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Effect of Lanthanum on Plants under Supplementary Ultraviolet-B Radiation: Effect of Lanthanum on Flavonoid Contents in Soybean Seedlings Exposed to Supplementary Ultraviolet-B Radiation
Available online at www.sciencedirect.com JOURNAL OF
RARE EARTHS www .elsevier,comllocateijre
JOURNAL OF RARE EARTHS 24 (2006) 613 - 616
Effect of Lanthanum on Plants under Supplementary Ultraviolet-B Radiation: Effect of Lanthanum on Flavonoid Contents in Soybean Seedlings Exposed to Supplementary Ultraviolet-B Radiation Liang Bin (% Q)' ' , Huang Xiaohua ( fk&+)2, Zhang Guangsheng ( Sk%&)', Zhang Feng (5% "$)I, Zhou Qing ( A *)' ( 1 . Key Laboratory of Industriul Bwtechnology , Ministry of Education, Southem Yangtze University, Wuxi 214036, China ; 2 . College of Chemistry and Environmental Science , Nanjing Normal University , Nanjing 21cK)97, China ) Received 24 September 2005; revised 10 April 2006
Abstract: The effect of La on flavonoids , chlorophyll contents, and phenylalanine ammonia-lyase (PAL) activity in soybean seedlings under supplementary ultraviolet-B radiation (UV-B , 280 320 nm) was studied. The results show that PAL activity, contents of flavonoids and chlorophyll in the plants pretreated with La (20 mg L - ) are higher than those in CK . UV-B radiation could result in an increase in flavonoid content and PAL activity, associated with a decrease in chlorophyll content. However, the increase in the range of PAL activity and flavonoid content in UV-B treatment are lesser than those in the La treatment. The changes of flavonoid contents and PAL activity in La + UV-B treatment are similar to those in UV-B treatment, and the increase in their range is higher than those in UV-B treatment. This shows that La can enhance the resistance of soybean seedling to UV-B radiation and alleviate the damage of UV-B radiation by increasing flavonoid content, chlorophyll content, and PAL activity.
-
- '
Key words: lanthanum; ultraviolet-B radiation ; soybean seedling; flavoniod ; rare earths CLC number: 0614.33; X503.231; 4946.8 Document code: A Article ID: 1002-0721(2006)05-613-04
-
The effects of solar ultraviolet.-B (UV-B, 280 320 nm) radiation on plants have been studied intensively over the last two decades, associated with the research on the biological impacts of stratospheric ozone depletion. There are a large number of reports about the effects of UV-B radiation on Flavonoid compounds, as secondary metabolites are considered to play a major role in protecting plants from UV-B damageL6'. These flavonoids generally absorb the light in the region of 280 320 nm and thus are capable of acting as UV filter, thereby protecting the photosynthetic tissues from damage.
-
*
we found that Ce can alIn the previous leviate the restraining effect of UV-B radiation on the growth of rape seedlings, and that this regulating effect is related to the improvement of photosynthesis resulting from Ce . Actually, the ecological course in plants is influenced by many physiological metabolisms. Consequently, we investigated the effect of La on the total flavonoid contents in soybean seedlings under UV-B radiation, aiming to reveal why RE can alleviate the damage of UV-B radiation on plants by regulating the synthesis of protecting compounds .
Corresponding author (E-mail: zhouqeco@yahoo. com.cn) Foundation item: Project supported b y the National Natural Science Foundation of China (20471030) and the Foundation of State Planning Committee (IFZ20051210)
Biography: Liang Bin (1979 - ) , Male, Master Copyright @ZOOS, by Editorial Committee of Journal of the Chinese
Rare Earths Society. Published by Elsevier B .V . All rights reserved.
