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Effects of enhanced ultraviolet-B radiation on crop structure, growth and yield components of spring wheat under field conditions
Field Crops Research 57 Ž1998. 253–263
Effects of enhanced ultraviolet-B radiation on crop structure, growth and yield components of spring wheat under field conditions Li Yuan 1, Yue Ming, Wang Xunling
)
Department of Biology, Lanzhou UniÕersity, Lanzhou 730000, China Received 20 May 1997; revised 25 October 1997; accepted 29 October 1997
Abstract Spring wheat ŽTriticum aestiÕum. was grown in the field for two consecutive seasons under ambient and supplemental levels of ultraviolet-B ŽUV-B, 280–315 nm. radiation to determine the potential for alterations in community structure, developmental stages, growth and yield components. The supplemental UV-B radiation simulated depletions of 12, 20, or 25% stratospheric ozone. Spring wheat is a potentially UV-B sensitive species, showing the greatest sensitivity to UV-B radiation at 5.31 kJ my2 . Delays in development and decrease in plant height were observed at early tillering stage under UV-B treatment, and slowly exacerbated during further development. UV-B radiation changed crop structure, by decreasing the total number of tillers produced and increasing dead shoot number, resulted in fewer head-bearing shoots at ripening stage, and decreased biomass and yield. UV-B radiation decreased the area of the last leaf and leaf area index, but increased specific leaf weight. UV-B radiation inhibited biomass accumulation and altered the patterns of biomass partitioning; these effects might be correlated with yield. Decreases in yield were the result of significant reductions in spike number, grain number per spike and thousand grain weight under UV-B. Generally, the effects of UV-B radiation on developmental stages and crop structure were the most important, they might change the other characteristics of spring wheat crop. The responses of spring wheat crop to enhanced UV-B radiation were assessed, decreases in some crop characteristics caused by a 10 or 20% global ozone depletion were predicted. Ozone depletion had the greatest decrease in yield and the least reduction in plant height. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Spring wheat; Stratospheric ozone depletion; UV-B radiation; Crop structure; Growth; Yield components
1. Introduction The rapid decline in stratospheric ozone concentrations has been confirmed by satellite measurements ŽMolina and Molina, 1992.. The most pro)
Corresponding author. Current address: Department of Environmental Science, Yunnan Agricultural University, Kunming 650201, China. 1
nounced thinning of the ozone layer has been measured over the Antarctic continent with up to 71% depletion during the Antarctic spring ŽKerr, 1993.. Recent mathematical models predict a further increase in solar ultraviolet-B ŽUV-B. irradiation in future years ŽMadronich et al., 1995.. UV-B effects on higher plants have been the subject of considerable research, approximately 350 papers have appeared ŽCaldwell et al., 1995.. An examination of
0378-4290r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 4 2 9 0 Ž 9 7 . 0 0 1 3 8 - X
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L. Yuan et al.r Field Crops Research 57 (1998) 253–263
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 ŽTeramura, 1983.. Plant species and even genotypes within a species can differ greatly in their responses to UV-B. The reasons for this are not clear ŽCaldwell and Flint, 1994.. Commonly observed UV-B effects on plants include physiological damage to the photosynthetic apparatus, damage to DNA, alteration in protein content and enzyme activity, effects on membranes and changes in leaf chemistry. Morphological damage may also result and be observed as plant stunting, leaf discoloration or reductions in vegetative biomass and grain yield ŽTeramura et al., 1990a.. On the other hand, most UV-B research in the past 2 decades has been conducted as short-term experiments in growth chambers and greenhouses where the unnatural spectral balance of radiation can lead to unrealistic conclusions, which may have substantially changed plant sensitivity to UV-B. It is important that experiments maintain a realistic balance between various spectral regions since both UV-A Ž315–400 nm. and visible Ž400–700 nm. radiation can have ameliorating effects on responses of plants to UV-B ŽCaldwell and Flint, 1994.. In growth chamber and greenhouse experiments, 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 ŽCaldwell et al., 1995.. 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 ŽCaldwell et al., 1995.. Vegetation communities function differently to plants in isolation and behavior of the former is not easily predicted from the latter. The history of research on elevated CO 2 certainly shows the ecosystem-level effects are not easily predicted from experiments with isolated plants ŽKorner, 1993.. If our ¨ thesis is correct, community-level effects of elevated UV-B radiation are not easily predicted from experiments with isolated plants. Community-level field experimentation is needed to evaluate realistic conse-
quences of increased solar UV-B resulting from ozone reduction. Wheat is one of the world’s major food crops ŽTeramura, 1983.; the effects of enhanced UV-B radiation on photosynthetic characteristics, morphology, total biomass and partitioning, tiller number and yield have been studied ŽTeramura, 1980; Teramura et al., 1990b; Barnes et al., 1990.. Unfortunately, only a few studies have been conducted under field conditions. In this study, we grew spring wheat in field under ambient and supplemental levels of UV-B radiation for two consecutive seasons with the objective to: Ž1. determine if UV-B radiation affects crop structure, growth and yield under field conditions; and Ž2. evaluate the effectiveness of UV-B radiation on spring wheat crops in the field.
