis mutants CHUP1 in which the chloroplasts do not redistribute normally in response to light intensity, the plastids suffer severe photodamage and become necrotic under continuous high light.

Stomatal response: Stomatal opening is induced by light, including blue and red light, and distinct mechanisms underly stomatal opening in response to these different wavelengths . Blue light acts as a signal and red light as both a signal and an energy source (Zeiger E. 1983). High light intensity during day time increase transpiration rate to reduce heat load. However long time effect reduces water potential which can reduce stomatal conductance and partially closing of stomata reduce transpiration rate (Midday depression).

Photoacclimation : Photosynthetic organisms have evolved a number of photoprotective mechanisms in response to excess light that involves both short-term and long-term responses, together involved in a process called photoacclimation (Anderson et al., 1995). Collectively, these nested, photoprotective responses ultimately attempt to prevent the production of reactive oxygen species (ROS) that can induce substantial photooxidative damage in photosynthetic cells (Li et al. 2009). Short-term photoacclimatory responses include energy dissipation via interconversion of carotenoids of the xanthophylls cycle (Demming-Adams &Adams 1996) and state transitions, where the antenna system is redistributed betweenthe photosystems. Both can also lead to a reduction, the Qa redox state of PSII in response to excess light exposure. Cyclic electron transport around PSI also alleviates some of the excess pressure on PSII (Niyogi 1999). Long-term acclimation mechanisms initiated by plants and green algae generally require changes in gene expression that ultimately reduce the organism’s light-harvesting capacity. Responses such as reduction in LHC antennae size, the PSI to PSII ratio, and the total number of reaction centres and LHC antennae are the result of transcriptional and translational regulation of the proteins making up these specific complexes. In addition to changes in gene expression, long-term responses include an increase in antioxidant metabolism, such as the accumulation of the lipid-soluble antioxidant a-tocopherol, more commonly known as vitamin E (Ledford et al. 2004).

Photoprotection : Oxygenic photosynthetic organisms have evolved multiple photoprotective mechanisms to cope with the potentially damaging effects of light. Some algae and plants avoid absorption of excessive light by movement of leaves, cells (negative phototaxis), or chloroplasts. Within the chloroplast, regulation of photosynthetic light harvesting and electron transport balances the absorption and utilization of light energy. For example, adjustments in light-harvesting antenna size and photosynthetic capacity can decrease light absorption and increase light utilization, respectively, during relatively long-term acclimation to excessive light. Alternative electron transport pathways and thermal dissipation can also help to remove excess absorbed light energy from the photosynthetic apparatus. Numerous antioxidant molecules and scavenging enzymes are present to deal with the inevitable generation of reactive molecules, especially reactive oxygen species (Ort D R 2001).

Antioxidant defence against ROS: Different antioxidant enzyme as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (POD), GPX, GST and glutathione reductase (GR) are generally elevated under hight light stress to cope oxidative stress (Asada K. 2006).

Conclusion : Excess light (EL) can be sensed directly by photoreceptors or indirectly through biochemical and metabolic signals. Photoreceptors mediate chloroplast avoidance movement, some changes in gene expression, and photophobic responses in algae. Because EL perturbs photosynthesis, responses in the chloroplast such as qE (pHdependent regulation of photosynthetic light harvesting) and several retrograde signalling pathways from the chloroplast to the nucleus have important roles in EL acclimation. Reactive oxygen species (ROS) generated as by-products of photosynthesis, especially singlet oxygen and H₂O₂, are signals involved in responses to EL. Integration of multiple signals and coordination with other environmental stress signal transduction pathways are important for acclimation to EL.

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About Author / Additional Info:
I am persuing ph.D degree from BHU varansi with UGC fellowship