JOURNAL OF RARE EARTHS, Vol. 24, No.4 , Aug . 2006
614
1 Materials and Methods 1.1 Plant culture The soybean ( Clycine nzux . ) seeds of "Kennong 18" were surface sterilized for 10 min with HgClz ( 0 . 1 % ) , and washed three times with deionized water. After being soaked for 4 h , the seeds were placed in a dish underlaid with three pieces of filter paper and germinated in the incubator at (25 i 1 ) 9:. When the length of the hypocotyl was about 2 cm, the seedlings were transplanted in plastic pots (diameter 10 cm , five plants per pot) filled with deionized water, under an illumination of 8 klx ( 12 h * d - ' ) . These were aired twice every day. When the first leaf developed, the seedlings were cultured in Arnon + Hoagland's solution. The nutrient solution was renewed every 3 d for pH stabilization. The seedlings that were 5-week-old were treated by RE and UV-B radiation.
1.2
Treatment
The optimum concentration of Lac& solution was 20 mg*L-', which was determined in the pre-experiments. The LaC13 solution was sprayed evenly on the leaves until drops began to fall. The same amount of distilled water was applied to another set, as the CK. After 48 h , half of the seedlings pretreated with La were placed under ultraviolet lamps. Enhanced UV-B radiation was performed using 40W UV-B lamps (produced by Nanjing Lamp Factory) hanging perpendicularly over the plants. The levels of UV-B radiation were 0. 15 and 0.45 W * m2, which were calibrated by an ultraviolet radiac (made by Photo-Electricity Instrument Factory of Beijing Normal University) The seedlings were irradiated for 5 h , from 1O:OO to 15:OO, for a total of 5 d . The height of lamps over the plants was adjusted to maintain consistent radiation intensity. There were sample sets in the experiments: controls ( sprayed with deionized water), La ( sprayed with LaC1, solution) , TI (irradiated with 0.15 W m-' UV-B) , T2 (irradiated with 0. 45 W m-' UV-B ) , La + TI (sprayed with hC13 solution and then exposed to 0. 15 W * m-2 UV-B radiation), and La + T2 (sprayed with Lac&solution and then exposed to 0.45 W-m-2 UV-B radiation). There were 3 replicates replicates for each set and 3 pots per treatment.
.
-
1.3 Determination Fresh samples of 0.5 g taken from the epicotyls were extracted in 10 ml of acidified methanol (metha-
nol-water-hydrochloric acid, 79:20 :1, V / V ) for UVB absorbing compounds, according to the Caldwell' s procedure"'. The hydrochloric acid used was 36% HC1. Extract absorbance at 300 nm was measured using a spectrophotometer (UV-3OOO; Hitachi , Japan) , and the absorbance was arbitrarily used for analysis. Chlorophyll content and PAL activity were determined according to Refs. [91 and [lo]. All data were analyzed with LSD test ( P c 0.05).
2 Results and Discussion 2.1 Effects of La on flavonoid contents in soybean seedlings exposed to UV-€3 radiation Flavonoids are a group of phenolics with strong absorbance in the short-wavelength region. As shown in Table 1 , the total flavonoid content in soybean seedlings with La treatment is increased by 34.3% compared to that with CK, and the La treatment is more beneficial since it aids in absorption of UV-B radiation and thereby shielding plant tissues from UV-B damage. Although flavonoid content is increased in UV-B treatment, the increased range is limited (17.9% 2 1 . 3 % ) . For the soybean seedlings with La + UV-B treatment, the flavonoid contents are increased by 47.4% and 28.3 % , respectively, higher than those with UV-B treatments. Furthermore, the difference in chlorophyll contents between La and UVB treatments indicates that although La and UV-B radiation can both increase flavonoid contents, La radiation actively enhances the pigment content and promotes the plant resistance to environmental stresses, whereas UV-B radiation is a passive stress for flavonoid production.