2. Materials and methods 2.1. Plant materials and growth conditions Field experiments were conducted on a loessial soil at Lanzhou University, Lanzhou, China during the 1996 and 1997 growing seasons. On March 25, 1996, soil was sampled at a depth of 0.3 m and analyzed at the Soil Testing Laboratory of Yunnan Province, Kunming, China. Based upon the results of analysis Žsoil properties: pH 7.5, organic matter 3.3%, available N 94.2 mg kgy1 , available P 164.6 mg kgy1 , available K 228.6 mg kgy1 , available Fe 17.2 mg kgy1 , available Zn 6.7 mg kgy1 and exchangeable Mg 563.9 mg kgy1 ., no fertilization was necessary during two consecutive seasons. Seeds of spring wheat ŽTriticum aestiÕum. cultivar 80101, the most grown spring wheat cultivar in the Lanzhou region, were obtained from Lanzhou Agricultural Science Research Institute, germinated in a chamber at 258C and then sown in rows spaced 0.1 m apart at a seeding density of 70 seeds my1 in 12 plots of 2.0 = 1.0 m2 each on March 30, 1996, and in six plots of 3.0 = 2.0 m2 each on March 11, 1997. 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 four UV-B treatments and three replications in 1996, and with two UV-B treatments and three replications in 1997. At the three-leaf stage,
L. Yuan et al.r Field Crops Research 57 (1998) 253–263
plants were thinned to 60 my1 for uniformity in growth. This planting density is within commonly used sowing practices for the Lanzhou region. 2.2. UV-B radiation Supplemental UV-B radiation was provided by filtered Qin brand ŽBaoji Lamp Factory, China. 30-W sunlamps following the procedure outlined in the work of Lydon et al. Ž1986.. Lamps were suspended above and perpendicular to the planted rows Žrows oriented in an east–west direction to minimize shading. and filtered with either 0.13-mm thick cellulose diacetate Žtransmission down to 290 nm. for supplemental UV-B radiation or 0.13-mm polyester plastic films Žabsorbs all radiation below 320 nm. as a control ŽSullivan 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 ŽOptronics Laboratories Orlando, FL, USA. spectroradiometer. The spectral irradiance was weighted with the general-
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ized plant response action spectrum ŽCaldwell, 1971. and normalized at 300 nm to obtain UV-B BE . We suspended 6 and 18 lamps above each plot in 1996 and 1997, respectively. Plants were irradiated for 7 h daily from three-leaf stage to ripening stage, centered around solar noon. Plants under polyester-filtered lamps received only ambient levels of UV-B radiation Ž8.85 kJ my2 UV-B BE 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 0.65, 0.5 or 0.35 m between the lamps and the top of the plants, and provided supplemental irradiances of 2.54, 4.25 or 5.31 effective kJ my2 UV-B BE , respectively in 1996. Only supplemental irradiance of 5.31 effective kJ my2 UV-B BE was used in 1997. These supplemental levels of 2.54, 4.25 or 5.31 kJ my2 UV-B radiation were similar to those which would be experienced at Lanzhou Ž368N, 1650 m. with a 12, 20 or 25% stratospheric ozone reductions during a clear day on the summer solstice Ž8.85 kJ my2 UV-B BE . according to a mathematical
Table 1 The effects of enhanced UV-B radiation on phenological development and plant height of spring wheat crops in 1996 and 1997 UV-B ŽkJ my2 .
Developmental stages Three-leaf
Early tillering
Tillering
Elongation
Booting
Heading
Flowering
Filling
Milk
Ripening
35 36 36 37
40 41 42 44
52 53 55 56
57 58 60 62
65 67 69 72
71 72 74 78
74 76 78 82
94 96 98 101
105 107 109 112
37 48
43 47
56 60
64 69
72 78
78 85
82 89
102 109
113 120
18.3a 18.7a 18.3a 18.5a
37.2a 32.5b 32.7b 31.3b
49.0a 46.9b 46.3b 46.0b
73.8a 70.7b 69.8bc 69.1c
80.3a 78.1b 77.6b 74.2c
38.3a 86.4b 85.5b 81.3c
36.0a 93.8b 93.4b 90.3c
97.5a 95.0b 94.9b 91.4c
99.9a 95.2b 94.8b 91.6c
110.9a 103.0b 99.1b 93.2c
18.9a 19.2b
36.4a 33.0b
48.4a 45.2b
74.6a 68.6b
83.5a 74.0b
89.2a 79.5b
96.5a 91.2b
99.5a 93.2b
100.2a 94.0b
113.4a 97.6b
Days after planting (day) 1996 0 23 2.54 23 4.25 23 5.31 23 1997 0 25 5.31 25 Plant height (cm) 1996 0 2.54 4.25 5.31 1997 0 5.31
Days after planting were defined as the completion of each developmental stage, respectively, by 50 of the main shoots. Plant height means in each column Žfor each year. followed by the same letter are not significantly different at P - 0.05, based on Duncan’s Multiple Range Test Ž n s 20..
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UV-B ŽkJ my2 .
Main shoots Žmy2 .
Tillers Žmy2 .
Total shoots at tillering stage Žmy2 .
Dead shoots Žmy2 .
% Dead shoots
Total shoots at ripening stage Žmy2 .
Head-bearing shoots Žmy2 .
Nonhead-bearing shoots Žmy2 .
1996 0 2.54 4.25 5.31
600 600 600 600
341.2a 307.6b 292.7b 160.4c
941.2a 907.6b 892.7b 760.4c
193.9c 321.4ab 348.5a 304.4b
20.5 35.4 39.0 40.0
747.3a 586.2b 544.2b 456.0c
660.2a 512.1b 504.1b 446.4c
87.1a 74.1b 40.1c 9.63d
1997 0 5.31
600 600
349.5a 158.4b
949.5a 758.4b
199.4b 312.4a
21.0 41.2
750.1a 446.0b
664.7a 425.3b
85.4a 20.7b
Means in each column for each year followed by the same letter are not significantly different at P - 0.05, based on Duncan’s Multiple Range Test Ž ns6.. Crop structure is expressed on a ground area basis. Tillers express the total number of tillers produced. The % dead shoots expresses a percentage of total shoots at tillering stage.
L. Yuan et al.r Field Crops Research 57 (1998) 253–263
Table 2 The effects of enhanced UV-B radiation on structure of spring wheat crops in 1996 and 1997
UV-B ŽkJ my2 .
Spikesa Žmy2 .
Grain yield a Žmy2 .
