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Intraspecific responses in crop growth and yield of 20 soybean cultivars to enhanced ultraviolet-B radiation under field conditions
Field Crops Research 78 (2002) 1–8
Intraspecific responses in crop growth and yield of 20 soybean cultivars to enhanced ultraviolet-B radiation under field conditions Li Yuana,b,*, Zu Yanqunb, Chen Jianjunb, Chen Haiyanb a
The Center for Agricultural Biodiversity Research and Training of Yunnan Province, Yunnan Agricultural University, Kunming 6502011, PR China b Eco-environment Research Institute, College of Resources and Environment, Yunnan Agricultural University, Kunming 6502011, PR China
Received 22 October 2001; received in revised form 15 April 2002; accepted 18 April 2002
Abstract Field studies were conducted to determine the potential for intraspecific responses in crop growth and grain yield of 20 soybean cultivars to enhanced ultraviolet-B (UV-B, 280–315 nm) radiation. The supplemental UV-B radiation was 5.00 kJ m2, simulating a depletion of 20% stratospheric ozone at Kunming (258N, 1950 m). Out of the 20 soybean cultivars tested, 17 and 15 showed significant change in plant height at 80 DAP (days after planting) and ripening stages, respectively. Sensitivity in plant height was greater at 80 DAP than at ripening. The plant height of 3 cultivars increased, and that of 17 cultivars decreased. Under UV-B radiation, LAI (leaf area index), biomass and grain yield decreased, respectively. The greatest percent decrease was 95.7, 93.9 and 92.8, respectively. RI (response index) was the sum of percent change in plant height at ripening, LAI, biomass and grain yield. The results showed that all 20 soybean cultivars had a negative RI, indicating inhibition by UV-B radiation on soybean growth. The RI of 6 tolerant cultivars was higher than 163.1 and 5 out of 6 originated from south China (low latitude). The RI of the most tolerant cultivars, Yunnan 97801, was 72.4. Meanwhile, the RI of 5 sensitive cultivars was lower than 256.9 and 4 out of the 5 originated from north China (high latitude). The RI of the most sensitive cultivar, Huanxianhuangdou, was 295.7. These UV-B tolerant cultivars identified in this study might be useful in breeding programs. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Soybean; Stratospheric ozone depletion; UV-B radiation; Response index; Crop growth; Grain yield
1. Introduction The rapid decline in stratospheric ozone concentrations has been confirmed by satellite measurements (Molina and Molina, 1992). The most pronounced thinning of the ozone layer has been measured over
*
Corresponding author. Present address: College of Resources and Environment, Yunnan Agricultural University, Kunming 6502011, PR China.
the Antarctic continent with up to 71% depletion during the Antarctic spring (Kerr, 1993). Recent mathematical models predict a further increase in solar ultraviolet-B (UV-B) irradiation in future years (Madronich et al., 1995). UV-B effects on plants have been the subject of considerable research; approximately 600 papers have been published (Caldwell et al., 1998). An examination of more than 200 plant species reveals that roughly 20% are sensitive, 50% are mildly sensitive or tolerant and 30% are completely insensitive to UV-B radiation (Teramura, 1983).
0378-4290/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 4 2 9 0 ( 0 2 ) 0 0 0 8 4 - 9
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Whilst the impact of enhanced UV-B radiation on plant physiology, morphology, growth, nutrition and biomass has been investigated extensively (Li et al., 1998, 1999; Yue et al., 1988), little is known about intraspecific responses of plants to enhanced UV-B. Intraspecific responses in maize and wheat (Biggs and Kossuth, 1978; Li et al., 2000a,b), broad bean (Bennett, 1981), cucumber (Murali and Teramura, 1986), soybean (Teramura and Murali, 1986; Sullivan and Teramura, 1990; Teramura et al., 1990a; D’surney et al., 1993), rice (Teramura et al., 1991; Barnes et al., 1993; Dai et al., 1994) have been reported. Plant species and even genotypes within species can differ greatly in their responses to UV-B. Reasons for this are not clear (Caldwell and Flint, 1994). Due to our lack of understanding of the basis of intraspecific responses to UV-B radiation, further studies on its importance would be worthwhile. Most of the UV-B research in the past two decades has been conducted as short-term experiments in growth chambers and greenhouses, where an unnatural spectral balance of radiation may have substantially changed plant sensitivity to UV-B. It is important in experiments to maintain a realistic balance between various spectral regions, since both UVA (315–400 nm) and visible (400–700 nm) radiation can have ameliorating effects on responses of plants to UV-B (Caldwell et al., 1995). In growth chamber and greenhouse experiments, the visible and UV-A radiation is usually much less than in sunlight; thus, even if realistic levels of UV-B are used in simulating ozone reduction, the plant response may be exaggerated relative to field conditions. Unfortunately, only 15% of the studies have been conducted under field conditions. While the laboratory and glasshouse studies provide information on mechanisms and processes of UV-B action, only field studies can provide realistic assessments of what will happen as the stratospheric ozone layer thins (Caldwell et al., 1995). Vegetation communities function differently to isolated plants and the behavior of the former is not easily predicted from the latter. Besides, the responses of soybean crops to UV-B are more complicated than those of plants isolated (Teramura and Murali, 1986; Sullivan and Teramura, 1990; Teramura et al., 1990a,b; D’surney et al., 1993). Community-level field experimentation is needed to evaluate realistic
consequences of increased solar UV-B resulting from ozone reduction. Soybean is one of the major world food crops. The effects of enhanced UV-B radiation on photosynthetic characteristics, growth, morphology, intraspecific responses, total biomass and yield have already been studied (Sullivan and Teramura, 1990; Teramura, 1980, 1983; Teramura and Murali, 1986; Teramura et al., 1990a,b; Lydon et al., 1980; D’surney et al., 1993). Unfortunately, only few studies have been conducted under field conditions (Teramura et al., 1990a). In this study we grew 20 soybean cultivars in the field under ambient and supplemental levels of UV-B radiation for a season with the objective of evaluating intraspecific responses in crop growth and yield. We hypothesized that enhanced UV-B radiation would affect crop growth and yield and result in intraspecific responses under field conditions.