-
Table 1 Effects of La on flavonoid and chlorophyll contents, the PAL activity in soybean seedlings under UV-B radiation stress
0.3290 f 0.0187d (100.0)
1.82 to.& (100.0)
4.22*0.1%
La
0.4419* 0 . W b (134.3)
2.28*0.1ld (125.4)
4.79t0.11a (113.5)
TI
0.38&0*0.0368c (117.9)
2.09 t 0.17~ (115.1)
3.61 * 0.17~
La+Ti
0.4851+0.03%s ( 147.4)
2.39*0.2& (131.a)
3.88*0.22c (91.W)
TZ
0.3991 to.Cbl2c (121.3)
2.49 t 0. la (136.7)
3.37 t 0.17d (79.88)
La + T2
0.4221 t O.LM96b
2.96tO.25a (162.7)
3.82t0.24~ (90.43)
CK
(128.3)
(100.0)
(85.54)
h n g 3 et al . La on Fhvonoid Contents in Soybean Seedlings Exposed to Ultraviolet-B Radiation
2.2
Effects of La on PAL activity in soybean seedlings exposed to W - B radiation
Phenylalanine ammonia-lyase (PAL) is the key enzyme that catalyzes the first reaction of the flavonoid biosynthetic pathway'"' '*I. As it induces the key enzymes of the phenyl propanoid pathway, UV-B radiation leads to the accumulation of flavonoids. Compared with that of C K , PAL activity in La, UV-B, and La + UV-B treatments is increased by 25. 4% ( L a ) , 1 5 . 1 % ( T , ) , 36.7% ( T 2 ) , 3 1 . 6 % ( L a + T,) , and 6 2 . 7 % (La + T2) respectively. This shows that La treatment can effectively promote PAL activity in the plant and then enhance the resistance to UV-B radiation damage, as the increase in range with La + UV-B treatments is greater than that with UV-B. Furthermore, Table l suggests that the change of PAL activity is basically coincident with that of flavonoid content, and the correlation is not simply linear. Thus the increase of PAL activity is beneficial for accumulating flavonoid, however, PAL is not the key enzyme restricting the reaction velocity of flavonoid synthesis. The regulating effect of La on flavonoid synthesis is related to other key enzymes such as Chalcone Synthase (CHS) and the regulating sites in the flavonoid biosynthetic pathway are more than one.
2.3
Effects of La on chlorophyll contents in soybean seedlings exposed to UV-B radiation
Chlorophyll is the structural and functional substance of photosynthesis in plants. Mirecki et al. [13' proved that UV-B radiation could damage chloroplast stroma lamella and grana , hinder chlorophyll synthesis, and accelerate chlorophyll decomposition in plants. When compared with CK, chlorophyll content in the La treatment is increased by 1 3 . 5 % . Inversely, there is an obvious decrease in chlorophyll content of leaves exposed to UV-B radiation. In addition, the decrease in the range of chlorophyll in La + UV-B treatment is 8.06% ( T a + T 1 ) and 9.57% (La+T,), lower than that with UV-B treatment, where it is 14.46% ( T1) and 20.12 % (T2) . The results indicate that La can alleviate the damage of UV-B radiation at a certain degree, which is coincident with the results of our previous studies", I 4 I .
2.4
Dynamic effects of La on flavonoid contents in soybean seedlings under W - B radiation stress Dynamic changes of flavonoid contents in soybean
615
seedlings with six treatments during 5 d are shown in Fig. 1 ( a ) . Flavonoid contents in La treatment are higher than those in CK throughout the experiment. It suggests that La can increase the flavonoid content in plants and maintain it at a higher level. Flavonoid content in UV-B treatments began to increase from the 2nd day, which is possibly because of the time lag of plant response to stress. Furthermore, the decrease in the range of flavonoid content in treatment T2 is greater than that in lreatment T I , whereas the increased range in the UV-B treatment is lesser than that in the La treatment, indicating that the ability of soybean seedlings against UV-B radiation stress is limited"51. The trend of flavonoid content in La + UV-B treatment is similar to that in the UV-B treatment. However, the increased range in the treatment with La + UV-B is obviously greater than that in UV-B, and the increased range in La + Tl is greater than that in La + T2. The result shows that La can increase the resistance of soybean seedlings to UV-B through accumulation of flavonoids, and the protective effect of La on soybean seedlings is better at the TI level than at the T2level of UV-B radiation.