Spike length Žcm.
Grain number per spike Žspikey1 .
Grain length Žcm.
Thousand grain weight Žg=1000 grainy1 .
HI a Ž%.
1996 0 2.54 4.25 5.31
658.7a 510.1b 503.2b 442.3c
425.0a 350.1b 290.2c 189.7d
8.6a 7.7b 7.1b 5.6c
29.4a 27.9a 23.7b 22.5b
0.66a 0.65a 0.64b 0.61c
31.3a 28.0b 27.0b 20.4c
34.3 33.8 32.3 24.6
1997 0 5.31
662.9a 423.4b
418.2a 179.8b
8.2a 5.8b
28.5a 22.1b
0.66a 0.61b
30.0a 21.2b
32.5 23.2
Means in each column for each year followed by the same letter are not significantly different at P - 0.05, based on Duncan’s Multiple Range Test. a Indicates ns6 and is expressed on a ground area basis, and ns 20 in the other columns. HI: harvest index Ž%..
L. Yuan et al.r Field Crops Research 57 (1998) 253–263
Table 3 The effects of enhanced UV-B radiation on yield components of spring wheat crops in 1996 and 1997
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L. Yuan et al.r Field Crops Research 57 (1998) 253–263
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Table 4 The effects of enhanced UV-B radiation on area of the last leaf, leaf area index ŽLAI. and specific leaf weight ŽSLW. of spring wheat at various developmental stages in 1996 UV-B ŽkJ my2 .
0 2.54 4.25 5.31
SLW Žg my2 .
Area of the last leaf Žm2 my2 .
LAI
Tillering
Flowering
Early tillering
Tillering
Booting
Flowering
Early tillering
Tillering
Booting
1.6a 1.3b 1.3b 1.1c
2.3a 2.2a 1.9b 1.6c
3.6a 3.3ab 3.1b 2.0c
4.2a 3.8b 3.6b 2.1c
6.2a 5.6b 5.4b 4.4c
5.1a 4.5b 4.3b 3.2c
23.5c 24.1bc 25.1b 28.1a
29.6c 30.0bc 31.2b 38.9a
46.9b 52.1a 51.7a 54.4a
Means in each column followed by the same letter are not significantly different at P - 0.05 based on Duncan’s Multiple Range Test Ž n s 6.. Area of the last leaf is expressed on a ground area basis.
model of Madronich et al. Ž1995.. Total daily photosynthetic photon fluence ŽPPF between 400–700 nm. under lamp fixtures was 90% of that above the lamps. 2.3. Measurements and statistical analyses The phenological calendar Ždays after planting. was defined as completion of each developmental stage ŽTable 1., respectively, by 50 of the main shoots. We used 20 main shoots per plot for observing plant height. Plant height was expressed as the distance from soil surface to the shoot tips from three-leaf stage to milk stage, and to spike tips at ripening stage. Plants in two subplots of 0.5 = 0.5 m2 each were harvested from each plot to determine crop structure parameters ŽTable 2. at each developmental stage and yield components at maturity ŽTable 3.. Shoots
in each subplot were separated into leaves, stems, spikes and grains, and roots were carefully extracted by washing away the mineral matter from soil monoliths 0.5 = 0.5 = 0.3 m3 at maturity. All plant samples were oven-dried at 688C for 68 h and weighed. Harvest index ŽHI, 100 = grain yield divided by shoot mass Ž%.. was calculated. Total leaves, the last leaf Žthe uppermost fully expanded leaf. and a subsample of 20 leaves per subplot were collected at each developmental stage ŽTable 4., leaf area ŽLA. of this subsample was measured with a Li-Cor 3100 ŽLi-Cor, Lincoln, NE, USA. area meter, then all leaves were oven dried at 688C for 68 h and weighed. A regression relationship was developed between leaf weight and leaf area Ž r s 0.5813, P - 0.01.. This linear regression was used to determine total LA and area of the last leaf of crops, then LAI and SLW Žthe ratio of leaf mass to area. were determined.
Table 5 The regression analyses between spring wheat crop characteristics Ž y . and enhanced UV-B radiation Ž x, kJ my2 ., and the predicted percent reductions of the effectiveness of UV-B radiation at a simulated 10 or 20% ozone depletion at ripening stage Characteristics
Regression models
r
F-value
Mean error Ž%.
PPR 10 Ž%.
PPR 20 Ž%.
Grain yield Žg my2 . Total biomass Žg my2 . Leaf biomass Žg my2 . Total shoots Žno. my2 . Spikes Žno. my2 . Plant height Žcm.
y s 438.91 y 41.38 x y s 1387.83 y 89.19 x y s 144.11ey0.0589 x y s 746.80 ey0.0869 x y s 644.86 ey0.0693 x y s 111.34 ey0 .0310 x
y0.96 y0.96 y0.98 y0.98 y0.96 y0.99
18.9 19.6 88.5 177.3 18.8 109.7
9.3 4.6 1.9 3.0 3.6 1.0
17.4 12.6 11.9 16.9 15.5 6.0
38.1 26.4 22.3 31.0 27.1 12.0
All correlation coefficient Ž r . and F-values are significant at P - 0.05 Ž n s 4.. All characteristics except plant height are expressed on a ground area basis. PPR 10 or PPR 20 : the predicted percent reductions of the effectiveness of UV-B radiation at a simulated 10 or 20% ozone depletion.
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Statistical differences between absolute treatment means of any measured parameter were determined by Duncan’s Multiple Range Test at the P - 0.05 level. Regression models were used to predict the effectiveness of supplemental UV-B radiation in altering each measured parameter ŽTable 5.. The Fvalue and correlation coefficient were utilized to test the significance of the regression models.
