Effect of UV Radiation on Crop Plants
Authors: Chongtham Allaylay Devi and Brij Bihari Pandey
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Ultraviolet radiation (UVR) is a type of solar radiation with wavelengths between 100 and 400 nm. It ends where the colours of rainbow start. The atmosphere absorbs all UV-C (< 280 nm), a significant part of UV-B (280 – 315 nm) but transmits most of the UV-A radiation (315 – 400 nm). UVR represents only about 7-9 % of total solar radiation reaching the biosphere, but unlike other types of solar radiation, UVR is highly energetic radiation. This means that UVR can cause reactions between molecules that are hit by such radiation.
Plants use sunlight for photosynthesis and, as a consequence, are exposed to the ultraviolet (UV) radiation that is present in sunlight. UV radiation is generally divided into three classes: UV-C, UV-B, and UV-A. The UV-C region of the UV spectrum includes wavelengths below 280 nm; these highly energetic wavelengths are effectively absorbed by ozone in the stratosphere and, thus, are not present in sunlight at the earth’s surface. UV-C wavelengths will be removed from the light reaching the earth’s surface so long as there is any ozone present (Caldwell et al., 1989). In contrast, UV radiation in the UV-B region, from 280 to 320 nm, does reach ground level. The UV-B portion of sunlight has received much attention in recent years because irradiation from this spectral region (especially 297 to 310 nm) will increase as the stratospheric ozone concentration decreases (Caldwell et al., 1989).
Currently, ozone decreases result from chlorofluorocarbon contamination of the stratosphere (McFarland and Kaye, 1992). UV wavelengths from 320 to 390 nm, which make up the UV-A region of the spectrum, are not attenuated by ozone, so their fluency will be unaffected by ozone layer reduction. Like all living organisms, plants sense and respond to UV radiation, both the wavelengths present in sunlight (UV-A and UV-B) and the wavelengths below 280 nm (UV-C). AI1 types of UV radiation are known to damage various plant processes. Such damage can be classified into two categories: damage to DNA (which can cause heritable mutations) and damage to physiological processes.
Plant Physiological Responses To UV-B Radiation
Many different plant responses to supplemental UV-B radiation have been observed. For instance, changes seen after supplemental UV-B radiation include biomass reductions, decreases in the percentage of pollen germination , changes in the ability of crop plants to compete with weeds, epidermal deformation, changes in cuticular wax composition and increased flavonoid levels .
These changes could result from any number of primary UV-B events: DNA damage, direct photosynthetic damage, membrane changes, protein destruction, hormone inactivation, signal transduction through phytochrome or signal transduction via a UV-B photoreceptor. To determine precisely which factor or factors are involved, the action spectrum and the kinetics of the response must be established. For example, direct DNA damage is detectable shortly after irradiation, whereas chalcone synthase (CHS) gene expression requires many minutes. Plants were kept in red light to isolate this response from the similar response through phytochrome (Baskin and lino, 1987), the action of phytochrome in this UV-6 response cannot be excluded. In the experiments with cucumber, shielding the actively growing tissues from UV radiation did not affect the magnitude of the decrease in hypocotyl length, so direct effects on cell division or elongation would not explain the UV-B-induced growth inhibition. Recovery after return to un inducing conditions was rapid, again suggesting a true photo-morphogenic response to UV-B.
Possible receptors for anthocyanin and flavonoid induction after UV-6 exposure have been analyzed in sorghum and parsley. The action spectrum for anthocyanin induction in sorghum has severa1 maxima, indicating that both phytochrome and a separate UV-6 photoreceptor are involved in anthocyanin induction. There is no peak in the blue region of the spectrum, ruling out the involvement of a blue light photoreceptor. The action spectrum peak (263 nm) for anthocyanin inhibition is photoreactivatible, and this inhibition does not involve
Plants are well ‘equipped‘ to use solar radiation efficiently
Plants, as primary producers, are fully dependent on solar radiation. Light is their source of energy, driving photosynthesis and directing plant development from germination to flowering. However, light is not just beneficial, but it can also exert warming and destructive effects. Therefore proper ’equipment‘ to exploit solar radiation efficiently without suffering the damage is crucial for plants. This holds especially true for radiation in the UV range.