2. Materials and methods 2.1. Plant materials and growth conditions The field experiment was conducted at Yunnan Agricultural University, Kunming, China. No fertilization was needed during the season. Seeds of 20 soybean (Glycine max) cultivars were obtained from Yunnan Academic of Agricultural Sciences, Gansu Academic of Agricultural Sciences and Henan Academic of Agricultural Sciences. Seeds were sown in rows spaced 0.4 m apart at a density of 50 seeds m2 in 120 plots of 2:0 1:0 m2 each on 10 April 1999. Five border rows were sown round each plot in order to minimize heterogeneity in microclimate. The overall experimental design was a randomized complete block with two UV-B treatments and three replications. At 30 DAP (days after planting), plants were thinned to 40 m2 for uniformity in growth. This planting density is within commonly used sowing practices for the Kunming region. 2.2. UV-B radiation Supplemental UV-B radiation was provided by filtered Gucun brand 30 W sunlamps (Gucun Instrument Factory, Shanghai, China) following the
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procedure outlined in Lydon et al. (1986). Lamps were suspended above and perpendicular to the planted rows (rows oriented in an east–west direction to minimize shading) and filtered with either 0.13 mm thick cellulose diacetate (transmission down to 290 nm) for supplemental UV-B radiation or 0.13 mm polyester plastic films (absorbs all radiation below 320 nm) as a control (Sullivan and Teramura, 1990). Cellulose diacetate filters were presolarized for 8 h and changed weekly to ensure uniformity of UV-B transmission. The spectral irradiance from the lamps was determined with an Optronics Model 742 (Optronics Laboratories, Orlando, FL, USA) spectroradiometer. The spectral irradiance was weighted with the generalized plant response action spectrum (Caldwell, 1971) and normalized at 300 nm to obtain UVBBE. Six lamps were installed above each plot. Plants were irradiated for 7 h daily from 30 DAP to ripening stage, centered around solar noon. Plants under polyester-filtered lamps received only ambient levels of UVB radiation (10.00 kJ m2 UV-BBE during clear sky conditions on the summer solstice). Plants beneath the cellulose diacetate filters received ambient plus supplemental levels of UV-B. The lamp height above the plants was adjusted weekly to maintain a distance of 0.45 m between the lamps and the top of the canopy, and provided supplemental irradiances of 5.00 effective kJ m2 UV-BBE. This supplemental level was similar to that which would be experienced at Kunming (258N, 1950 m) with a 20% stratospheric ozone reduction during a clear day on the summer solstice (10.00 kJ m2 UV-BBE) according to a mathematical model of Madronich et al. (1995). Total daily photosynthetic photon fluence (PPF between 400 and 700 nm) under lamp fixtures was 90% of that above the lamps. 2.3. Measurements and statistical analyses Fifteen plants per plot were used to observe plant height measured as the distance from the soil surface to shoot tip at 80 DAP and ripening. Plants in two subplots of 0:5 0:5 m2 each were harvested from each plot to determine leaf area index (LAI) at 55 DAP, total biomass (including roots) per subplot and grain yield per subplot at ripening. Total leaves and a subsample of 15 leaves per subplot were collected at 55 DAP, and LA (area per leaf) of this
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subsample was measured with a Li-Cor 3100 (Li-Cor, Lincoln, NE, USA) area meter. All leaves were then oven-dried at 68 8C for 68 h and weighed. A regression relationship was developed between leaf weight and leaf area ðP < 0:01Þ. This linear regression was used to determine total LA per subplot, then LAI was determined. LAI is leaf area per unit ground area. Total biomass and grain were oven-dried at 68 8C for 68 h and weighed. The response index (RI) according to Dai et al. (1994), Teramura and Murali (1986) and Li et al. (2000a) used to evaluate the overall response of soybean to enhanced UV-B radiation which was calculated as follows: PHt PHc LAIt LAIc RI ¼ þ PHc LAIc TBt TBc GYt GYc þ þ 100% TBc GYc where RI is the response index, PH the plant height at ripening stage, LAI the leaf area index, TB the total biomass and GY the grain yield under t (UV-B radiation) and c (control). Statistical differences between means of control and UV-B radiation treatment of any measured parameter were determined by t-test at the P < 0:05 or 0.01 level. Correlation between the percent change of different parameters were determined at the P < 0:05 or 0.01 level.
3. Results 3.1. Plant height UV-B radiation had obvious effects on the plant height of 20 soybean cultivars at 80 DAP and ripening under field conditions (Table 1). At 80 DAP, UV-B radiation had a positive effect on Yunnan 97929 ðP < 0:05Þ, and a negative effect on 16 cultivars (P < 0:05 or 0.01). At ripening, UV-B radiation increased the height of Lanyin 20 ðP < 0:05Þ, and significantly decreased the height of 14 cultivars (P < 0:05 or 0.01). Out of 20 cultivars, 17 and 15 showed significant differences (P < 0:01 or 0.05) at 80 DAP and at ripening, respectively. Sensitivity in plant height of 20 soybean cultivars to enhanced UV-B
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Table 1 Intraspecific sensitivity to UV-B radiation based on plant height (cm) of 20 soybean cultivars under field conditionsa Cultivar name
Yunnan 97801 Lanyin 20 Yunnan 97929 Yudou 10 Yudou 18 Yudou 8 Lvgundou Heidadou Yunnan 97501 Pitiaohuangdou Yunnan 97506 Yunnan 97701 Longdou 1 E0138 Yunnan 96510 Df-1 Lingtaihuangdou Kanglehuangdou Tuhuangdou 1 Huanxianhuangdou
80 DAP
Ripening stage
Control
þUV-B
% Change
Control
þUV-B
% Change
41.2 21.5 43.3 46.4 54.2 49.5 34.9 85.2 62.3 64.2 45.2 53.8 76.6 57.1 52.5 59.4 39.5 33.9 66.4 37.3
37.7 22.3 49.5 41.1 46.4 40.3 34.8 65.8 48.1 50.9 36.8 43.6 46.4 40.4 38.2 45.7 27.2 29.6 49.5 26.8
8.5 3.7 14.3* 11.4* 14.4** 18.6** 0.3 22.8** 22.8** 18.4** 18.6** 19.0** 39.4** 29.3** 27.2** 23.1** 31.1** 12.7* 25.5** 28.2**
46.8 18.7 57.0 58.6 61.9 58.3 49.9 88.8 68.2 60.4 46.9 49.0 79.8 68.4 53.0 72.1 39.7 34.3 73.6 37.4
46.3 23.3 58.8 46.4 48.1 42.2 49.8 66.3 47.6 51.9 39.1 43.4 52.4 52.3 39.4 44.9 32.4 36.7 40.5 31.0
1.1 24.6* 3.2 20.8** 22.3** 27.6** 0.2 25.3* 30.2** 14.1* 16.6** 11.4 34.3** 23.5** 25.7** 37.7** 18.4** 6.7 45.0** 17.1**
a
Measurements were made on 15 plants per plot. Significant differences between control and UV-B radiation at P < 0:05 according to t-test. ** Significant differences between control and UV-B radiation at P < 0:01 according to t-test. *
radiation varied with time, being greater at 80 DAP than at ripening stage. 3.2. Leaf area index Under field conditions at 55 DAP, LAI of 20 soybean cultivars decreased significantly by UV-B radiation (Table 2). Of 20 cultivars, Yunnan 97801 and Yunnan 97501 showed significance at P < 0:05 while another 18 cultivars were significant at P < 0:01. UV-B radiation did not have positive effects on LAI for any of the cultivars. 3.3. Total biomass UV-B radiation significantly decreased total biomass of 20 soybean cultivars (Table 2). Of the 20 cultivars, Yunnan 97801, Yunnan 97929 and Yudou 10 were significant at P < 0:05, while another 17 cultivars were significant at P < 0:01. UV-B radiation did not have positive effects on total biomass for any of the cultivars.