3. Results 3.1. DeÕelopmental stages Each developmental stage was shorter in 1996 than that in 1997, with 0 and 5.31 kJ my2 UV-B radiation, respectively. From early tillering to ripening in 1996 and 1997, developmental stages were delayed by supplemental UV-B radiation, depending strongly on UV-B radiation levels. The delay was expressed more after flowering ŽTable 1.. With treatment of 2.54, 4.25 or 5.31 kJ my2 UV-B radiation, developmental stages were postponed by 1, 3 or 4 to 7 days, respectively from early tillering to flowering, and were delayed by 2, 4 or 7 to 8 days respectively after flowering. 3.2. Plant height Spring wheat plants were significantly dwarfed by UV-B radiation in 1996 and 1997 ŽTable 1.. There was no significant difference in plant height between treatments when supplemental UV-B radiation was provided at the three-leaf stage, but it was significantly decreased from early tillering to ripening. Reductions in plant height were closely related to developmental stage, and were more obvious after stem elongation. Reductions in plant height were the greatest with highest UV-B radiation Ž5.31 kJ my2 . after stem elongation. For spring wheat exposed to 2.54, 4.25 or 5.31 kJ my2 UV-B radiation by ripening, plant height decreased by a 7.2, 10.6 or 16.2%, respectively, and differences in plant height were the most obvious, because plant height was measured at the tips of spikes, including the length of spike and spike stalk, which was decreased a 8.5, 9.4 or 32.3%, respectively, and significant differences Žbased on
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Duncan’s Multiple Range Test, P - 0.05, n s 20. were observed between treatments. 3.3. Crop structure The main shoot number of spring wheat was 600 my2 in all plots at tillering. Enhanced UV-B radiation affected crop structure in 1996 and 1997 ŽTable 2.. UV-B radiation significantly decreased the total number of tillers produced, this resulted in reductions in total shoot number at tillering stage. Increases in dead shoot number and the percentage of dead shoots were observed, and dead shoot number was more than the total number of tillers produced with enhanced UV-B radiation levels. In fact, a few main shoots died, and the number of dead main shoot was increased by enhanced UV-B radiation. Most dead shoots occurred during the tillering and elongation stages. Those changes reduced the number of head-bearing and nonhead-bearing shoots. Differences in crop structure except nonhead-bearing shoot number were not observed between 2.54 and 4.25 kJ my2 UV-B radiation. Changes in crop structure were greatest at 5.31 kJ my2 UV-B radiation. 3.4. Leaf area dynamics and specific leaf weight The experiment showed a significant decline in area of the last leaf at tillering and flowering and LAI at early tillering, tillering, booting and flowering ŽTable 4.. Both the area of last leaf and LAI were least at 5.31 kJ my2 UV-B radiation. At early tillering, tillering and booting, SLW was increased by enhanced UV-B radiation, although the differences were not significant between 2.54, 4.25 and 5.31 kJ my2 UV-B radiation at booting ŽTable 4.. SLW was not sensitive to UV-B radiation levels at booting. 3.5. Crop biomass and partitioning patterns Because most dead shoots occurred during tillering and stem elongation, the biomass of dead tissue was small, and was included in total biomass. UV-B radiation was effective in reducing crop total biomass and the biomass of leaves, stems, roots and spikes as well as in producing large shifts in biomass partition-
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Table 6 The effects of enhanced UV-B radiation on biomass and partitioning patterns of spring wheat crops at ripening stage in 1996 and 1997 UV-B ŽkJ my2 .
Total biomass Žg my2 .
Leaves
Stems
Roots
Biomass Žg my2 .
%
Biomass Žg my2 .
%
Biomass Žg my2 .
1996 0 2.54 4.25 5.31 1997 0 5.31
Spikes %
Biomass Žg my2 .
%
1350.8a 1163.8ab 1087.5b 850.0c
144.4a 122.0b 116.1c 103.4c
10.5 10.5 10.7 12.2
520.8a 448.4b 424.8b 340.0c
38.0 38.5 39.1 40.0
113.0ab 129.0a 98.4b 80.2c
9.7 11.1 9.1 9.4
572.6a 464.4b 448.2b 326.4c
41.8 39.9 41.4 38.4
1417.7a 848.5b
155.9a 118.7b
11.0 14.0
544.7a 340.4b
38.4 40.1
128.4a 73.5b
8.7 8.7
588.7a 315.9b
39.8 37.3
Means in each column for each year followed by the same letter are not significantly different at P - 0.05, based on Duncan’s Multiple Range Test Ž n s 6.. Biomass is expressed on a ground area basis, and biomass partitioning patterns are expressed as a percentage of total biomass.
ing at ripening stage in 1996 and 1997 ŽTable 6.. A 37.1% and 40.2% decrease in total biomass was observed with 5.31 kJ my2 UV-B radiation in 1996 and 1997, respectively. The treatment of 5.31 kJ my2 UV-B radiation resulted in obvious shifts in biomass partitioning patterns, a larger proportion of biomass was in leaves and stems and less in spikes, whereas the proportion of roots did not show marked change. 3.6. Yield components Enhanced UV-B radiation was shown to affect yield components in 1996 and 1997 ŽTable 3.. Significant reductions in crop spike number, crop grain yield, grain number per spike, spike length, grain length and thousand grain weight were observed, grain number per spike and grain length were unaffected with 2.54 kJ my2 UV-B radiation. In addition, UV-B radiation significantly decreased spike biomass ŽTable 6.. HI did not change with 2.54 or 4.25 kJ my2 UV-B radiation, but it decreased obviously at 5.31 kJ my2 UV-B radiation. Reductions in yield components were the greatest with 5.31 kJ my2 UV-B radiation. 3.7. Regression models and prediction Based on experimental results in 1996, regression models were used to predict the effectiveness of
supplemental UV-B radiation in altering spring wheat crop characteristics at ripening. The most appropriate predictor models for UV-B effectiveness differed for each parameter ŽTable 5., both correlation coefficient Ž r . and F-value of all predictor models were significant Ž P - 0.05., so the predictor models were useable. The effectiveness of UV-B radiation in simulating a 10 or 20% stratospheric ozone depletion on grain yield, total biomass, leaf biomass, total shoot number, spike number and plant height at ripening were predicted ŽTable 5.. Ozone depletion effected the greatest decrease in grain yield and the least reduction in plant height.
