Effects of stimulated Ultraviolet Radiation on the growth of Maize Seedlings

Effects of stimulated Ultraviolet Radiation on the
growth of Maize Seedlings
Keywords:
UV radiation, Zea mays, Chlorophyll concentration, Growth Parameters.
ABSTRACT:
The effects of ultraviolet-A and ultraviolet-B radiation on the growth of maize
seedlings have been studied under controlled conditions. Maize (Zea mays) seeds
were planted and exposed to ultraviolet-A and ultraviolet-B radiation for 0-10 hours.
The results showed a decrease in the concentration of chlorophyll a and b for both
ultraviolet A and B as time of exposure increases. There is also a decrease in the
height, diameter of stem, and number of leaves in the seedlings exposed to UV-A and
UV-B with the increase in the time of exposure. The decrease in chlorophyll a and b
concentrations was more pronounced in plants exposed to UV-B. The study indicates
that UV radiation pose a serious threat to plants and this might lead to significant loss
of production or reduced quality of products in agricultural sectors.
098-103 | JRPS | 2012 | Vol 1 | No 2
This article is governed by the Creative Commons Attribution License (http://creativecommons.org/
licenses/by/2.0), which gives permission for unrestricted use, non-commercial, distribution, and
reproduction in all medium, provided the original work is properly cited.
www.plantsciences.info
Journal of Research in
Plant Sciences
An International Scientific
Research Journal
Authors:
Marius Hedimbi1
,
Natalia Naikaku1
and
Shyam Singh2
.
Institution:
1. Department of Biological
Sciences, University of
Namibia, P/Bag 13301,
Windhoek, Namibia.
2. Department of Physics,
University of Namibia,
P/Bag 13301, Windhoek,
Namibia.
Corresponding author:
Marius Hedimbi.
Email:
mhedimbi@unam.na/
mhedimbi@yahoo.com.
Phone No:
(+264) (61) 206 3425.
Fax :
(+264) (61) 206 3791.
Web Address:
http://plantsciences.info/
documents/PS0020.pdf.
Dates:
Received: 27 Jan 2012 Accepted: 08 Feb 2012 Published: 16 Jun 2012
Article Citation:
Marius Hedimbi, Natalia Naikaku and Shyam Singh.
Effects of stimulated Ultraviolet Radiation on the growth of Maize Seedlings.
Journal of Research in Plant Sciences (2012) 1(2): 098-103
Original Research
Journal of Research in Plant Sciences
JournalofResearchinPlantSciences An International Scientific Research Journal

INTRODUCTION
Ozone depletion has caused an increase in the
amount of ultraviolet-A and ultraviolet-B radiation
reaching the earth’s surface. UV-A has a Wavelength
with a range of 320-440 nm and UV-B has a wavelength
with a range of of 280-320nm. Since it has a shorter
wavelength, UV-B is the most damaging ultraviolet
radiation. The negative effects of UV-A and UV-B on
the growth of maize seedlings have been reported by
Singh et al., (1998), Singh (2000), and Van Kent and
Singh (2008). Numerous investigations have
demonstrated that the effect of UVB radiation
enhancement on plants includes reduction in grain yield,
alteration in species competition, susceptibility to
diseases and changes in plant structure and pigmentation
(Gao et al., 2004). UV-B radiation causes mutations in
plants by promoting the formation of cyclobutane
pyrimidine dimmers between adjacent pyrimidine bases
(Britt 1996).
Maize is one of the most important staple foods
in the world. Therefore the study of maize seedlings to
stimulated UV-A and UV-B light under laboratory
conditions is very important for assessment of the
responses of maize plants to this stress factors in the
field. This may be used to get an overview of what
happens when maize plants are growing in UV regions
under field conditions, and help in finding measures to
protect crops from increasing levels of UV-B and UV-A
radiations.
The objective of this study was to evaluate the
effects of UV radiation on the growth of maize seedlings.
The study evaluated the effects of UV-A and UV-B
radiation on the concentration of chlorophyll a and b in
maize leaves and whether the time of exposure to UV
light has a negative effect on the growth of maize
seedling.
MATERIALS AND METHODS
Growing maize
Fifteen pots were filled with sandy soil. The
depth of the seeds in the soil was kept at 1 cm and the
pots were left to stand under normal light during the day
and under a dark mode during night time by switching
the lights off. Each pot contained three seeds. The pots
were watered with 100ml of water every 3rd
day for two
weeks. The seedlings were divided into two groups. The
first group was used for the determination of chlorophyll
concentration, and the second group was used for health
parameter measurements. Each group was further
subdivided into three groups. One group was exposed to
normal light, another group was exposed to UV-A
(Philips, F40T, 12/B1, Actinic) and another group was
exposed to UV-B (Philips, TL. 40W/12RS, J2) lamb
lights. The exposure was done for 0-10 hours. Each
experiment was done using three replicates.
Extraction of chlorophyll a and b
Eight grams of fresh maize leaves were crushed
using mortar and pestle and mixed with eight milliliters
of 80% acetone. The mixture was left to stand for 20
minutes in the dark and the mixture was then filtered.
The solid particles were discarded and the residue
containing chlorophyll a and b was then transferred to
cuvettes and absorbance was measured with a
spectrophotometer. 80% acetone was used to serve as a
control, which was used to reset the spectrophotometer to
zero before reading the absorbance of the samples
(treatments). Absorbance of chlorophyll a was measured
at wavelength of 645nm and that of chlorophyll b was
measured at a wavelength of 663nm. The number of
chlorophyll was calculated using a modified formula that
was reported by Dere et al., (1998): Chlorophyll a (mg/g
leaf)=[(12.7×A663)-(2.6×A645)] and Chlorophyll b (mg/g
leaf)=[(22.9×A645)–(4.68×A663)].
Measurements of the health parameters
Parameters such as, the width of leaves, height of
the seedling, diameter of stem and dry mass of the plant
Hedimbi et al., 2012
099 Journal of Research in Plant Sciences (2012) 1(2): 098-103