3.4. Grain yield The effect of UV-B radiation on grain yield of 20 soybean cultivars under field conditions is presented in Table 2. Of the 20 cultivars, UV-B radiation had no significant effect on Lanyin 20, Yudou 8, Lvgundou, Heidadou and Longdou 1, and had a negative effect on the other 15 cultivars (P < 0:05 or 0.01). 3.5. Response index The RI is an integration of the effect on plant height, LAI, total biomass and grain yield, which could reflect the overall sensitivity of soybean cultivars to enhanced UV-B radiation. In this case, all 20 cultivars had a negative RI, indicating an overall inhibition of UV-B radiation on soybean growth (Table 3). Huanxianhuangdou was most adversely affected (RI, 295.7), while Yunnan 97801 was least affected (RI, 72.4). Across all cultivars tested in the present study, five cultivars had negative RI less than 256.9, the most sensitive cultivars were Huanxianhuangdou,
L. Yuan et al. / Field Crops Research 78 (2002) 1–8
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Table 2 Intraspecific sensitivity to UV-B radiation based on LAI, total biomass and grain yield of 20 wheat cultivars under field conditionsa Cultivar name
Control Yunnan 97801 Lanyin 20 Yunnan 97929 Yudou 10 Yudou 18 Yudou 8 Lvgundou Heidadou Yunnan 97501 Pitiaohuangdou Yunnan 97506 Yunnan 97701 Longdou 1 E0138 Yunnan 96510 Df-1 Lingtaihuangdou Kanglehuangdou Tuhuangdou 1 Huanxianhuangdou
Total biomass (g m2)
LAI
1.287 1.709 3.455 2.982 1.850 3.290 3.824 2.204 1.655 0.543 1.879 1.932 5.478 1.951 2.082 2.788 2.873 1.351 1.455 2.417
þUV-B 0.976 0.285 1.078 1.057 0.735 1.181 0.390 0.551 1.069 0.269 0.621 0.734 0.238 0.330 0.753 0.413 0.156 0.106 0.254 0.189
% Change *
24.2 83.3** 68.8** 64.5** 60.3** 68.9** 89.8** 75.0** 35.4* 50.5** 67.0** 62.0** 95.7** 83.1** 63.8** 85.2** 94.6** 92.2** 82.5** 92.2**
Control 396.8 201.2 617.5 446.2 412.4 474.2 698.5 615.3 833.3 283.5 593.3 416.9 1042.4 674.4 392.2 971.6 387.4 343.4 326.1 845.1
þUV-B 312.2 105.0 357.1 282.2 193.8 237.6 255.5 272.5 185.5 69.7 294.4 122.1 277.4 155.0 59.6 254.8 45.3 46.9 66.3 51.7
Grain yield (g m2) % Change *
21.3 47.8** 42.2* 36.8* 53.0** 49.9** 63.4** 55.7** 77.7** 75.4** 50.4** 70.7** 73.4** 77.0** 84.8** 73.8** 88.3** 86.3** 79.7** 93.9**
Control
þUV-B
% Change
98.9 68.5 133.0 42.2 36.6 45.5 82.3 130.7 36.3 33.8 161.5 39.1 142.6 50.8 10.4 195.5 42.5 92.8 105.3 292.0
73.4 56.5 105.5 33.0 26.8 37.9 68.9 109.4 22.5 17.0 64.3 16.9 125.9 30.6 5.2 77.8 16.0 6.7 17.5 22.0
25.8* 17.5 20.7* 21.8* 26.8* 16.7 16.3 16.3 38.0* 49.7** 60.2** 56.8** 11.7 39.8* 50.0** 60.2** 62.4** 92.8** 83.4** 92.5**
a LAI was determined on three plots at 55 DAP. Total biomass and grain yield are expressed on a ground area basis. Measurements were made on three subplots per plot. Shoot biomass and grain were determined at ripening. * Significant differences between control and UV-B radiation at P < 0:05 according to t-test. ** Significant differences between control and UV-B radiation at P < 0:01 according to t-test.
Table 3 RI of 20 wheat cultivars under enhanced UV-B radiation, together with their origin and habitata Rank
Cultivar name
Origin
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Yunnan 97801 Lanyin 20 Yunnan 97929 Yudou 10 Yudou 18 Yudou 8 Lvgundou Heidadou Yunnan 97501 Pitiaohuangdou Yunnan 97506 Yunnan 97701 Longdou 1 E0138 Yunnan 96510 Df-1 Lingtaihuangdou Kanglehuangdou Tuhuangdou 1 Huanxianhuangdou
South North South South South South North North South North South South North South South South North North North North
a
China China China China China China China China China China China China China China China China China China China China
Ranking 1–20 is in the order of increasing sensitivity to UV-B radiation.
Habitat
RI
High elevation, upland Middle elevation High elevation, upland Low elevation, lowland Low elevation, lowland Low elevation, lowland Middle elevation, upland Middle elevation, upland High elevation, upland Middle elevation High elevation, upland High elevation, upland Middle elevation, upland High elevation, upland High elevation, upland High elevation, upland Middle elevation, upland Middle elevation, upland Middle elevation, upland Middle elevation, upland
72.4 124.0 128.5 143.9 162.4 163.1 169.7 172.3 181.3 189.7 194.2 200.9 215.1 223.4 224.3 256.9 263.7 264.6 290.8 295.7
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Tuhuangdou 1, Kanglehuangdou, Lingtaihuangdou and Df-1. The 6 most tolerant cultivars were Yunnan 97801, Lanyin 20, Yunnan 97929, Yudou 10, Yudou 18, Yudou 8, with response indices higher than 163.1.