4. Discussion This is the first report to suggest that UV-B radiation may postpone developmental stages of spring wheat in the field. This is supported by earlier finding that UV-B radiation may delay developmental stages of alpine plants and the onset of flowering of other plants ŽCaldwell, 1968; Ziska et al., 1992; Musil, 1995; Mark et al., 1996.. UV-B radiation may directly affect cell division and some intrinsic growth characteristics, and delay the rate of plant development, this general growth delay has been recognized as one means of protection from UV-B radiation ŽBeggs et al., 1985.. It may be associated with many
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of the changes observed following UV-B exposure, such as the changes in plant height, grain yield, total biomass, tiller and leaf area dynamics. The delay in flowering was the most important, it resulted in yield reduction in maize ŽMark et al., 1996.. In the case that high UV-B radiation delays flowering and thus harvestable yield, this may have severe economic and ecological consequences for many crops and natural ecosystems as well. In many areas, delay in harvest of crops might not be possible due to changing climate conditions. On the other hand, in natural ecosystems pollinators may not be available when flowers are in their final reproductive phase. This may lead to changes in biodiversity. The reduction in plant height has often been the index used to assess the degree of UV-B radiation sensitivity ŽBiggs and Kossuth, 1978.. Although UV-B radiation had no consistent effect on wheat height in a greenhouse experiment ŽTeramura, 1980., in this study, UV-B radiation significantly dwarfed spring wheat, primarily due to shorter internodes rather than node number, because there were six nodes in plants in all treatments. This can be due to a photo-oxidative destruction of the phytohormone indole acetic acid followed by reduced cell wall extensibility as demonstrated in sunflower seedlings ŽRos and Tevini, 1995.. The levels of ethylene, which promotes radial growth and reduces elongation, are increased after irradiation with UV-B ŽCaldwell et al., 1995.. Total biomass accumulation is a good indicator of UV-B radiation effects on growth ŽTeramura, 1983.. Decreases in total biomass in this study are similar to those found in greenhouses ŽTeramura, 1980, 1983; Barnes et al., 1990.. Other greenhouse experiments have reported either increase or no change in biomass of wheat with enhanced UV-B radiation ŽTeramura, 1983.. This study suggests that reduction in total biomass might be often accompanied by substantial modification in the partitioning of biomass into component plant organs. A similar conclusion in wheat has been reported ŽTeramura, 1980.. A larger proportion of biomass was invested as leaves and stems, and less as spikes, these changes might be related to the translocation of dry matter from leaves and stems into spikes. The larger proportion of biomass partitioned to leaves might be a result of increases in SLW. Increasing SLW, leaf thickening ŽSullivan and
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Teramura, 1990. may be an adaptation to UV-B radiation, with upper leaf tissue acting as an anatomical screen or filter to decrease UV-B transmission into sensitive underlying tissue ŽTeramura, 1983.. Very little is known about UV-B effects on grain yield of wheat ŽTeramura, 1983., only an 8% decrease has been reported in a greenhouse study ŽTeramura et al., 1990b.. In the present study, grain yield was positively correlated with crop spike number Ž r s 0.9728, P - 0.05., grain number per spike Ž r s 0.9517, P - 0.05. and thousand grain weight Ž r s 0.9784, P - 0.05.. Crop spike number was the result of the change of crop structure. Grain number per spike and thousand grain weight might be related to an integration of all phenological and physiological presses that affect growth ŽTeramura, 1980, 1983.. Although UV-B radiation increased secondary tiller number of isolated wheat plants in the greenhouse ŽTeramura, 1980; Barnes et al., 1990., in this experiment, it significantly decreased crop tiller number under field conditions. The same effect has been observed in 14 of 16 rice cultivars ŽTeramura et al., 1991.. On the other hand, UV-B radiation exacerbated intraspecific competition and self-thinning of a spring wheat crop, increasing dead shoot number. In this way, head-bearing shoot number was decreased at ripening stage, reducing total biomass, grain yield, LAI and area of the last leaf. Total shoot number at ripening was positively correlated with total biomass Ž r s 0.9771, P - 0.05. and grain yield Ž r s 0.9640, P - 0.05.. Additionally, reduced photosynthetic rates due to reduction in enzyme activity ŽJordan et al., 1992., in the efficiency of photosystem II ŽStrid et al., 1990. and in stomatal conductance ŽNegash and Bjorn, 1986. might contribute to the decrease in the grain yield and total biomass. Previous studies concerning the effects of UV-B radiation on wheat have been conducted with isolated plants as short-term experiments in greenhouse, the responses of plant height and tiller number to UV-B radiation ŽTeramura, 1980; Barnes et al., 1990. do not coincide with those of this study with crops under field conditions. Because plants respond differently to UV-B in each environment ŽTeramura and Murali, 1985., such as the differences between greenhouses and field conditions ŽCaldwell et al., 1995.. In addition to the genetic differences inherent
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in different cultivars, 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 ŽTeramura et al., 1991.. Besides, intraspecific competition and self-thinning in spring wheat crops have been observed in this study ŽTable 2., so the responses of spring wheat crops to UV-B radiation may be more complicated than those of plants in isolation.
