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Figure 2. LIF spectrum of maize leaves exposed to 4 hours of UV-A radiation.

Figure 3. LIF spectrum of maize leaves exposed to 6 hours of UV-A radiation.

concentrations of flavonoids and anthocyanins, which absorb the UV radiation safely. The UV treatment is affected by photosynthetically active radiation (PAR) levels [13,14]. Whether the presence or absence of activated photomorphogenic, photosynthetic and photoprotective systems during the UV treatment affect plant sensitivity to UV radiation or not has not been investigated so far. For example, the possible role of photosynthetic process and of xanthophyll cycle activated by wavelengths > 500

Figure 4. LIF spectrum of maize leaves exposed to 8 hours of UV-A radiation.

Figure 5. LIF spectrum of maize leaves exposed to 10 hours of UV-A radiation.

nm has not been determined because UV irradiation is always been given in the presence of PAR [11].

UV-B radiation causes a multitude of physiological and biochemical changes in plants, including inhibition

Figure 6. LIF spectrum of maize leaves exposed to 2 hours of UV-B radiation.

Figure 7. LIF spectrum of maize leaves exposed to 4 hours of UV-B radiation.

of photosynthesis [15,16]. This inhibition is due to reduced levels of chlorophyll (Chl), chloroplast proteins such as Rubisco (ribulose-1,5-bisphosphate carboxylase/ oxygenase) and LHCII (light-harvesting chlorophyll a/b-binding protein of photosystem II) [16,17], and pho-

Figure 8. LIF spectrum of maize leaves exposed to 6 hours of UV-B radiation.

Figure 9. LIF spectrum of maize leaves exposed to 8 hours of UV-B radiation.

tosynthesis-related gene expression [16,18]. Allen et al. [19] reported that loss of Rubisco is a primary factor in UV-B inhibition of photosynthesis in oilseed rape. Further,

Figure 10. LIF spectrum of maize leaves exposed to 10 hours of UV-B radiation.

Table 1. The fluorescence intensity ratios of maize leaves exposed to UV-A with their respective standard errors of three replicates.

Table 2. The fluorescence intensity ratios of maize leaves exposed to UV-B with their respective standard errors of three replicates.

it was apparent that the UV-B induced reduction in Rubisco was greater in UV-sensitive, than in UV-resistant strains [16].

It has been reported that, in chloroplasts under illumination, the large subunit (LSU) of Rubisco is directly fragmented into two polypeptides by reactive oxygen species (ROS) [20,21]. Other authors have demonstrated that UV-B generated ROS induce photodamage to Rubisco [22], and that ROS cause proteolytic degradation of the LSU [23]. Thus, the generation of ROS is thought to be involved in UV-B induced degradation of Rubisco in rice. John et al. [24] reported that, in Arabidopsis, exposure to UV-B radiation induces expression of senescence-associated genes (SAGs), including SAG12, which encodes a cysteine protease [16,25]. This result suggests that the formation of some kind of protease may also be involved in the enhancement of Rubisco degradation in plants [16].

Singh et al. [10] studied the effects of UV-A radiation on the growth of maize plants and their fluorescence spectra. The emission spectra of the second leaf from the bottom side of each maize plant excited by 337 nm were obtained using spex 1680, 0.22 m double monochromator known as spexfluorolog made in USA. The spectra consisted of two peak bands in the blue-green region at 435 nm (F435) and 525 nm (F525) respectively and two peak bands in the red region at 684 nm (F684) and 740 nm (F740) respectively. The ratios of these peaks as (F435/F525), (F435/F684) and (F684/F740) were calculated in each case. The ratio of blue to red (F435/F684) and chlorophyll fluorescence (F684/F740) ratio were found be decreased. This might be due to the fact that the intensity of red peaks was increased due to reabsorption of light when the plants were treated with UV-A, and hence the ratios (F435/F684) and (F684/F740) were reduced [3,5,10].

LIF obtained from intact leaves under different conditions and their quantitative relation to stress detection has been discussed in detail by Theisen [26]. Fluorescence of plant tissues is considered a serious problem in immuno fluorescence microscopy investigation of signified tissues and tissues with vacuolar deposits of phenolic compounds and in some cases it has been eliminated by the use of strong reducing substances [3,5]. Oxidized chlorophylls that emit at 540 nm are released to vacuoles from chloroplasts [3,5]. Broglia [26] found a decrease of laser induced green fluorescence and increase of chlorophyll fluorescence in Vicia faba when epidermal tissue had been removed. This shows that the epidermis is both an important protection against UV radiation and a possible topological source of green fluorescence [3,5].

There are two potential primary mechanisms involved in UVB-induced physiological and biochemical damage. DNA lesions, such as cyclobutane pyrimidine dimer (CPD) and pyrimidine (6-4) pyrimidone photoproducts [(6-4) photoproduts], interfere with DNA replication and transcription [16,27]. The second mechanism is through modification of proteins by photo-oxidation, or by reactive oxygen species (ROS) and free radicals produced during photosensitization [16,22,27]. These modifications include cross-linking, aggregation, denaturation and degradation [16,27].

Gao et al. [2] investigated the effects of supplementary UV-B radiation on the growth, yield and seed qualities of maize under field conditions. Increased UV-B radiation caused a significant reduction in dry matter and yield, and affected seed quality as follows: protein, sugar and starch levels decreased, whereas lysine levels increased. Taken together, the results of this study suggest that the enhanced solar UV-B radiation as predicted by atmospheric models will result in reduction of growth and yield of maize crops in the future. This has been confirmed from this study that the enhanced UV-A and UV-B radiation causes a significant reduction in the growth of maize plants and hence in the yield in turn. However, to have proper protection, the excessive exposure of both UV-A and UV-B should be avoided to have better growth of yield. Also, it has been observed that maize can sustain certain level of UV-A, but excessive exposure will be dangerous to plants whereas exposure of UV-B of any level is harmful to plants.

4. ACKNOWLEDGEMENTS

We are grateful to the department of Physics and the department of Biological Sciences at the University of Namibia for laboratory space and equipment.

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