4. Discussion This is the first report identifying intraspecific responses in crop growth and yield of 20 soybean cultivars to enhanced UV-B radiation under field conditions. This is supported by earlier findings of intraspecific responses in soybean (Teramura and Murali, 1986; Sullivan and Teramura, 1990; Teramura et al., 1990a; D’surney et al., 1993), maize and wheat (Biggs and Kossuth, 1978; Li et al., 2000a,b), horsebean (Bennett, 1981), cucumber (Murali and Teramura, 1986), rice (Teramura et al., 1991; Barnes et al., 1993; Dai et al., 1994). Reduction in plant height has often been used as an index to assess the degree of UV-B radiation sensitivity (Biggs and Kossuth, 1978). In this field study, UV-B radiation had significant effects on the height of 20 soybean cultivars at 80 DAP and ripening (Table 1). Sensitivity in height of 20 soybean cultivars to enhanced UV-B radiation was greater at 80 DAP than at the ripening. The correlation between height at 80 DAP and height at ripening was significant (r ¼ 0:754, P < 0:01). Height at ripening was used in the RI. In a greenhouse experiment, UV-B radiation decreased plant height of soybean (Teramura, 1980). UV-B radiation significantly dwarfed soybean, primarily due to shorter internodes rather than smaller node number (Teramura, 1980). A similar result was observed in wheat (Li et al., 1998). This could be due to photo-oxidative destruction of the phytohormone indole acetic acid followed by reduced cell wall extensibility as demonstrated in sunflower seedlings (Ros and Tevini, 1995). The levels of ethylene, which promote radial growth and reduce elongation, are increased after irradiation with UV-B (Caldwell et al., 1995). However, the mechanism for UV-B radiation to increase plant height is still not clear. Although UV-B radiation had no significant effects on leaf area and total leaf number of soybean in an early study (Teramura, 1980), in this study, LAI of 20 soybean cultivars responded significantly to UV-B
radiation (Table 2). Under UV-B radiation, sensitivity in LAI was significant. So, LAI may be used as a parameter for selection of UV-B tolerance. UV-B radiation may directly affect cell division and some intrinsic growth characteristics (Beggs et al., 1985). It may be associated with many of the changes observed following UV-B exposure, such as the changes in leaf area dynamics. LAI was contributed by area per leaf and leaf number per subplot. In this study, leaf number per subplot and area per leaf decreased under UV-B radiation, respectively. Similarly, UV-B radiation decreased LA of soybean (Sullivan and Teramura, 1990) and this was the reason why soybean LAI decreased. Total biomass is a good indictor of the effects of UV-B radiation on growth (Teramura, 1983). Total biomass represents a long-term integration of all biochemical, physiological, and growth parameters. Therefore, even subtle UV-B induced effects on physiological processes could accumulate and result in significant effects on biomass. Other studies indicated UV-B radiation decreased total biomass of soybean (Teramura, 1983; Teramura et al., 1990b; Sullivan and Teramura, 1990). Decrease in total biomass of soybean in this study are similar to those found in both greenhouse (Teramura, 1980, 1983) and field (Sullivan and Teramura, 1990) studies. No greenhouse experiments have reported increases or no changes in biomass of soybean with enhanced UV-B radiation (Teramura, 1983). The effect of UV-B radiation on total biomass may be dependent on the concentration of UV-absorbing compounds that attenuate incoming radiation and affectively limit damage to cellular components including the genetic material (D’surney et al., 1993). Early experiments on effects of UV-B radiation on crops were conducted in growth chambers and greenhouses. Owing to space limitations, it is impractical to grow plants to reproductive maturity in such facilities. Thus, very little is known about UV-B effects on grain yield of soybean (Teramura, 1983). No changes were reported in a greenhouse study (Teramura et al., 1990b) and a field study (Sullivan and Teramura, 1990), grain yield of soybean cultivars Essex and Williams decreased when combined over the 6-year field study (Teramura et al., 1990a). Reduced grain yield has important economic implications; consequently, using grain yield as a parameter in a RI is
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necessary. In addition, comparisons between greenhouse and field observations revealed that UV-B sensitivity based on seed yield of soybean differs markedly, indicating that field validations are ultimately necessary for assessing the potential impacts of increased solar UV-B radiation (Teramura and Murali, 1986). The RI was a good indicator to assess plant sensitivity to enhanced UV-B radiation (Dai et al., 1994). In previous studies, parameters used in response indices were different. Only in the soybean field study of Teramura and Murali (1986), was grain yield included as a parameter, and response sensitivity of soybean to UV-B radiation differed markedly when based on seed yield. Additionally, these parameters were only observed with isolated plants of cucumber and rice during short-term experiments in the greenhouse (Murali and Teramura, 1986; Teramura et al., 1991; Barnes et al., 1993; Dai et al., 1994). Because plants respond differently to UV-B in different environments (Teramura and Murali, 1986) and inherent genetic differences in different cultivars also contribute, variability between experiments could also be due to differences in growth conditions, length of UV-B radiation, stage of growth and the ratio of incident PPF to UV-B radiation, all of which have been demonstrated to greatly modify UV-B responsiveness (Teramura et al., 1991). Besides, the responses of soybean crops to UV-B were more complicated than those of isolated plants (Teramura and Murali, 1986; Sullivan and Teramura, 1990; Teramura et al., 1990a,b; D’surney et al., 1993). Intraspecific differences of soybean to UV-B radiation were observed in the greenhouse and the field (Teramura and Murali, 1986; D’surney et al., 1993). In this study, the parameters of the RI included plant height, LAI, total biomass and grain yield. The correlations between LAI and total biomass (r ¼ 0:500, P < 0:05), total biomass and grain yield (r ¼ 0:672, P < 0:01) were significant, respectively. And the parameters were measured in crops under field conditions. Then the results of this study might provide realistic assessments of intraspecific responses among 20 soybean cultivars to enhanced UV-B radiation. As ambient UV-B radiation level is greater at lower latitudes than that at higher latitudes, it is generally assumed that crop cultivars originating near the equator are more tolerant to UV-B radiation. In this study,
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out of the 6 tolerant cultivars, 5 cultivars originated from south China (low latitude). The most sensitive cultivars, i.e. Huanxianhuangdou (RI-295.7), Tuhuangdou 1 (RI-290.8), Kanglehuangdou (RI-264.6) and Lingtaihuangdou (RI-263.7), originated in north China (high latitude). Similar results were observed in wheat in a field study (Li et al., 2000a). However, rice cultivars originating from regions with high ambient UV-B were not necessarily more tolerant to enhanced UV-B radiation (Barnes et al., 1993; Dai et al., 1994). The UV-B tolerant cultivars identified in our study might be used as possible donors in breeding programs; however, the genetic basis for these differences must be further examined. In conclusion, enhanced UV-B radiation had significant effect on plant height, LAI, total biomass and grain yield of soybean crops under field conditions. The RI presented here showed that soybean is potentially a UV-B sensitive species. This result was similar to the previous study (Biggs and Kossuth, 1978). UVB sensitivity might be associated with UV-B radiation fluences and long-term accumulation (Li et al., 1998). Soybean is an economically important crop and its intraspecific response to UV-B radiation under field conditions is still not clearly understood. Effects of UV-B radiation on plants are related to other environmental factors, including PPF, CO2, drought, phosphorus nutrition, temperature, ozone fumigation and heavy metal (Teramura et al., 1990b; Caldwell et al., 1995). Further field studies are needed to improve the understanding of the relationship between UV-B radiation effectiveness and other environmental variables. This would greatly enhance our ability to more realistically assess intraspecific responses of soybean to increased levels of UV-B radiation.
Acknowledgements This work was supported by the National Natural Science Foundation of China, the National Research Foundation for Post-Doctorate Studies in China, the Applied Base Research Foundation of Yunnan Province, China, and the Research Foundation for Academic Leaders in Yunnan Province, China. We wish to thank Tan Lingling of the Department of Biology, Lanzhou University, for supporting the experimental work.