2.5
Dynamic effects of La on chlorophyll contents in soybean seedlings under UV-B radiation stress
Dynamic changes of chlorophyll content in soybean seedlings with six treatments during 5 d are shown in Fig. 1 ( b ) . Because the stroma lamella and grana of the photosynthetic apparatus are damaged, chlorophyll synthesis is restrained, and decomposition is accelerated by UV-B radiation[13]. During the experimental period with UV-B treatment, the chlorophyll content in leaves is constantly decreased. In addition, the inhibitory effect of UV-B is more at T2level than that at TI level, which agrees with the concentration-dependent rule. The trend of chlorophyll content in La + UV-B treatment is similar to that in UV-B treatment. However, the decrease range in La + W - B is obviously smaller than that in UV-B, and the decrease range in La + TI is lower than that in La + T2. This shows that La can allewiate the damage of UV-B radiation on stmma lamella and grana of chloroplasts, and prevent chlorophyll from decomposing, with better protective effect at a low level of UV-B radiation than at a high level. Chlorophyll content in the La treatment increased steadily and was higher than that in CK during the experiment, which indicates that La can protect photosynthetic apparatus and chlorophyll to a certain degree.
.
JOURNAL OF RARE EARTHS, Vol. 24, No.4 , Aug 2006
616
0
1
3
2
4
5
dd Fig. 1
ttd
Dynamic effects of La on content of flavonoids (a) and chlorophyll (b) in soybean seedlings under UV-B stress -0-CK; --.-La; -A-Ti; -A-La + T1; +T2; +-La + T2
3 Conclusions 1. La can increase chlorophyll content in soybean seedlings, alleviate the damage of UV-B radiation on photosynthetic apparatus and stroma lamella, and prevent the decomposition of chlorophyll. 2. La can alleviate the damage of UV-B radiation on the plant by enhancing the flavonoid content in it. Furthermore, there is a correlation between the increase of flavonoid content and PAL activity on account of La regulating, but it is not linear. 3. La can alleviate the damage of UV-B radiation on soybean seedlings by enhancing flavonoid content and inducing PAL activity. 4. According to the different changes of chlorophyll content in the seedlings with L a and UV-B treatments, it can be concluded that although both La and UV-B radiation can increase flavonoid content and induce PAL activity in the plant, the regulation mechanisms of the two treatments are different. La treatment actively enhances the pigment content for promoting the resistance of plants to environmental stresses, whereas UV-B treatment is a passive stress inducing flavonoid production and UV-B radiation adaptation. As for the effects of La on key enzymes such as CHS and other regulating sites in flavanone metabolism pathways, further research is needed.
References :
He D L, Wang C H, He Y H, et al. The effect of reduction of ultraviolet-B irradiance on the content of flavonoid in leaves of wheat [J] . Chinese Journal of Agromteomlogy, #103, 24(4): 32. ZhaoXL,ZhangRG,ZhengYF, etal. Effectsofenhanced W - B radiation on growth and physiological indicators of spinach [J] . Journal of Agm-Environment Science, B(1): 26. Hanns Fmhnuyer, Domthee Staiger. Ultraviolet-B radiatiowmediated respom in plants. balancing damage and pmtection [J].plant Physiology, 2003, 133(4): 1420. L i a n g C J , H u a n g X H , T a o W Y , e t a l . Effectsofrare earths on plant under elevated ultraviolet 2B radiation stress I. effects of cerium ( on growth and photosynthesis in rape d i n g s under elevated ultraviolet 2B radiation stress [J]. Journal of Rare Earths, W ,23(3): 351. Caldwell M M . Ihe effects of solar UV-B radiations (280 -315) on higher plants: implication of stratwpheric ozone reduction. Research in Photobiology [M] New Yo&: PlenumPress, 1 9 n . 335. Zhang Z L. Guide to Ekperiments of Plants F'hysiology [MI (in Chin.). Beijing: High Educational Press, 2003. 67. Liu Y G , Li H Y, Yang Q K. Study on the relationship between resistance of soybean and activity of PAL in leaves of soybean infected by cercospora sojina hara J 1. Soybean Science, 2002, 21(3): 195. Bmach6 M , Fant C, Bergkvist S W, et al. Molecular markers for W - B streas in plants: alteration of the q r e s sion of four classes of genes in Pisum sativum and the formation of hi& molecular mass RNA adducts [J l. Biochem. Biophys. Acta, 1999, 1447(2): 185. Jenkins G I. W and blue li&t signal d u c t i o n in h a bidopsis [J] Plant Cell Environ., 1997, 20(6): 773. Mirecki R M, Tenu~luraA H. Effects of ultraviolet-B irradiance on soybean [J]. Plant F'hysiol., 1984, 74(2): 475. Huang X H, Zhou Q , Zhang G S. Advances on rare earth application in pollution ecology [J] . J d of Rare Eaahs, 2005, B(1):5 . Zhao D, Reddy K R, Kadani V G, et al. Growth and physiological responses of &on ( Gxspiwn hirsuum L .) to elevated carbon dioxide and ultraviolet-B radiation under comlled environment mnditions [J] Plant Cell Envimn . , m ,26(5): 771.
