Response of Seed Germination of Safflower to UV-B Radiation
Increased levels of solar ultraviolet-B (UV-B) have been measured at temperate and polar regions of the Earth surface as a consequence of decreases in the stratospheric ozone layer. Although, UV-B represents only a small part of the solar radiation reaching the earth, its impact on biological processes can be very deleterious, owing to its high energy level. Treatment of safflower (Carthamus tinctorious) seeds with artificial UV-B radiation caused changes in seed germination and seedling growth. In all treatments, UV light sped the germination of these seeds but the subsequent growth of the seedlings was markedly retarded. Pictures are presented to show this latter effect and the possible effects of UV-irradiation on seed germination are discussed. Therefore, continual exposure to UV-B could have similar effects on seeds. The hypocotyls become quite short and there is less above shoot biomass.
The stratospheric ozone layer is the principal agent absorbing ultraviolet
radiation in the Earths atmosphere and the thinning of this layer has
led to an increase in solar UV-B radiation (280-320 nm) reaching the Earths
surface. The increase in solar ultraviolet-B (UVB) radiation (280-320 nm) reaching
the earth's surface, because of depletion of the stratospheric ozone layer,
is a serious environmental concern (Madronich et al.,
1995). Assuming full compliance with the montreal protocol on Substances
That deplete the ozone layer United Nations Environment Program, Na. 87-6106),
O3 depletion was predicted to peak around 1998 and then slowly recover
by about 2045 (WMO, 1995). However, numerous researchers
have questioned the validity of the assumptions upon which these predictions
are based (Allen et al., 1998; Greene,
1995) and have expressed concern regarding the delayed recovery of stratospheric
O3 (Shindell et al., 1998). The adverse
effects of UVB radiation on biologically important molecules (e.g., nucleic
acids, proteins, pigments and lipids), plant morphology and physiological processes
have been extensively studied (Kovacs and Keresztes, 2002;
Koti et al., 2007; Jansen
et al., 1998; Runeckles and Krupa, 1994;
Liu et al., 2004).When plants are exposed to
UV-B radiation the enhancement of the secondary metabolite production has been
detected. In fact, an increase in flavonoids and anthocyanins has been reported
and it has been assumed that plants can use phytochemical defenses to avoid
this harmful radiation (Ruhland and Day, 2000; Staaij
et al., 2002). However, Pal et al. (1995)
found no influence of UVB radiation on the germination of seeds of several crop
species. plants accumulate unidentified UV-B absorbing compounds (reported as
absorbance at 300 nm) after irradiation with UV-B light. In this way, plants
respond to UV-B stress by synthesizing greater amounts of vacuolar metabolites
which can efficiently dissipate high levels of this radiation (Xiong
and Day, 2001).
High and rapid seed germination and vigorous seedling growth are important
for the establishment and competitive ability of plant species. Plant seeds
on the soil surface may get exposed to solar irradiance, including biologically
effective UVB radiation, before, during and immediately after germination. Although,
the shoot is the primary target of UVB radiation, the radicle may be exposed
to this radiation immediately after germination in the case of a seed located
on the soil surface. Little information is available on seed germination and
early-seedling growth response of oil-seed species to UVB radiation. In nature,
plant seedlings may be exposed to an enhanced level of solar UVB radiation for
a short duration and whether the damaging effect of this exposure on plant growth
continues in the low-UVB environment (e.g., under cloudy conditions) that follows
is not known. Safflower is one the most important oil- seed crop and despite
the potential impact of UV-B radiation on field performance and therefore, on
the overall productivity of this species, few studies concerning its response
to this environmental stress have been done (Correia et
al., 1998). Most of the reports describe the effects of UV-B on seedlings
and individual plants grown in pots. Other developmental stages like germination,
flowering and seed-set are less well documented. These stages in plant development
however, might prove to be susceptible to UV-B. In a study by Musil
(1995), dicotyledonous Asteraceae exhibited delayed flowering, decreased
flower production and reduced numbers of seeds set in response to elevated UV-B.
In addition, the latter study as well as several earlier studies (Flint
and Caldwell, 1984; Feder and Shrier, 1990), demonstrated
the UV-B sensitivity of pollen germination and pollen tube growth. It is reasonable
to assume that developmental stages may have differential UV-B sensitivity,
regarding that the maximal UV-B exposure of different developmental stages may
vary considerably (Teramura and Sullivan, 1994).
The objectives of this study were to investigate the influence of UVB radiation on seed germination and early-seedling elongation in Safflower.