were measured. The width and height were measured
using a ruler while the diameter was measured using a
vernier calliper. The dry mass was determined by putting
fresh seedlings in an oven at 90ºC for 48 hours or until
all the water has evaporated from the seedlings. All
measurements were done using three replicates.
Data Analysis:
A t-test was carried out to test the significance in
differences between means. The tests were done at
confidence interval of 95% and means were considered
to be significantly different when p<0.05.
RESULTS
Chlorophyll concentrations
There was a decrease in the number of
chlorophyll a and chlorophyll b in both in UVA and
UVB as the time of exposure increased (Figure 1). The
level of chlorophyll b was higher (p<0.05) than
Chlorophyll a after exposure to UV radiation.
Furthermore, the level of both chlorophyll a and b were
significantly lower (p<0.05) in seedlings exposed to
UV-B than those exposed to UV-A. There was a
significant decrease in the level of chlorophyll b
(p<0.05) as the time of exposure to both types of UV
radiation increased while there was no significant
difference (p>0.05) in the level of chlorophyll a as the
time of exposure to both types of UV radiation increased
(Figure 1).
Direct effect of UV radiation on the growth of maize
seedlings.
The results showed that the width of leaves,
diameter of the stem and height of the seedling decrease
as the time of exposure both to UV-A and UV-B
increases compared to the control that was not exposed,
whose parameters were higher (Table 1). The decrease in
health parameters appears to be more pronounced with
increased exposure to UV-B and less pronounced with
increased exposure to UV-A (Table 1). There was a
decrease in dry mass of maize seedlings with increase in
exposure to both UV-A and UV-B radiations (Figure 2).
There was no difference (p>0.05) in the dry mass of
plants exposed to UV-A and UV-B at 0, 6 and 8 hours of
exposure to UV radiation (Figure 2).
DISCUSSION
The chlorophyll concentration has been
decreasing as the time of exposure to UV-A and UV-B
radiation has been increasing. It has also been found that
there was more chlorophyll in the leaves exposed to
UV-A than the leaves exposed to UV-B (p<0.05).
Chlorophyll is the green pigment that is found in the
Journal of Research in Plant Sciences (2012) 1(2): 098-103 100
Figure 2: Percentage (%) dry mass of maize
seedlings after exposure to UV-A and UV-B
radiation for 0-10 hours. Means ± Standard errors of
three replicates are presented.
Figure 1. Concentration of chlorophyll a and b in
maize seedling leaves after exposure to UV-A and
UV-B for 0-10 hours. Means ± Standard errors of
three replicates are presented.