m,
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Kakani V G, Reddy K R, 7hao D, et al. Field cmp mspnses to ultraviolet-B radiation [J] Agricultural and Forest Meteorology, 2003, 120: 191. Bilger W, Johnsen T, Schreiber U. W-excited chlomphyll fuoreacence as a tool for the assessment of W-protection by the epidermis of plants [J]. J. Exp. Botany, 2Mll. 52: 2007. Feng H Y , An L Z , Xu S J , et al. Effect of enhanced ultraviolet-B radiation on growth, development, pigments and yield of soybean (Glycine max) [J] . Acta Agronomica Sinica, 2001, 27(3): 319.
.
.
RARE EARTHS www .elsevier,comllocateijre
JOURNAL OF RARE EARTHS 24 (2006) 613 - 616
Effect of Lanthanum on Plants under Supplementary Ultraviolet-B Radiation: Effect of Lanthanum on Flavonoid Contents in Soybean Seedlings Exposed to Supplementary Ultraviolet-B Radiation Liang Bin (% Q)' ' , Huang Xiaohua ( fk&+)2, Zhang Guangsheng ( Sk%&)', Zhang Feng (5% "$)I, Zhou Qing ( A *)' ( 1 . Key Laboratory of Industriul Bwtechnology , Ministry of Education, Southem Yangtze University, Wuxi 214036, China ; 2 . College of Chemistry and Environmental Science , Nanjing Normal University , Nanjing 21cK)97, China ) Received 24 September 2005; revised 10 April 2006
Abstract: The effect of La on flavonoids , chlorophyll contents, and phenylalanine ammonia-lyase (PAL) activity in soybean seedlings under supplementary ultraviolet-B radiation (UV-B , 280 320 nm) was studied. The results show that PAL activity, contents of flavonoids and chlorophyll in the plants pretreated with La (20 mg L - ) are higher than those in CK . UV-B radiation could result in an increase in flavonoid content and PAL activity, associated with a decrease in chlorophyll content. However, the increase in the range of PAL activity and flavonoid content in UV-B treatment are lesser than those in the La treatment. The changes of flavonoid contents and PAL activity in La + UV-B treatment are similar to those in UV-B treatment, and the increase in their range is higher than those in UV-B treatment. This shows that La can enhance the resistance of soybean seedling to UV-B radiation and alleviate the damage of UV-B radiation by increasing flavonoid content, chlorophyll content, and PAL activity.
-
- '
Key words: lanthanum; ultraviolet-B radiation ; soybean seedling; flavoniod ; rare earths CLC number: 0614.33; X503.231; 4946.8 Document code: A Article ID: 1002-0721(2006)05-613-04
-
The effects of solar ultraviolet.-B (UV-B, 280 320 nm) radiation on plants have been studied intensively over the last two decades, associated with the research on the biological impacts of stratospheric ozone depletion. There are a large number of reports about the effects of UV-B radiation on Flavonoid compounds, as secondary metabolites are considered to play a major role in protecting plants from UV-B damageL6'. These flavonoids generally absorb the light in the region of 280 320 nm and thus are capable of acting as UV filter, thereby protecting the photosynthetic tissues from damage.