DNA Damage and Repair in Plants
UV-C radiation has been used as a mutagenic agent in plants, and it is known to reactivate the maize Mutator transposable element (Walbot, 1992). To prevent mutation and/or cell death, UV radiation-induced DNA damage must be repaired before DNA replication. Repair of UV radiation-induced lesions may be of particular importance in plant pollen, especially in wind pollinated species (Jackson, 1987). DNA damage must also be repaired to allow transcription (Sauerbier and Hercules, 1978).
Although UV-C damage is not physiologically relevant for plants growing in the sun, short-wavelength (UV-C) radiation from germicidal lamps has often been used to study DNA damage in animals and bacteria, as well as in plants. UV-C has been used because DNA has a strong absorption maximum in the UV-C range (at 260 nm); UV-C photons are highly energetic, and high levels of damage can thus be created quickly. The best-studied UV radiation-induced DNA lesion is the cyclobutane-type pyrimidine dimer (CPD). Other types of DNA damage are the pyrimidine(6,4) pyrimidone dimer, diverse rare DNA photoproducts, and indirect types such as DNA-protein crosslinks and singlet oxygen damage (Peak and Peak, 1986).
UV radiation-induced DNA damage can be repaired by three mechanisms: photoreactivation, excision repair, or recombinational repair. CPDs can be repaired by all three methods, but the other UV radiation- induced DNA lesions can be repaired only by excision or recombinational repair.
UVR triggers production of UV-absorbing filters in plants
One of the most consistent morphogenic responses of plants to solar UVR is synthesis and accumulation of UV absorbing compounds. The diversity and complexity of these substances in plants has increased through evolution. UV-protective compounds in plants include mycosporine-like amino acids (MAAs), which are found in algae and variety of phenolic substances synthesised in vascular plants.
Phenolic substances (phenolics) are plant secondary metabolites comprising around 8000 naturally occurring compounds, possessing one common structural feature, a phenolic (aromatic) ring. Besides photoprotection, phenolics have many other functions: they provide defence against injury, infection and stress (frost, high temperatures, drought), protect plants against herbivory and the improve the survival of plants in soils rich with toxic metals.
UV absorbing compounds accumulate in the epidermal cells of leaves and act as selective sunscreens to reduce the penetration of UVR into the leaf tissue. At the same time, they do not affect the penetration of visible light, which is essential for photosynthesis. They work similarly to the sunscreens which humans use to protect our skin from UVR.
In many cases the production of UV-B absorbing compounds is not only dependent on the UV-B dose. Plants growing in open places, tropical and high altitude environments already contain high levels of these phenolics, and enhanced UV doses do not contribute to increased production.
UVR may affect plant growth
Different studies have shown that UVR induces diverse growth responses in plants. Many of these responses are found in alpine plant species. Scientists presume that alpine flora have adapted in this way partly due to enhanced UVR at high elevations. It has been argued that each of these architectural changes allows plants to efficiently scatter and reflect UVR, protecting their cells for damage. However, researchers are still debating whether this is really the case.
Indeed, some scientists have even argued that these UV-induced changes in plant shape are only to help the plant survive heat and drought. The reason for increased drought tolerance of UVR treated plants is morphogenetic changes (especially smaller leaves) which increase water use efficiency in plants, while phenolic compounds also protect tissues from damage.
Exposure of plants to UVR might alleviate negative impacts of other environmental constraints
Several studies have shown that plant treatment with UVR may increase the plant’s tolerance to drought and vice versa, plants that are more tolerant to drought are also likely to be more tolerant to UVR. UVR may also reduce plant infections with pathogens, since fungi and bacteria are generally more sensitive to damage by UVR than are higher plants. Moreover, many of UV-induced phenolic substances also have an antimicrobial activity.