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Li, Y., Zu, Y.Q., Chen, J.J., Chen, H.Y., Yang, J.L., Hu, Z.D., 2000b. Intraspecific differences in physiological responses of 20 wheat cultivars to enhanced ultraviolet-B radiation under field conditions. Environ. Exp. Bot. 44 (2), 95–103. Lydon, J., Teramure, A.H., Summers, E.G., 1986. Effects of ultraviolet-B radiation on growth and productivity of fieldgrown soybean. In: Worrest, R.C. (Ed.), Stratospheric Ozone Reduction, Solar Ultraviolet Radiation and Plant Life. Springer, Berlin, pp. 313–325. Madronich, S., Mckenzie, R.L., Caldwell, M.M., Bjorn, L.O., 1995. Changes in ultraviolet radiation reaching the earth’s surface. Ambio 24, 143–153. Molina, M.J., Molina, L.T., 1992. Stratospheric ozone. ACS Symp. Ser. 483, 24–35. Murali, N.S., Teramura, A.H., 1986. Intraspecific differences in Cucumis sativus sensitivity to ultraviolet-B radiation. Physiol. Plant. 68, 673–677. Ros, J., Tevini, M., 1995. Interaction of UV-radiation and IAA during growth of seedling and hypocotyl segments of sunflower. J. Plant Physiol. 146, 295–302. Sullivan, J.H., Teramura, A.H., 1990. Field study of the interaction between solar ultraviolet-B radiation and drought on photosynthesis and growth in soybean. Plant Physiol. 92, 141–146. Teramura, A.H., 1980. Effects of ultraviolet-B irradiances on soybean. I. Importance of photosynthetically active radiation in evaluating ultraviolet-B irradiance effects on soybean and wheat growth. Physiol. Plant. 48, 333–339. Teramura, A.H., 1983. Effects of ultraviolet-B radiation on the growth and yield of crop plants. Physiol. Plant. 58, 415–427. Teramura, A.H., Murali, N.S., 1986. Intraspecific differences in growth and yield of soybean exposed to ultraviolet-B radiation under greenhouse and field conditions. Environ. Exp. Bot. 26, 89–95. Teramura, A.H., Sullivan, J.H., Lydon, J., 1990a. Effects of UV-B radiation on soybean yield and seed quality: a 6-year field study. Physiol. Plant. 80, 5–11. Teramura, A.H., Sullivan, J.H., Ziska, L.H., 1990b. Interaction of elevated ultraviolet-B radiation and CO2 on productivity and photosynthetic characteristics in wheat. Plant Physiol. 94, 470– 475. Teramura, A.H., Ziska, L.H., Sztein, A.E., 1991. Changes in growth and photosynthetic capacity of rice with increased UVB radiation. Physiol. Plant. 83, 373–380. Yue, M., Li, Y., Wang, X.L., 1988. Effects of enhanced ultraviolet-B radiation on plant nutrients and decomposition of spring wheat under field conditions. Environ. Exp. Bot. 40, 187–196.
Intraspecific responses in crop growth and yield of 20 soybean cultivars to enhanced ultraviolet-B radiation under field conditions Li Yuana,b,*, Zu Yanqunb, Chen Jianjunb, Chen Haiyanb a
The Center for Agricultural Biodiversity Research and Training of Yunnan Province, Yunnan Agricultural University, Kunming 6502011, PR China b Eco-environment Research Institute, College of Resources and Environment, Yunnan Agricultural University, Kunming 6502011, PR China
Received 22 October 2001; received in revised form 15 April 2002; accepted 18 April 2002
Abstract Field studies were conducted to determine the potential for intraspecific responses in crop growth and grain yield of 20 soybean cultivars to enhanced ultraviolet-B (UV-B, 280–315 nm) radiation. The supplemental UV-B radiation was 5.00 kJ m2, simulating a depletion of 20% stratospheric ozone at Kunming (258N, 1950 m). Out of the 20 soybean cultivars tested, 17 and 15 showed significant change in plant height at 80 DAP (days after planting) and ripening stages, respectively. Sensitivity in plant height was greater at 80 DAP than at ripening. The plant height of 3 cultivars increased, and that of 17 cultivars decreased. Under UV-B radiation, LAI (leaf area index), biomass and grain yield decreased, respectively. The greatest percent decrease was 95.7, 93.9 and 92.8, respectively. RI (response index) was the sum of percent change in plant height at ripening, LAI, biomass and grain yield. The results showed that all 20 soybean cultivars had a negative RI, indicating inhibition by UV-B radiation on soybean growth. The RI of 6 tolerant cultivars was higher than 163.1 and 5 out of 6 originated from south China (low latitude). The RI of the most tolerant cultivars, Yunnan 97801, was 72.4. Meanwhile, the RI of 5 sensitive cultivars was lower than 256.9 and 4 out of the 5 originated from north China (high latitude). The RI of the most sensitive cultivar, Huanxianhuangdou, was 295.7. These UV-B tolerant cultivars identified in this study might be useful in breeding programs. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Soybean; Stratospheric ozone depletion; UV-B radiation; Response index; Crop growth; Grain yield
1. Introduction The rapid decline in stratospheric ozone concentrations has been confirmed by satellite measurements (Molina and Molina, 1992). The most pronounced thinning of the ozone layer has been measured over
*
Corresponding author. Present address: College of Resources and Environment, Yunnan Agricultural University, Kunming 6502011, PR China.
the Antarctic continent with up to 71% depletion during the Antarctic spring (Kerr, 1993). Recent mathematical models predict a further increase in solar ultraviolet-B (UV-B) irradiation in future years (Madronich et al., 1995). UV-B effects on plants have been the subject of considerable research; approximately 600 papers have been published (Caldwell et al., 1998). An examination of more than 200 plant species reveals that roughly 20% are sensitive, 50% are mildly sensitive or tolerant and 30% are completely insensitive to UV-B radiation (Teramura, 1983).
0378-4290/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 4 2 9 0 ( 0 2 ) 0 0 0 8 4 - 9
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Whilst the impact of enhanced UV-B radiation on plant physiology, morphology, growth, nutrition and biomass has been investigated extensively (Li et al., 1998, 1999; Yue et al., 1988), little is known about intraspecific responses of plants to enhanced UV-B. Intraspecific responses in maize and wheat (Biggs and Kossuth, 1978; Li et al., 2000a,b), broad bean (Bennett, 1981), cucumber (Murali and Teramura, 1986), soybean (Teramura and Murali, 1986; Sullivan and Teramura, 1990; Teramura et al., 1990a; D’surney et al., 1993), rice (Teramura et al., 1991; Barnes et al., 1993; Dai et al., 1994) have been reported. Plant species and even genotypes within species can differ greatly in their responses to UV-B. Reasons for this are not clear (Caldwell and Flint, 1994). Due to our lack of understanding of the basis of intraspecific responses to UV-B radiation, further studies on its importance would be worthwhile. Most of the UV-B research in the past two decades has been conducted as short-term experiments in growth chambers and greenhouses, where an unnatural spectral balance of radiation may have substantially changed plant sensitivity to UV-B. It is important in experiments to maintain a realistic balance between various spectral regions, since both UVA (315–400 nm) and visible (400–700 nm) radiation can have ameliorating effects on responses of plants to UV-B (Caldwell et al., 1995). In growth chamber and greenhouse experiments, the visible and UV-A radiation is usually much less than in sunlight; thus, even if realistic levels of UV-B are used in simulating ozone reduction, the plant response may be exaggerated relative to field conditions. Unfortunately, only 15% of the studies have been conducted under field conditions. While the laboratory and glasshouse studies provide information on mechanisms and processes of UV-B action, only field studies can provide realistic assessments of what will happen as the stratospheric ozone layer thins (Caldwell et al., 1995). Vegetation communities function differently to isolated plants and the behavior of the former is not easily predicted from the latter. Besides, the responses of soybean crops to UV-B are more complicated than those of plants isolated (Teramura and Murali, 1986; Sullivan and Teramura, 1990; Teramura et al., 1990a,b; D’surney et al., 1993). Community-level field experimentation is needed to evaluate realistic
consequences of increased solar UV-B resulting from ozone reduction. Soybean is one of the major world food crops. The effects of enhanced UV-B radiation on photosynthetic characteristics, growth, morphology, intraspecific responses, total biomass and yield have already been studied (Sullivan and Teramura, 1990; Teramura, 1980, 1983; Teramura and Murali, 1986; Teramura et al., 1990a,b; Lydon et al., 1980; D’surney et al., 1993). Unfortunately, only few studies have been conducted under field conditions (Teramura et al., 1990a). In this study we grew 20 soybean cultivars in the field under ambient and supplemental levels of UV-B radiation for a season with the objective of evaluating intraspecific responses in crop growth and yield. We hypothesized that enhanced UV-B radiation would affect crop growth and yield and result in intraspecific responses under field conditions.