MATERIALS AND METHODS
The experiment was conducted in Faculty of Science, Islamic Azad University, Mashad Branch, Iran (2008-2009). Safflower Seeds were placed on one 5.5 cm Whatman No. 1 filter disk on a 5.5 cm absorbent pad in a 6 cm open petri dish with a 5-mm-diam hole at the bottom. A completely randomized design with the three UVB treatments described previously was used. The treatments were given by exposing seeds to UVB radiation 2,4 and 8 h. UV radiation was provided by four fluorescent tubes (Philips TL 20W/12; 1220x1000 mm2) positioned 30 cm above Petri dishes. There were four replicates per UVB treatment. Each replicate consisted of four dishes with 25 seeds per dish. To each dish was added 20 mL of distilled water. Water lost because of evaporation was replenished daily. Seed germination was periodically recorded. Seedlings with ≥5 mm radicle length were considered germinated. The experiment was repeated twice.
Data reported are the Mean±SE of three independent experiments. Samples
from at least four plants were taken for each replication in all measurements.
Results were examined by ANOVA. Means in individual experiments were tested
for significance using lower-significant-difference (LSD, p≤0.05).
By 22 h, 7 vs. 8 seeds had germinated in the Safflower 2 h UV-irradiated vs. control dishes. The roots were approximately 22.5 vs. 16 mm long for the controls. However, hypocotyls were 12.7 vs. 23 mm for the controls. Furthermore, the roots of the UV treated were denser and more curly (Fig. 1a, b).
|| Safflower (a) 2 h UV-irradiated and (b) control
|| Safflower seeds (4 h treatment)
|| Safflower seeds (8 h UV-irradiated)
For the 4 h treatment, 10 vs. 8 had germinated by 22 h in the UV treated vs. control groups. The roots were as long as to control (approximately 16.2 vs. 16 mm). Again hypocotyls of the UV treated were shorter than the controls(19.7 vs. 23 mmfor the controls) (Fig. 2).
The same was found for the 8 h treatment. Here, by 24 h, 9 vs. 8 seeds had germinated in the UV treated vs. control groups. Again, the roots were denser and curlier and hypocotyls were far shorter than the controls (approximately 15 vs. 23 mm) (Fig. 3).
This study shows that UV-irradiation causes more rapid germination of seeds.
This is probably due to the fact that UV-B photons (280-320 nm) are more energetic
than visible light photons (>400 nm) and hence, have a stronger effect on
the surface of plant cells (Kovacs and Keresztes, 2002).
This causes the ultimate breakdown of seed coating allowing germination to occur.
Receiving 7.1 kJ/m2/day UV-BBE, seed germination of radish
was reduced by 26%, while germination of cucumber and bean was reduced by 23%
when compared with control plants receiving 0.2 kJ/m2/day UV-BBE
(Tevini et al., 1983). In contrast, Krizek
(1975) observed no adverse effects on the percentage of germination for
a range of crop species after 72 h of continuous exposure to UV-B irradiance.
Germination of Silene vulgaris seeds originating from a highland and
a lowland population was also unaffected by enhanced UV-B irradiance (Staaij,
1994). Musil (1995) reported enhanced germination
for several dicotyledonous and monocotyledonous arid-environment ephemerals.
However, once germinated the effects of continued UV-irradiation are damaging.
Thus, it has been shown by Day et al. (2001)
that exposure to UV-B reduced the vegetative growth of Antarctic vascular plants
(Colobanthus quitensis and Deschampsia antarctica). This was,
primarily, due to slower leaf elongation rates, which led to shorter fully expanded
leaves. Krizek (1975) reported no adverse effects on
dry weight accumulation of a variety of crop species after 72 h of continuous
UV-B irradiance. A marked decrease of dry weight was reported by Tevini
et al. (1983) for bean and cucumber seedlings.
In natural environments several factors may influence the maximal UV-B exposure of germinating seeds. First of all timing of germination is important. In the field, germination may occur largely in the dark, avoiding solar irradiance. Secondly, the position of the seed in the soil profile is important. Thirdly, vegetation density may markedly influence the UV-B climate at soil level. Even in the relatively sun exposed dune grassland ecosystem, germination is expected to occur at a relatively low UV-B level.
Therefore, we conclude that continual exposure to UV-B can have similar effects on seeds. The hypocotyls become quite short and there is less above shoot biomass. Hence, depletion of the ozone layer, which causes increased UV-irradiation, seems to be devastating to vegetative growth as it is to most all other cellular organisms. UV-B irradiated plants showed morphological, biochemical and genetic differences compared to non-irradiated plants. These findings support the idea that plants respond to UV-B radiation, using different strategies. The integration of molecular, chemical, physiological and biochemical information could provide some details about the actual responses to this radiation.
This work was financially supported by Islamic Azad University,Mashhad Branch (grant No. Ms project/1387). Thanks are due to Islamic Azad University,Science and Research Campus.
Allen, D., S. Nogues and N. Baker, 1998.
Ozone depletion and increased UV-B radiation is there a real threat to photosynthesis. J. Exp. Bot., 49: 1775-1788.PubMed |
Day, T.A., C.T. Ruhland and F.S. Xiong, 2001.