5. Conclusion In conclusion, enhanced UV-B radiation had significant effects on spring wheat crops under field conditions. The effects on developmental stages and crop structure were the most important, they might change growth, biomass partitioning and grain yield. The data presented here show that spring wheat is a potentially UV-B sensitive species, although wheat has been previously shown to be UV-B resistant ŽBiggs and Kossuth, 1978.. UV-B sensitivity might be associated with UV-B radiation fluences and long-term accumulation ŽTable 1.. Spring wheat is an economically important crop. Its response to UV-B radiation under field conditions is not clearly understood and will be studied intensively. Since the effects of UV-B radiation on plants are related to other environmental factors, including PPF, CO 2 , drought, phosphorus nutrition, temperature, ozone fumigation and heavy metal ŽTeramura et al., 1990a; Caldwell et al., 1995., the field studies and an understanding of the relationship between UV-B radiation effectiveness and other environmental variables would greatly enhance our ability to more realistically assess the impacts of increased levels of solar UV-B radiation resulting from stratospheric ozone depletion.
Acknowledgements This work was supported by the National Natural Science Foundation of China. We wish to thank Elisabeth Kessler, the editor-in-chief of Ambio, for providing Ambio Vols. 24 No. 2 and 3, 1995.
References Barnes, P.W., Flint, S.D., Caldwell, M.M., 1990. Morphological responses of crop and weed species of different growth forms to ultraviolet-B radiation. Am. J. Bot. 77 Ž10., 1354–1360. Beggs, C.J., Schneider-Zeibert, R., Wellman, E., 1985. UV-B radiation and adaptive mechanisms in plants. In: Worrest, R.C. ŽEd.., Stratospheric Ozone Reduction, Solar Ultraviolet Radiation and Plant Life. Springer-Verlag, Berlin, pp. 235–250. Biggs, R.H., Kossuth, S.V., 1978. Impact of solar UV-B radiation on crop productivity. Final Report of UV-B Biological and Climate Effects Research. Terrestrial FY 1977, pp. 11–77. Caldwell, M.M., 1968. Solar ultraviolet radiation as an ecological factor for alpine plants. Ecol. Monogr. 38, 243–268. Caldwell, M.M., 1971. Solar UV-B irradiation and the growth and development of higher plants. In: Giese, A.C. ŽEd.., Photophysiology, Vol 6. Academic Press, New York, NY, pp. 131–171. Caldwell, M.M., Flint, S.D., 1994. Stratospheric ozone reduction, solar UV-B radiation and terrestrial ecosystem. Clim. Change 27, 375–394. Caldwell, M.M., Teramura, A.H., Tevini, M., Bornman, J.F., Bjorn, L.O., Kulandaivelu, G., 1995. Effects of increased solar ultraviolet radiation on terrestrial plants. Ambio 24 Ž3., 166– 173. Jordan, B.B., He, J., Chow, W.S., Anderson, J.M., 1992. Changes in mRNA levels and polypeptide subunits of ribulose 1,5-bisphosphate carboxylase in response to supplementary ultraviolet-B radiation. Plant Cell Environ. 15, 91–98. Kerr, R.A., 1993. The ozone hole reaches a new low. Science 262, 501. Korner, C., 1993. CO 2 fertilization: the great uncertainty in future ¨ vegetation development. In: Soloman, A.M., Shugart, H.H. ŽEds.., Vegetation Dynamics and Global Change. Chapman & Hall, London, pp. 53–70. 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. ŽEd.., Stratospheric Ozone Reduction, Solar Ultraviolet Radiation and Plant Life. Springer-Verlag. 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. Mark, U., Saile-Mark, M., Tevini, M., 1996. Effects of solar UV-B radiation on growth, flowering and yield of Central and Southern European maize cultivars Ž Zea mays L... Photochem. Photobiol. 64 Ž3., 457–462. Molina, M.J., Molina, L.T., 1992. Stratospheric ozone. A. C.S. Symp. Ser. Am. Chem. Soc. Washington, DC, 483: 24–35. Musil, C.F., 1995. Differential effects of elevated ultraviolet-B radiation on the photochemical and reproductive performances of dicotyledonous and monocotyledonous arid-environmental ephemerals. Plant Cell Environ. 18, 844–854. Negash, L., Bjorn, L.O., 1986. Stomatal closure by ultraviolet radiation. Physiol. Plant. 53, 19–26. Ros, J., Tevini, M., 1995. Interaction of UV-radiation and IAA
L. Yuan et al.r Field Crops Research 57 (1998) 253–263 during growth of seedling and hypocotyl segments of sunflower. J. Plant Physiol. 146, 295–302. Strid, A., Chow, W.S., Anderson, J.M., 1990. Effects of supplementary ultraviolet-B radiation on photosynthesis in Pisum satiÕum. Biochim. Biophys. Acta 1020, 260–268. 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., 1985. Intraspecific differences in
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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 CO 2 on productivity and photosynthetic characteristics in wheat, rice and soybean. 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 UV-B radiation. Physiol. Plant. 83, 373–380. Ziska, L.H., Teramura, A.H., Sullivan, J.H., 1992. Physiological sensitivity of plants along an elevational gradient to UV-B radiation. Am. J. Bot. 79, 863–871.