2. Materials and methods 2.1. Plant materials and growth conditions The field experiment was conducted at Yunnan Agricultural University, Kunming, China. No fertilization was needed during the season. Seeds of 20 soybean (Glycine max) cultivars were obtained from Yunnan Academic of Agricultural Sciences, Gansu Academic of Agricultural Sciences and Henan Academic of Agricultural Sciences. Seeds were sown in rows spaced 0.4 m apart at a density of 50 seeds m2 in 120 plots of 2:0 1:0 m2 each on 10 April 1999. Five border rows were sown round each plot in order to minimize heterogeneity in microclimate. The overall experimental design was a randomized complete block with two UV-B treatments and three replications. At 30 DAP (days after planting), plants were thinned to 40 m2 for uniformity in growth. This planting density is within commonly used sowing practices for the Kunming region. 2.2. UV-B radiation Supplemental UV-B radiation was provided by filtered Gucun brand 30 W sunlamps (Gucun Instrument Factory, Shanghai, China) following the
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procedure outlined in Lydon et al. (1986). Lamps were suspended above and perpendicular to the planted rows (rows oriented in an east–west direction to minimize shading) and filtered with either 0.13 mm thick cellulose diacetate (transmission down to 290 nm) for supplemental UV-B radiation or 0.13 mm polyester plastic films (absorbs all radiation below 320 nm) as a control (Sullivan and Teramura, 1990). Cellulose diacetate filters were presolarized for 8 h and changed weekly to ensure uniformity of UV-B transmission. The spectral irradiance from the lamps was determined with an Optronics Model 742 (Optronics Laboratories, Orlando, FL, USA) spectroradiometer. The spectral irradiance was weighted with the generalized plant response action spectrum (Caldwell, 1971) and normalized at 300 nm to obtain UVBBE. Six lamps were installed above each plot. Plants were irradiated for 7 h daily from 30 DAP to ripening stage, centered around solar noon. Plants under polyester-filtered lamps received only ambient levels of UVB radiation (10.00 kJ m2 UV-BBE during clear sky conditions on the summer solstice). Plants beneath the cellulose diacetate filters received ambient plus supplemental levels of UV-B. The lamp height above the plants was adjusted weekly to maintain a distance of 0.45 m between the lamps and the top of the canopy, and provided supplemental irradiances of 5.00 effective kJ m2 UV-BBE. This supplemental level was similar to that which would be experienced at Kunming (258N, 1950 m) with a 20% stratospheric ozone reduction during a clear day on the summer solstice (10.00 kJ m2 UV-BBE) according to a mathematical model of Madronich et al. (1995). Total daily photosynthetic photon fluence (PPF between 400 and 700 nm) under lamp fixtures was 90% of that above the lamps. 2.3. Measurements and statistical analyses Fifteen plants per plot were used to observe plant height measured as the distance from the soil surface to shoot tip at 80 DAP and ripening. Plants in two subplots of 0:5 0:5 m2 each were harvested from each plot to determine leaf area index (LAI) at 55 DAP, total biomass (including roots) per subplot and grain yield per subplot at ripening. Total leaves and a subsample of 15 leaves per subplot were collected at 55 DAP, and LA (area per leaf) of this
3
subsample was measured with a Li-Cor 3100 (Li-Cor, Lincoln, NE, USA) area meter. All leaves were then oven-dried at 68 8C for 68 h and weighed. A regression relationship was developed between leaf weight and leaf area ðP < 0:01Þ. This linear regression was used to determine total LA per subplot, then LAI was determined. LAI is leaf area per unit ground area. Total biomass and grain were oven-dried at 68 8C for 68 h and weighed. The response index (RI) according to Dai et al. (1994), Teramura and Murali (1986) and Li et al. (2000a) used to evaluate the overall response of soybean to enhanced UV-B radiation which was calculated as follows: PHt PHc LAIt LAIc RI ¼ þ PHc LAIc TBt TBc GYt GYc þ þ 100% TBc GYc where RI is the response index, PH the plant height at ripening stage, LAI the leaf area index, TB the total biomass and GY the grain yield under t (UV-B radiation) and c (control). Statistical differences between means of control and UV-B radiation treatment of any measured parameter were determined by t-test at the P < 0:05 or 0.01 level. Correlation between the percent change of different parameters were determined at the P < 0:05 or 0.01 level.
3. Results 3.1. Plant height UV-B radiation had obvious effects on the plant height of 20 soybean cultivars at 80 DAP and ripening under field conditions (Table 1). At 80 DAP, UV-B radiation had a positive effect on Yunnan 97929 ðP < 0:05Þ, and a negative effect on 16 cultivars (P < 0:05 or 0.01). At ripening, UV-B radiation increased the height of Lanyin 20 ðP < 0:05Þ, and significantly decreased the height of 14 cultivars (P < 0:05 or 0.01). Out of 20 cultivars, 17 and 15 showed significant differences (P < 0:01 or 0.05) at 80 DAP and at ripening, respectively. Sensitivity in plant height of 20 soybean cultivars to enhanced UV-B
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Table 1 Intraspecific sensitivity to UV-B radiation based on plant height (cm) of 20 soybean cultivars under field conditionsa Cultivar name
Yunnan 97801 Lanyin 20 Yunnan 97929 Yudou 10 Yudou 18 Yudou 8 Lvgundou Heidadou Yunnan 97501 Pitiaohuangdou Yunnan 97506 Yunnan 97701 Longdou 1 E0138 Yunnan 96510 Df-1 Lingtaihuangdou Kanglehuangdou Tuhuangdou 1 Huanxianhuangdou
80 DAP
Ripening stage
Control
þUV-B
% Change
Control
þUV-B
% Change
41.2 21.5 43.3 46.4 54.2 49.5 34.9 85.2 62.3 64.2 45.2 53.8 76.6 57.1 52.5 59.4 39.5 33.9 66.4 37.3
37.7 22.3 49.5 41.1 46.4 40.3 34.8 65.8 48.1 50.9 36.8 43.6 46.4 40.4 38.2 45.7 27.2 29.6 49.5 26.8
8.5 3.7 14.3* 11.4* 14.4** 18.6** 0.3 22.8** 22.8** 18.4** 18.6** 19.0** 39.4** 29.3** 27.2** 23.1** 31.1** 12.7* 25.5** 28.2**
46.8 18.7 57.0 58.6 61.9 58.3 49.9 88.8 68.2 60.4 46.9 49.0 79.8 68.4 53.0 72.1 39.7 34.3 73.6 37.4
46.3 23.3 58.8 46.4 48.1 42.2 49.8 66.3 47.6 51.9 39.1 43.4 52.4 52.3 39.4 44.9 32.4 36.7 40.5 31.0
1.1 24.6* 3.2 20.8** 22.3** 27.6** 0.2 25.3* 30.2** 14.1* 16.6** 11.4 34.3** 23.5** 25.7** 37.7** 18.4** 6.7 45.0** 17.1**
a
Measurements were made on 15 plants per plot. Significant differences between control and UV-B radiation at P < 0:05 according to t-test. ** Significant differences between control and UV-B radiation at P < 0:01 according to t-test. *
radiation varied with time, being greater at 80 DAP than at ripening stage. 3.2. Leaf area index Under field conditions at 55 DAP, LAI of 20 soybean cultivars decreased significantly by UV-B radiation (Table 2). Of 20 cultivars, Yunnan 97801 and Yunnan 97501 showed significance at P < 0:05 while another 18 cultivars were significant at P < 0:01. UV-B radiation did not have positive effects on LAI for any of the cultivars. 3.3. Total biomass UV-B radiation significantly decreased total biomass of 20 soybean cultivars (Table 2). Of the 20 cultivars, Yunnan 97801, Yunnan 97929 and Yudou 10 were significant at P < 0:05, while another 17 cultivars were significant at P < 0:01. UV-B radiation did not have positive effects on total biomass for any of the cultivars.