-
*
we found that Ce can alIn the previous leviate the restraining effect of UV-B radiation on the growth of rape seedlings, and that this regulating effect is related to the improvement of photosynthesis resulting from Ce . Actually, the ecological course in plants is influenced by many physiological metabolisms. Consequently, we investigated the effect of La on the total flavonoid contents in soybean seedlings under UV-B radiation, aiming to reveal why RE can alleviate the damage of UV-B radiation on plants by regulating the synthesis of protecting compounds .
Corresponding author (E-mail: zhouqeco@yahoo. com.cn) Foundation item: Project supported b y the National Natural Science Foundation of China (20471030) and the Foundation of State Planning Committee (IFZ20051210)
Biography: Liang Bin (1979 - ) , Male, Master Copyright @ZOOS, by Editorial Committee of Journal of the Chinese
Rare Earths Society. Published by Elsevier B .V . All rights reserved.
JOURNAL OF RARE EARTHS, Vol. 24, No.4 , Aug . 2006
614
1 Materials and Methods 1.1 Plant culture The soybean ( Clycine nzux . ) seeds of "Kennong 18" were surface sterilized for 10 min with HgClz ( 0 . 1 % ) , and washed three times with deionized water. After being soaked for 4 h , the seeds were placed in a dish underlaid with three pieces of filter paper and germinated in the incubator at (25 i 1 ) 9:. When the length of the hypocotyl was about 2 cm, the seedlings were transplanted in plastic pots (diameter 10 cm , five plants per pot) filled with deionized water, under an illumination of 8 klx ( 12 h * d - ' ) . These were aired twice every day. When the first leaf developed, the seedlings were cultured in Arnon + Hoagland's solution. The nutrient solution was renewed every 3 d for pH stabilization. The seedlings that were 5-week-old were treated by RE and UV-B radiation.
1.2
Treatment
The optimum concentration of Lac& solution was 20 mg*L-', which was determined in the pre-experiments. The LaC13 solution was sprayed evenly on the leaves until drops began to fall. The same amount of distilled water was applied to another set, as the CK. After 48 h , half of the seedlings pretreated with La were placed under ultraviolet lamps. Enhanced UV-B radiation was performed using 40W UV-B lamps (produced by Nanjing Lamp Factory) hanging perpendicularly over the plants. The levels of UV-B radiation were 0. 15 and 0.45 W * m2, which were calibrated by an ultraviolet radiac (made by Photo-Electricity Instrument Factory of Beijing Normal University) The seedlings were irradiated for 5 h , from 1O:OO to 15:OO, for a total of 5 d . The height of lamps over the plants was adjusted to maintain consistent radiation intensity. There were sample sets in the experiments: controls ( sprayed with deionized water), La ( sprayed with LaC1, solution) , TI (irradiated with 0.15 W m-' UV-B) , T2 (irradiated with 0. 45 W m-' UV-B ) , La + TI (sprayed with hC13 solution and then exposed to 0. 15 W * m-2 UV-B radiation), and La + T2 (sprayed with Lac&solution and then exposed to 0.45 W-m-2 UV-B radiation). There were 3 replicates replicates for each set and 3 pots per treatment.
.
-
1.3 Determination Fresh samples of 0.5 g taken from the epicotyls were extracted in 10 ml of acidified methanol (metha-
nol-water-hydrochloric acid, 79:20 :1, V / V ) for UVB absorbing compounds, according to the Caldwell' s procedure"'. The hydrochloric acid used was 36% HC1. Extract absorbance at 300 nm was measured using a spectrophotometer (UV-3OOO; Hitachi , Japan) , and the absorbance was arbitrarily used for analysis. Chlorophyll content and PAL activity were determined according to Refs. [91 and [lo]. All data were analyzed with LSD test ( P c 0.05).