Influence of solar ultraviolet-B radiation on antarctic terrestrial plants results from a 4-year field study. J. Photochem. Photobiol. B., 62: 78-87.PubMed |
Feder, W.A. and R. Shrier, 1990.
Combination of U.V.-B and ozone reduces pollen tube growth more than either stress alone. Environ. Exp. Bot., 30: 451-454.Direct Link |
Flint, S.D. and M.M. Caldwell, 1984.
Partial inhibition of in vitro
pollen germination by simulated solar ultraviolet-B radiation. Ecology, 65: 792-795.CrossRef | Direct Link |
Greene, O., 1995.
Emerging challenges for the montreal protocol. Globe, 27: 5-6.Direct Link |
Jansen, M.A.K., V. Gaba and B.M. Greenberg, 1998.
Higher plants and UV-B radiation: balancing damage, repair and acclimation. Trends Plant Sci., 3: 131-135.CrossRef | Direct Link |
Koti, S., K.R. Reddy, V.G. Kakani, D. Zhao and W. Gao, 2007.
Effects of carbon dioxide temperature and ultra violet B and their interactions on soybean growth and development. Environ. Exp. Bot., 60: 1-10.CrossRef |
Kovacs, E. and A. Keresztes, 2002.
Effect of gamma and UV-B/C radiation on plant cells. Micron, 33: 199-210.CrossRef | PubMed | Direct Link |
Krizek, D.T., 1975.
Influence of ultraviolet radiation on germination and early seedling growth. Physiol. Plant, 34: 182-186.CrossRef |
Liu, Q., T.V. Callaghan and Z. Yuanyuan, 2004.
Effects of elevated solar UV-B radiation from ozone depletion on terrestrial ecosystems. J. Mountain Sci., 1: 276-288.CrossRef |
Madronich, S., R.L. McKenzie, M.M. Caldwell and L.O. Bjorn, 1995.
Changes in ultraviolet radiation reaching the Earth's surface. Ambio, 24: 143-152.
Musil, C.F., 1995.
Differential effects of elevated ultraviolet-B radi-ation on the photochemical and reproductive performances of dicotyledonous and monocotyledonous arid-environment ephem-erals. Plant Cell Environ., 18: 844-854.Direct Link |
Pal, M., A. Sharma and U.K. Sengupta, 1995.
Effect of UV-B radiation on germination and seedling growth in mungbean (Vigna radiata
L. Wilczek). Indian J. Plant Physiol., 38: 293-297.
Ruhland, C.T. and T.A. Day, 2000.
Effects of ultraviolet-B radiation on leaf elongation, production and phenylpropanoid concentration of Deschampsia antarctica
and Colobanthus quitensis
in Antarctica. Physiol. Plant., 109: 244-251.CrossRef |
Runeckles, V.C. and S.V. Krupa, 1994.
The impact of UV-B radiation and ozone on terrestrial vegetation. Environ. Pollut., 83: 191-213.Direct Link |
Shindell, D.T., D. Rind and P. Lonergan, 1998.
Increased polar stratospheric ozone losses and delayed eventual recovery owing to increasing greenhouse-gas concentration. Nature, 292: 589-592.Direct Link |
Teramura, A.H. and J.H. Sullivan, 1994.
Effects of UV-B radiation on photosynthesis and growth of terrestrial plants. Photosynth. Res., 39: 463-473.CrossRef | Direct Link |
Tevini, M., U. Thoma and W. Iwanzik, 1983.
Effects of enhanced UV-B radiation on germination, seedling growth, leaf anatomy and pigments of some crop plants. Z. Pflanzenphysiol., 109: 435-448.
Staaij, V.J.W.M., 1994.
The impact of elevated UV-B radi-ation levels on the reproductive biology of a highland and a low-land population of Silene vulgaris
. Doctorate Thesis, Vrije Universiteit, Amsterdam, The Netherlands.
Staaij, J.V., N.V.J. Bakker, A. Oosthoek, R. Broekman and A.V. Beem et al
Flavonoid concentration in three grass species and a sedge grown in the field and under controlled environment conditions in response to enhanced UV-B radiation. J. Photochem. Photobiol. Biol., 66: 21-29.CrossRef | Direct Link |
Scientific assessment of ozone depletion. World Meteorological Organization Global Ozone Research and Monitoring Projects, Report No. 37, WMO, Geneva, Switzerland.
Xiong, S.F. and A.T. Day, 2001.
Effect of Solar Ultraviolet-B Radiation during springtime ozone depletion on photosynthesis and biomass production of antarctic vascular plants. Plant Physiol., 125: 738-751.PubMed | Direct Link |
Correia, C.M., E.L.V. Areal, M.S. Torres-Pereira and J.M.G. Torres-Pereira, 1998.
Intraspecific variation in sensitivity to ultraviolet-B radiation in maize grown under field conditions. I. Growth and morphological aspects. Field Crops Res., 59: 81-89.CrossRef | Direct Link |