Effects of enhanced ultraviolet-B radiation on crop structure, growth and yield components of spring wheat under field conditions Li Yuan 1, Yue Ming, Wang Xunling
)
Department of Biology, Lanzhou UniÕersity, Lanzhou 730000, China Received 20 May 1997; revised 25 October 1997; accepted 29 October 1997
Abstract Spring wheat ŽTriticum aestiÕum. was grown in the field for two consecutive seasons under ambient and supplemental levels of ultraviolet-B ŽUV-B, 280–315 nm. radiation to determine the potential for alterations in community structure, developmental stages, growth and yield components. The supplemental UV-B radiation simulated depletions of 12, 20, or 25% stratospheric ozone. Spring wheat is a potentially UV-B sensitive species, showing the greatest sensitivity to UV-B radiation at 5.31 kJ my2 . Delays in development and decrease in plant height were observed at early tillering stage under UV-B treatment, and slowly exacerbated during further development. UV-B radiation changed crop structure, by decreasing the total number of tillers produced and increasing dead shoot number, resulted in fewer head-bearing shoots at ripening stage, and decreased biomass and yield. UV-B radiation decreased the area of the last leaf and leaf area index, but increased specific leaf weight. UV-B radiation inhibited biomass accumulation and altered the patterns of biomass partitioning; these effects might be correlated with yield. Decreases in yield were the result of significant reductions in spike number, grain number per spike and thousand grain weight under UV-B. Generally, the effects of UV-B radiation on developmental stages and crop structure were the most important, they might change the other characteristics of spring wheat crop. The responses of spring wheat crop to enhanced UV-B radiation were assessed, decreases in some crop characteristics caused by a 10 or 20% global ozone depletion were predicted. Ozone depletion had the greatest decrease in yield and the least reduction in plant height. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Spring wheat; Stratospheric ozone depletion; UV-B radiation; Crop structure; Growth; Yield components
1. Introduction The rapid decline in stratospheric ozone concentrations has been confirmed by satellite measurements ŽMolina and Molina, 1992.. The most pro)
Corresponding author. Current address: Department of Environmental Science, Yunnan Agricultural University, Kunming 650201, China. 1
nounced thinning of the ozone layer has been measured over the Antarctic continent with up to 71% depletion during the Antarctic spring ŽKerr, 1993.. Recent mathematical models predict a further increase in solar ultraviolet-B ŽUV-B. irradiation in future years ŽMadronich et al., 1995.. UV-B effects on higher plants have been the subject of considerable research, approximately 350 papers have appeared ŽCaldwell et al., 1995.. An examination of
0378-4290r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 4 2 9 0 Ž 9 7 . 0 0 1 3 8 - X
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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 ŽTeramura, 1983.. Plant species and even genotypes within a species can differ greatly in their responses to UV-B. The reasons for this are not clear ŽCaldwell and Flint, 1994.. Commonly observed UV-B effects on plants include physiological damage to the photosynthetic apparatus, damage to DNA, alteration in protein content and enzyme activity, effects on membranes and changes in leaf chemistry. Morphological damage may also result and be observed as plant stunting, leaf discoloration or reductions in vegetative biomass and grain yield ŽTeramura et al., 1990a.. On the other hand, most UV-B research in the past 2 decades has been conducted as short-term experiments in growth chambers and greenhouses where the unnatural spectral balance of radiation can lead to unrealistic conclusions, which may have substantially changed plant sensitivity to UV-B. It is important that experiments maintain a realistic balance between various spectral regions since both UV-A Ž315–400 nm. and visible Ž400–700 nm. radiation can have ameliorating effects on responses of plants to UV-B ŽCaldwell and Flint, 1994.. In growth chamber and greenhouse experiments, 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 ŽCaldwell et al., 1995.. 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 ŽCaldwell et al., 1995.. Vegetation communities function differently to plants in isolation and behavior of the former is not easily predicted from the latter. The history of research on elevated CO 2 certainly shows the ecosystem-level effects are not easily predicted from experiments with isolated plants ŽKorner, 1993.. If our ¨ thesis is correct, community-level effects of elevated UV-B radiation are not easily predicted from experiments with isolated plants. Community-level field experimentation is needed to evaluate realistic conse-
quences of increased solar UV-B resulting from ozone reduction. Wheat is one of the world’s major food crops ŽTeramura, 1983.; the effects of enhanced UV-B radiation on photosynthetic characteristics, morphology, total biomass and partitioning, tiller number and yield have been studied ŽTeramura, 1980; Teramura et al., 1990b; Barnes et al., 1990.. Unfortunately, only a few studies have been conducted under field conditions. In this study, we grew spring wheat in field under ambient and supplemental levels of UV-B radiation for two consecutive seasons with the objective to: Ž1. determine if UV-B radiation affects crop structure, growth and yield under field conditions; and Ž2. evaluate the effectiveness of UV-B radiation on spring wheat crops in the field.
2. Materials and methods 2.1. Plant materials and growth conditions Field experiments were conducted on a loessial soil at Lanzhou University, Lanzhou, China during the 1996 and 1997 growing seasons. On March 25, 1996, soil was sampled at a depth of 0.3 m and analyzed at the Soil Testing Laboratory of Yunnan Province, Kunming, China. Based upon the results of analysis Žsoil properties: pH 7.5, organic matter 3.3%, available N 94.2 mg kgy1 , available P 164.6 mg kgy1 , available K 228.6 mg kgy1 , available Fe 17.2 mg kgy1 , available Zn 6.7 mg kgy1 and exchangeable Mg 563.9 mg kgy1 ., no fertilization was necessary during two consecutive seasons. Seeds of spring wheat ŽTriticum aestiÕum. cultivar 80101, the most grown spring wheat cultivar in the Lanzhou region, were obtained from Lanzhou Agricultural Science Research Institute, germinated in a chamber at 258C and then sown in rows spaced 0.1 m apart at a seeding density of 70 seeds my1 in 12 plots of 2.0 = 1.0 m2 each on March 30, 1996, and in six plots of 3.0 = 2.0 m2 each on March 11, 1997. 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 four UV-B treatments and three replications in 1996, and with two UV-B treatments and three replications in 1997. At the three-leaf stage,
L. Yuan et al.r Field Crops Research 57 (1998) 253–263
plants were thinned to 60 my1 for uniformity in growth. This planting density is within commonly used sowing practices for the Lanzhou region. 2.2. UV-B radiation Supplemental UV-B radiation was provided by filtered Qin brand ŽBaoji Lamp Factory, China. 30-W sunlamps following the procedure outlined in the work of Lydon et al. Ž1986.. Lamps were suspended above and perpendicular to the planted rows Žrows oriented in an east–west direction to minimize shading. and filtered with either 0.13-mm thick cellulose diacetate Žtransmission down to 290 nm. for supplemental UV-B radiation or 0.13-mm polyester plastic films Žabsorbs all radiation below 320 nm. as a control ŽSullivan 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 ŽOptronics Laboratories Orlando, FL, USA. spectroradiometer. The spectral irradiance was weighted with the general-
255
ized plant response action spectrum ŽCaldwell, 1971. and normalized at 300 nm to obtain UV-B BE . We suspended 6 and 18 lamps above each plot in 1996 and 1997, respectively. Plants were irradiated for 7 h daily from three-leaf stage to ripening stage, centered around solar noon. Plants under polyester-filtered lamps received only ambient levels of UV-B radiation Ž8.85 kJ my2 UV-B BE 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 0.65, 0.5 or 0.35 m between the lamps and the top of the plants, and provided supplemental irradiances of 2.54, 4.25 or 5.31 effective kJ my2 UV-B BE , respectively in 1996. Only supplemental irradiance of 5.31 effective kJ my2 UV-B BE was used in 1997. These supplemental levels of 2.54, 4.25 or 5.31 kJ my2 UV-B radiation were similar to those which would be experienced at Lanzhou Ž368N, 1650 m. with a 12, 20 or 25% stratospheric ozone reductions during a clear day on the summer solstice Ž8.85 kJ my2 UV-B BE . according to a mathematical
Table 1 The effects of enhanced UV-B radiation on phenological development and plant height of spring wheat crops in 1996 and 1997 UV-B ŽkJ my2 .