3.4. Grain yield The effect of UV-B radiation on grain yield of 20 soybean cultivars under field conditions is presented in Table 2. Of the 20 cultivars, UV-B radiation had no significant effect on Lanyin 20, Yudou 8, Lvgundou, Heidadou and Longdou 1, and had a negative effect on the other 15 cultivars (P < 0:05 or 0.01). 3.5. Response index The RI is an integration of the effect on plant height, LAI, total biomass and grain yield, which could reflect the overall sensitivity of soybean cultivars to enhanced UV-B radiation. In this case, all 20 cultivars had a negative RI, indicating an overall inhibition of UV-B radiation on soybean growth (Table 3). Huanxianhuangdou was most adversely affected (RI, 295.7), while Yunnan 97801 was least affected (RI, 72.4). Across all cultivars tested in the present study, five cultivars had negative RI less than 256.9, the most sensitive cultivars were Huanxianhuangdou,
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Table 2 Intraspecific sensitivity to UV-B radiation based on LAI, total biomass and grain yield of 20 wheat cultivars under field conditionsa Cultivar name
Control Yunnan 97801 Lanyin 20 Yunnan 97929 Yudou 10 Yudou 18 Yudou 8 Lvgundou Heidadou Yunnan 97501 Pitiaohuangdou Yunnan 97506 Yunnan 97701 Longdou 1 E0138 Yunnan 96510 Df-1 Lingtaihuangdou Kanglehuangdou Tuhuangdou 1 Huanxianhuangdou
Total biomass (g m2)
LAI
1.287 1.709 3.455 2.982 1.850 3.290 3.824 2.204 1.655 0.543 1.879 1.932 5.478 1.951 2.082 2.788 2.873 1.351 1.455 2.417
þUV-B 0.976 0.285 1.078 1.057 0.735 1.181 0.390 0.551 1.069 0.269 0.621 0.734 0.238 0.330 0.753 0.413 0.156 0.106 0.254 0.189
% Change *
24.2 83.3** 68.8** 64.5** 60.3** 68.9** 89.8** 75.0** 35.4* 50.5** 67.0** 62.0** 95.7** 83.1** 63.8** 85.2** 94.6** 92.2** 82.5** 92.2**
Control 396.8 201.2 617.5 446.2 412.4 474.2 698.5 615.3 833.3 283.5 593.3 416.9 1042.4 674.4 392.2 971.6 387.4 343.4 326.1 845.1
þUV-B 312.2 105.0 357.1 282.2 193.8 237.6 255.5 272.5 185.5 69.7 294.4 122.1 277.4 155.0 59.6 254.8 45.3 46.9 66.3 51.7
Grain yield (g m2) % Change *
21.3 47.8** 42.2* 36.8* 53.0** 49.9** 63.4** 55.7** 77.7** 75.4** 50.4** 70.7** 73.4** 77.0** 84.8** 73.8** 88.3** 86.3** 79.7** 93.9**
Control
þUV-B
% Change
98.9 68.5 133.0 42.2 36.6 45.5 82.3 130.7 36.3 33.8 161.5 39.1 142.6 50.8 10.4 195.5 42.5 92.8 105.3 292.0
73.4 56.5 105.5 33.0 26.8 37.9 68.9 109.4 22.5 17.0 64.3 16.9 125.9 30.6 5.2 77.8 16.0 6.7 17.5 22.0
25.8* 17.5 20.7* 21.8* 26.8* 16.7 16.3 16.3 38.0* 49.7** 60.2** 56.8** 11.7 39.8* 50.0** 60.2** 62.4** 92.8** 83.4** 92.5**
a LAI was determined on three plots at 55 DAP. Total biomass and grain yield are expressed on a ground area basis. Measurements were made on three subplots per plot. Shoot biomass and grain were determined at ripening. * Significant differences between control and UV-B radiation at P < 0:05 according to t-test. ** Significant differences between control and UV-B radiation at P < 0:01 according to t-test.
Table 3 RI of 20 wheat cultivars under enhanced UV-B radiation, together with their origin and habitata Rank
Cultivar name
Origin
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Yunnan 97801 Lanyin 20 Yunnan 97929 Yudou 10 Yudou 18 Yudou 8 Lvgundou Heidadou Yunnan 97501 Pitiaohuangdou Yunnan 97506 Yunnan 97701 Longdou 1 E0138 Yunnan 96510 Df-1 Lingtaihuangdou Kanglehuangdou Tuhuangdou 1 Huanxianhuangdou
South North South South South South North North South North South South North South South South North North North North
a
China China China China China China China China China China China China China China China China China China China China
Ranking 1–20 is in the order of increasing sensitivity to UV-B radiation.
Habitat
RI
High elevation, upland Middle elevation High elevation, upland Low elevation, lowland Low elevation, lowland Low elevation, lowland Middle elevation, upland Middle elevation, upland High elevation, upland Middle elevation High elevation, upland High elevation, upland Middle elevation, upland High elevation, upland High elevation, upland High elevation, upland Middle elevation, upland Middle elevation, upland Middle elevation, upland Middle elevation, upland
72.4 124.0 128.5 143.9 162.4 163.1 169.7 172.3 181.3 189.7 194.2 200.9 215.1 223.4 224.3 256.9 263.7 264.6 290.8 295.7
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Tuhuangdou 1, Kanglehuangdou, Lingtaihuangdou and Df-1. The 6 most tolerant cultivars were Yunnan 97801, Lanyin 20, Yunnan 97929, Yudou 10, Yudou 18, Yudou 8, with response indices higher than 163.1.