The vision of pollinators and optical properties of flowers are a result of long lasting co-evolution
The vision of bees, butterflies and some other insects, for example, extends into the ultraviolet range. Therefore, many insects can see the accumulation of UV-absorbing pigments by plants. It appears that plants exploit animal vision in the UV range for advertising their flowers and vice versa, insects gained further adaptations to see flower patterns which are visible in UV range only. In many flowers ultraviolet light uncovers secret paths and "landing strips" that lead to delicious food. These markings are visible only to selected insects.
Thus, on the evolutionary scale the colour vision of some insects and the spectral properties of flowers have developed in to mutual plant – pollinator relationships. Some carnivorous plants, however, use these ultraviolet markings for a more sinister reason. They attract pollinators to their insect traps, by imitating the UV-visible patterns of flowers.
Plants provide protective substances for humans
Plants also produce many important protective substances and vitamins that are indispensable for them, but also benefit us, since our bodies are not able to synthesise them. The most important group of chemicals are the phenolics that exhibit a wide variety of beneficial biological roles, including antiviral, antibacterial, immune-stimulating, anti-allergic, anti-inflammatory, anti-carcinogenic and others. They are also powerful antioxidants scavenging reactive oxygen species and free radicals and can bind (chelate) with metal ions such as iron and copper, enabling our bodies to use these important micro-nutrients. Important sources of phenolics are different herbs (i.e. medical plants), fruits, vegetables, grains (i.e. buckwheat, wild rice), tea, coffee beans, bee pollen (propolis), and red wine.
UV might increase the amount of active substances in medical plants
Many studies have shown that enhanced UVR, especially UV-B radiation, increases the amount of active substances in many plant species. We have already mentioned the importance of different phenolics (i.e. flavonoids) and vitamin D, production of which is stimulated by UV-B. Vitamin D is also synthesised in plankton, which is then ingested by fish and can eventually become human food rich with vitamin D and beneficial to health. UVR stimulation has also been shown to increase plant production of different (phenolic) alkaloids, essential oils and terpenoids, that have known medicinal properties.
UVR enhances plant food quality
Human efforts to increased food production and to control plant production have changed basic environmental conditions for plant growth. Plant breeding has increased the yield of plant cultivars, which require irrigation and fertilisation during the entire growth season to ensure favourable harvests. Plants are also cultured in greenhouses to avoid different pests and weeds and to prolong the growing season. Because of long distance food transportation we often consume unripe fruits that are usually poor in phenolics and vitamins.
(1) If plants are bred to grow in a favourable environment they will lose the natural genetic adaptations needed to cope with adverse environmental conditions. Therefore, it may happen during drought that poorly ’equipped‘ plants will be more susceptible compared with plants growing under natural conditions
(2) Plants subjected to intensive breeding might have lower potential to produce beneficial phenolic substances following exposure to UV-B
(3) Many studies show a negative effect on food quality when the natural UVR dose is reduced in greenhouses. Culturing plants in greenhouses might have two adverse consequences: less radiation at visible wavelengths for photosynthesis and less or no UVR. This latter reduces the production of UV-induced phenolic substances.
Negative effects of enhanced UV-B on productivity are found in many agricultural plants, but rarely in plants from natural environments
The effects of enhanced UVR, especially UV-B radiation on plants, have been widely studied. The negative effects depend on the species and on the balance between potential damage and the induction of protective and repair mechanisms. As already mentioned, the most common response of field-grown plants to elevated level of UV-B is an increase in levels of different UV-absorbing phenolics.
Changes in metabolism affect the timing of seasonal changes in plant activity (phenology), together with biomass and seed production. Studies have also shown that UV-B radiation can cause damage to DNA and affect photosynthesis, respiration, water management, growth and development.
1. UV-B Radiation: From Environmental Stressor to Regulator of Plant Growth- Vijay Pratap Singh, Samiksha Singh, Sheo Mohan Prasad and Parul Parihar
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