leaves of plants which absorbs sunlight from the
environment and uses its energy to synthesis
carbohydrates from carbon dioxide and water. A
decrease in chlorophyll content in plants is therefore
likely to lead the plant to become more susceptible to
diseases and poisoning by inorganic pollutants such as
arsenic. Britto et al., (2011) reported that uptake of
arsenic by plants plays an important role in transfer of
this toxic element into the food chain. Additionally
inorganic arsenic species are phytotoxic and the elevated
concentration of arsenic in the soil causes a significant
reduction in crop yield (Meharg, 2004, Britto et al.,
2011).
The reduction in health status of plant due to
ultraviolet radiation may cause increased uptake of heavy
metals by the plant (Upadhyaya et al., 2011). Heavy
metal ions, such as copper, play essential roles in many
physiological processes of plants. In trace amounts,
several of these ions are required for metabolism,
growth, and development (Upadhyaya et al., 2011).
However, problems arise when cells are confronted with
an excess of these vital ions or with non-nutritional ions
that lead to cellular damage (Tewari et al., 2006, Zhang
et al., 2008, Panda, 2008, Britto et al., 2011). Heavy
metal toxicity comprises inactivation of biomolecules by
either blocking essential functional groups or by
displacement of essential metal ions (Upadhyaya et al.
2011). In addition, auto-oxidation of redox-active heavy
metals and production of Reactive Oxygen Species
(ROS) by the Fenton reaction causes cellular injury
(Cobbett, 2003, Choudhury and Panda, 2005, Azevedo
and Azevedo, 2006, Upadhyaya et al., 2011).
In the parameters that were measured, the same
trend of a decrease in the parameters measured for both
UV-A and UV-B as the time of exposure increases were
found. The number of leaves of the seedlings where
found to be decreasing as the time of exposure increased
and it was also found that there were more leaves in the
seedlings that were exposed to UV-A than the ones that
were exposed to UV-B radiation. These finding could be
supported by the damage of the chlorophyll caused by
the UV-B. If most of the chlorophyll has been damaged,
there will be reduced absorption of the light for the
seedling to carry out photosynthesis. Therefore the
exposed plant will not grow as unexposed plant, leading
to decrease of the number of leaves in the exposed
seedlings. A reduction in leaves is likely to lead to high
production of stress proteins in plants in response to
increased exposure to UV radiation. Proteins which are
regulated by stress conditions (stress proteins) have been
observed in response to high and low temperatures,
salinity, droughts, and several other stress factors (Pareek
et al., 1997; De Britto et al., 2011).
Singh et al., (1998) did a fluorescence study of
maize irradiated by UV-A. The effects of UV-A on the
growth of maize (Zea mays) plants were investigated by
growing them under two different controlled conditions.
It was found that the growth of the plant was reduced
under UV-A treatment at an early age. A clear-cut
reduction in the growth of the plants was also observed
Table 1. Effects of UV-A and UV-B on the growth of maize seedlings.
Means ± Standard errors of three replicates are presented.
Width of leaves (cm) Stem diameter (cm) Height of the plant (cm)
Exposure time (h) UV-A UV-B UV-A UV-B UV-A UV-B
0 (control) 1.7±0.1 1.7±0.1 0.4±0.1 0.4±0.1 41 ±1.8 41 ±1.8
2 1.3±0.0 1.2±0.0 0.3±0.0 0.2±0.1 35.5±4.1 27.7±5.8
4 1.3±0.1 1.1±0.0 0.3±0.2 0.2±0.1 30.8±4.7 26.3±3.2
6 1.2±0.1 0.4±0.4 0.2±0.0 0.2±0.0 25.7±4.9 20.7±2.2
8 1.1±0.2 0.4±0.4 0.2±0.1 0.2±0.0 23.2±3.4 18.7±1.9
10 0.7±0.2 0.0±0.0 0.1±0.0 0.1±0.0 19.3±0.6 12 ±0.6

Journal of Research in Plant Sciences (2012) 1(2): 098-103 102
by Singh et al., (1998). Gao et al., (2004) carried out a
field experiment to study the effects of supplementary
Ultraviolet- B Irradiance on Maize Yield and qualities.
The field studies on UV-B radiation effect on plants have
been recommended to use the UV and
Photo-synthetically Active Radiation (PAR) irradiance
provided by natural light. The study by Gao et al., (2004)
reported the growth and yield responses of maize crop
exposed to enhanced UV-B radiation and the UV-B
effects on the seed qualities under field conditions.
Enhanced UV-B radiation caused a significant reduction
in the dry matter accumulation and the maize yield in
turn was affected. With an increased radiation, the
flavonoid accumulation in maize leaves increased and
the content of chlorophyll a and b of maize leaves were
reduced. The levels of proteins, sugar and starch of
maize seed decreased with enhanced UV-B radiation,
whereas the level of lysine increased with enhanced
UV-B radiation (Gao et al., 2004).
CONCLUSIONS
The study found that both UV-A and UV-B
radiations have a negative effect on chlorophyll content
and health parameter of maize seedlings. Such a decline
in health parameters of maize plants can lead to high
reduction in crop production, reduced grain yield and
increased susceptibility to diseases. With increase in
climate change, the devastating effect of ultraviolet
radiation on crops is likely to become more pronounced.
It is therefore important that farmers develop early
adaptation measures to mitigate crop losses due to
various factors of climate change. Development of maize
strains which are more tolerant to UV radiation damage
needs to be undertaken. The role of central governments
in launching awareness campaigns to educate farmers on
measures to be undertaken to protect their crops also
needs to be accelerated.
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