4. Discussion This is the first report identifying intraspecific responses in crop growth and yield of 20 soybean cultivars to enhanced UV-B radiation under field conditions. This is supported by earlier findings of intraspecific responses in soybean (Teramura and Murali, 1986; Sullivan and Teramura, 1990; Teramura et al., 1990a; D’surney et al., 1993), maize and wheat (Biggs and Kossuth, 1978; Li et al., 2000a,b), horsebean (Bennett, 1981), cucumber (Murali and Teramura, 1986), rice (Teramura et al., 1991; Barnes et al., 1993; Dai et al., 1994). Reduction in plant height has often been used as an index to assess the degree of UV-B radiation sensitivity (Biggs and Kossuth, 1978). In this field study, UV-B radiation had significant effects on the height of 20 soybean cultivars at 80 DAP and ripening (Table 1). Sensitivity in height of 20 soybean cultivars to enhanced UV-B radiation was greater at 80 DAP than at the ripening. The correlation between height at 80 DAP and height at ripening was significant (r ¼ 0:754, P < 0:01). Height at ripening was used in the RI. In a greenhouse experiment, UV-B radiation decreased plant height of soybean (Teramura, 1980). UV-B radiation significantly dwarfed soybean, primarily due to shorter internodes rather than smaller node number (Teramura, 1980). A similar result was observed in wheat (Li et al., 1998). This could be due to photo-oxidative destruction of the phytohormone indole acetic acid followed by reduced cell wall extensibility as demonstrated in sunflower seedlings (Ros and Tevini, 1995). The levels of ethylene, which promote radial growth and reduce elongation, are increased after irradiation with UV-B (Caldwell et al., 1995). However, the mechanism for UV-B radiation to increase plant height is still not clear. Although UV-B radiation had no significant effects on leaf area and total leaf number of soybean in an early study (Teramura, 1980), in this study, LAI of 20 soybean cultivars responded significantly to UV-B
radiation (Table 2). Under UV-B radiation, sensitivity in LAI was significant. So, LAI may be used as a parameter for selection of UV-B tolerance. UV-B radiation may directly affect cell division and some intrinsic growth characteristics (Beggs et al., 1985). It may be associated with many of the changes observed following UV-B exposure, such as the changes in leaf area dynamics. LAI was contributed by area per leaf and leaf number per subplot. In this study, leaf number per subplot and area per leaf decreased under UV-B radiation, respectively. Similarly, UV-B radiation decreased LA of soybean (Sullivan and Teramura, 1990) and this was the reason why soybean LAI decreased. Total biomass is a good indictor of the effects of UV-B radiation on growth (Teramura, 1983). Total biomass represents a long-term integration of all biochemical, physiological, and growth parameters. Therefore, even subtle UV-B induced effects on physiological processes could accumulate and result in significant effects on biomass. Other studies indicated UV-B radiation decreased total biomass of soybean (Teramura, 1983; Teramura et al., 1990b; Sullivan and Teramura, 1990). Decrease in total biomass of soybean in this study are similar to those found in both greenhouse (Teramura, 1980, 1983) and field (Sullivan and Teramura, 1990) studies. No greenhouse experiments have reported increases or no changes in biomass of soybean with enhanced UV-B radiation (Teramura, 1983). The effect of UV-B radiation on total biomass may be dependent on the concentration of UV-absorbing compounds that attenuate incoming radiation and affectively limit damage to cellular components including the genetic material (D’surney et al., 1993). Early experiments on effects of UV-B radiation on crops were conducted in growth chambers and greenhouses. Owing to space limitations, it is impractical to grow plants to reproductive maturity in such facilities. Thus, very little is known about UV-B effects on grain yield of soybean (Teramura, 1983). No changes were reported in a greenhouse study (Teramura et al., 1990b) and a field study (Sullivan and Teramura, 1990), grain yield of soybean cultivars Essex and Williams decreased when combined over the 6-year field study (Teramura et al., 1990a). Reduced grain yield has important economic implications; consequently, using grain yield as a parameter in a RI is
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necessary. In addition, comparisons between greenhouse and field observations revealed that UV-B sensitivity based on seed yield of soybean differs markedly, indicating that field validations are ultimately necessary for assessing the potential impacts of increased solar UV-B radiation (Teramura and Murali, 1986). The RI was a good indicator to assess plant sensitivity to enhanced UV-B radiation (Dai et al., 1994). In previous studies, parameters used in response indices were different. Only in the soybean field study of Teramura and Murali (1986), was grain yield included as a parameter, and response sensitivity of soybean to UV-B radiation differed markedly when based on seed yield. Additionally, these parameters were only observed with isolated plants of cucumber and rice during short-term experiments in the greenhouse (Murali and Teramura, 1986; Teramura et al., 1991; Barnes et al., 1993; Dai et al., 1994). Because plants respond differently to UV-B in different environments (Teramura and Murali, 1986) and inherent genetic differences in different cultivars also contribute, variability between experiments could also be due to differences in growth conditions, length of UV-B radiation, stage of growth and the ratio of incident PPF to UV-B radiation, all of which have been demonstrated to greatly modify UV-B responsiveness (Teramura et al., 1991). Besides, the responses of soybean crops to UV-B were more complicated than those of isolated plants (Teramura and Murali, 1986; Sullivan and Teramura, 1990; Teramura et al., 1990a,b; D’surney et al., 1993). Intraspecific differences of soybean to UV-B radiation were observed in the greenhouse and the field (Teramura and Murali, 1986; D’surney et al., 1993). In this study, the parameters of the RI included plant height, LAI, total biomass and grain yield. The correlations between LAI and total biomass (r ¼ 0:500, P < 0:05), total biomass and grain yield (r ¼ 0:672, P < 0:01) were significant, respectively. And the parameters were measured in crops under field conditions. Then the results of this study might provide realistic assessments of intraspecific responses among 20 soybean cultivars to enhanced UV-B radiation. As ambient UV-B radiation level is greater at lower latitudes than that at higher latitudes, it is generally assumed that crop cultivars originating near the equator are more tolerant to UV-B radiation. In this study,
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out of the 6 tolerant cultivars, 5 cultivars originated from south China (low latitude). The most sensitive cultivars, i.e. Huanxianhuangdou (RI-295.7), Tuhuangdou 1 (RI-290.8), Kanglehuangdou (RI-264.6) and Lingtaihuangdou (RI-263.7), originated in north China (high latitude). Similar results were observed in wheat in a field study (Li et al., 2000a). However, rice cultivars originating from regions with high ambient UV-B were not necessarily more tolerant to enhanced UV-B radiation (Barnes et al., 1993; Dai et al., 1994). The UV-B tolerant cultivars identified in our study might be used as possible donors in breeding programs; however, the genetic basis for these differences must be further examined. In conclusion, enhanced UV-B radiation had significant effect on plant height, LAI, total biomass and grain yield of soybean crops under field conditions. The RI presented here showed that soybean is potentially a UV-B sensitive species. This result was similar to the previous study (Biggs and Kossuth, 1978). UVB sensitivity might be associated with UV-B radiation fluences and long-term accumulation (Li et al., 1998). Soybean is an economically important crop and its intraspecific response to UV-B radiation under field conditions is still not clearly understood. Effects of UV-B radiation on plants are related to other environmental factors, including PPF, CO2, drought, phosphorus nutrition, temperature, ozone fumigation and heavy metal (Teramura et al., 1990b; Caldwell et al., 1995). Further field studies are needed to improve the understanding of the relationship between UV-B radiation effectiveness and other environmental variables. This would greatly enhance our ability to more realistically assess intraspecific responses of soybean to increased levels of UV-B radiation.
Acknowledgements This work was supported by the National Natural Science Foundation of China, the National Research Foundation for Post-Doctorate Studies in China, the Applied Base Research Foundation of Yunnan Province, China, and the Research Foundation for Academic Leaders in Yunnan Province, China. We wish to thank Tan Lingling of the Department of Biology, Lanzhou University, for supporting the experimental work.
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