2 Results and Discussion 2.1 Effects of La on flavonoid contents in soybean seedlings exposed to UV-€3 radiation Flavonoids are a group of phenolics with strong absorbance in the short-wavelength region. As shown in Table 1 , the total flavonoid content in soybean seedlings with La treatment is increased by 34.3% compared to that with CK, and the La treatment is more beneficial since it aids in absorption of UV-B radiation and thereby shielding plant tissues from UV-B damage. Although flavonoid content is increased in UV-B treatment, the increased range is limited (17.9% 2 1 . 3 % ) . For the soybean seedlings with La + UV-B treatment, the flavonoid contents are increased by 47.4% and 28.3 % , respectively, higher than those with UV-B treatments. Furthermore, the difference in chlorophyll contents between La and UVB treatments indicates that although La and UV-B radiation can both increase flavonoid contents, La radiation actively enhances the pigment content and promotes the plant resistance to environmental stresses, whereas UV-B radiation is a passive stress for flavonoid production.
-
Table 1 Effects of La on flavonoid and chlorophyll contents, the PAL activity in soybean seedlings under UV-B radiation stress
0.3290 f 0.0187d (100.0)
1.82 to.& (100.0)
4.22*0.1%
La
0.4419* 0 . W b (134.3)
2.28*0.1ld (125.4)
4.79t0.11a (113.5)
TI
0.38&0*0.0368c (117.9)
2.09 t 0.17~ (115.1)
3.61 * 0.17~
La+Ti
0.4851+0.03%s ( 147.4)
2.39*0.2& (131.a)
3.88*0.22c (91.W)
TZ
0.3991 to.Cbl2c (121.3)
2.49 t 0. la (136.7)
3.37 t 0.17d (79.88)
La + T2
0.4221 t O.LM96b
2.96tO.25a (162.7)
3.82t0.24~ (90.43)
CK
(128.3)
(100.0)
(85.54)
h n g 3 et al . La on Fhvonoid Contents in Soybean Seedlings Exposed to Ultraviolet-B Radiation
2.2
Effects of La on PAL activity in soybean seedlings exposed to W - B radiation
Phenylalanine ammonia-lyase (PAL) is the key enzyme that catalyzes the first reaction of the flavonoid biosynthetic pathway'"' '*I. As it induces the key enzymes of the phenyl propanoid pathway, UV-B radiation leads to the accumulation of flavonoids. Compared with that of C K , PAL activity in La, UV-B, and La + UV-B treatments is increased by 25. 4% ( L a ) , 1 5 . 1 % ( T , ) , 36.7% ( T 2 ) , 3 1 . 6 % ( L a + T,) , and 6 2 . 7 % (La + T2) respectively. This shows that La treatment can effectively promote PAL activity in the plant and then enhance the resistance to UV-B radiation damage, as the increase in range with La + UV-B treatments is greater than that with UV-B. Furthermore, Table l suggests that the change of PAL activity is basically coincident with that of flavonoid content, and the correlation is not simply linear. Thus the increase of PAL activity is beneficial for accumulating flavonoid, however, PAL is not the key enzyme restricting the reaction velocity of flavonoid synthesis. The regulating effect of La on flavonoid synthesis is related to other key enzymes such as Chalcone Synthase (CHS) and the regulating sites in the flavonoid biosynthetic pathway are more than one.
2.3
Effects of La on chlorophyll contents in soybean seedlings exposed to UV-B radiation
Chlorophyll is the structural and functional substance of photosynthesis in plants. Mirecki et al. [13' proved that UV-B radiation could damage chloroplast stroma lamella and grana , hinder chlorophyll synthesis, and accelerate chlorophyll decomposition in plants. When compared with CK, chlorophyll content in the La treatment is increased by 1 3 . 5 % . Inversely, there is an obvious decrease in chlorophyll content of leaves exposed to UV-B radiation. In addition, the decrease in the range of chlorophyll in La + UV-B treatment is 8.06% ( T a + T 1 ) and 9.57% (La+T,), lower than that with UV-B treatment, where it is 14.46% ( T1) and 20.12 % (T2) . The results indicate that La can alleviate the damage of UV-B radiation at a certain degree, which is coincident with the results of our previous studies", I 4 I .
2.4
Dynamic effects of La on flavonoid contents in soybean seedlings under W - B radiation stress Dynamic changes of flavonoid contents in soybean
615
seedlings with six treatments during 5 d are shown in Fig. 1 ( a ) . Flavonoid contents in La treatment are higher than those in CK throughout the experiment. It suggests that La can increase the flavonoid content in plants and maintain it at a higher level. Flavonoid content in UV-B treatments began to increase from the 2nd day, which is possibly because of the time lag of plant response to stress. Furthermore, the decrease in the range of flavonoid content in treatment T2 is greater than that in lreatment T I , whereas the increased range in the UV-B treatment is lesser than that in the La treatment, indicating that the ability of soybean seedlings against UV-B radiation stress is limited"51. The trend of flavonoid content in La + UV-B treatment is similar to that in the UV-B treatment. However, the increased range in the treatment with La + UV-B is obviously greater than that in UV-B, and the increased range in La + Tl is greater than that in La + T2. The result shows that La can increase the resistance of soybean seedlings to UV-B through accumulation of flavonoids, and the protective effect of La on soybean seedlings is better at the TI level than at the T2level of UV-B radiation.
2.5
Dynamic effects of La on chlorophyll contents in soybean seedlings under UV-B radiation stress
Dynamic changes of chlorophyll content in soybean seedlings with six treatments during 5 d are shown in Fig. 1 ( b ) . Because the stroma lamella and grana of the photosynthetic apparatus are damaged, chlorophyll synthesis is restrained, and decomposition is accelerated by UV-B radiation[13]. During the experimental period with UV-B treatment, the chlorophyll content in leaves is constantly decreased. In addition, the inhibitory effect of UV-B is more at T2level than that at TI level, which agrees with the concentration-dependent rule. The trend of chlorophyll content in La + UV-B treatment is similar to that in UV-B treatment. However, the decrease range in La + W - B is obviously smaller than that in UV-B, and the decrease range in La + TI is lower than that in La + T2. This shows that La can allewiate the damage of UV-B radiation on stmma lamella and grana of chloroplasts, and prevent chlorophyll from decomposing, with better protective effect at a low level of UV-B radiation than at a high level. Chlorophyll content in the La treatment increased steadily and was higher than that in CK during the experiment, which indicates that La can protect photosynthetic apparatus and chlorophyll to a certain degree.
.
JOURNAL OF RARE EARTHS, Vol. 24, No.4 , Aug 2006
616
0
1
3
2
4
5
dd Fig. 1
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Dynamic effects of La on content of flavonoids (a) and chlorophyll (b) in soybean seedlings under UV-B stress -0-CK; --.-La; -A-Ti; -A-La + T1; +T2; +-La + T2
3 Conclusions 1. La can increase chlorophyll content in soybean seedlings, alleviate the damage of UV-B radiation on photosynthetic apparatus and stroma lamella, and prevent the decomposition of chlorophyll. 2. La can alleviate the damage of UV-B radiation on the plant by enhancing the flavonoid content in it. Furthermore, there is a correlation between the increase of flavonoid content and PAL activity on account of La regulating, but it is not linear. 3. La can alleviate the damage of UV-B radiation on soybean seedlings by enhancing flavonoid content and inducing PAL activity. 4. According to the different changes of chlorophyll content in the seedlings with L a and UV-B treatments, it can be concluded that although both La and UV-B radiation can increase flavonoid content and induce PAL activity in the plant, the regulation mechanisms of the two treatments are different. La treatment actively enhances the pigment content for promoting the resistance of plants to environmental stresses, whereas UV-B treatment is a passive stress inducing flavonoid production and UV-B radiation adaptation. As for the effects of La on key enzymes such as CHS and other regulating sites in flavanone metabolism pathways, further research is needed.
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