Developmental stages Three-leaf
Early tillering
Tillering
Elongation
Booting
Heading
Flowering
Filling
Milk
Ripening
35 36 36 37
40 41 42 44
52 53 55 56
57 58 60 62
65 67 69 72
71 72 74 78
74 76 78 82
94 96 98 101
105 107 109 112
37 48
43 47
56 60
64 69
72 78
78 85
82 89
102 109
113 120
18.3a 18.7a 18.3a 18.5a
37.2a 32.5b 32.7b 31.3b
49.0a 46.9b 46.3b 46.0b
73.8a 70.7b 69.8bc 69.1c
80.3a 78.1b 77.6b 74.2c
38.3a 86.4b 85.5b 81.3c
36.0a 93.8b 93.4b 90.3c
97.5a 95.0b 94.9b 91.4c
99.9a 95.2b 94.8b 91.6c
110.9a 103.0b 99.1b 93.2c
18.9a 19.2b
36.4a 33.0b
48.4a 45.2b
74.6a 68.6b
83.5a 74.0b
89.2a 79.5b
96.5a 91.2b
99.5a 93.2b
100.2a 94.0b
113.4a 97.6b
Days after planting (day) 1996 0 23 2.54 23 4.25 23 5.31 23 1997 0 25 5.31 25 Plant height (cm) 1996 0 2.54 4.25 5.31 1997 0 5.31
Days after planting were defined as the completion of each developmental stage, respectively, by 50 of the main shoots. Plant height means in each column Žfor each year. followed by the same letter are not significantly different at P - 0.05, based on Duncan’s Multiple Range Test Ž n s 20..
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UV-B ŽkJ my2 .
Main shoots Žmy2 .
Tillers Žmy2 .
Total shoots at tillering stage Žmy2 .
Dead shoots Žmy2 .
% Dead shoots
Total shoots at ripening stage Žmy2 .
Head-bearing shoots Žmy2 .
Nonhead-bearing shoots Žmy2 .
1996 0 2.54 4.25 5.31
600 600 600 600
341.2a 307.6b 292.7b 160.4c
941.2a 907.6b 892.7b 760.4c
193.9c 321.4ab 348.5a 304.4b
20.5 35.4 39.0 40.0
747.3a 586.2b 544.2b 456.0c
660.2a 512.1b 504.1b 446.4c
87.1a 74.1b 40.1c 9.63d
1997 0 5.31
600 600
349.5a 158.4b
949.5a 758.4b
199.4b 312.4a
21.0 41.2
750.1a 446.0b
664.7a 425.3b
85.4a 20.7b
Means in each column for each year followed by the same letter are not significantly different at P - 0.05, based on Duncan’s Multiple Range Test Ž ns6.. Crop structure is expressed on a ground area basis. Tillers express the total number of tillers produced. The % dead shoots expresses a percentage of total shoots at tillering stage.
L. Yuan et al.r Field Crops Research 57 (1998) 253–263
Table 2 The effects of enhanced UV-B radiation on structure of spring wheat crops in 1996 and 1997
UV-B ŽkJ my2 .
Spikesa Žmy2 .
Grain yield a Žmy2 .
Spike length Žcm.
Grain number per spike Žspikey1 .
Grain length Žcm.
Thousand grain weight Žg=1000 grainy1 .
HI a Ž%.
1996 0 2.54 4.25 5.31
658.7a 510.1b 503.2b 442.3c
425.0a 350.1b 290.2c 189.7d
8.6a 7.7b 7.1b 5.6c
29.4a 27.9a 23.7b 22.5b
0.66a 0.65a 0.64b 0.61c
31.3a 28.0b 27.0b 20.4c
34.3 33.8 32.3 24.6
1997 0 5.31
662.9a 423.4b
418.2a 179.8b
8.2a 5.8b
28.5a 22.1b
0.66a 0.61b
30.0a 21.2b
32.5 23.2
Means in each column for each year followed by the same letter are not significantly different at P - 0.05, based on Duncan’s Multiple Range Test. a Indicates ns6 and is expressed on a ground area basis, and ns 20 in the other columns. HI: harvest index Ž%..
L. Yuan et al.r Field Crops Research 57 (1998) 253–263
Table 3 The effects of enhanced UV-B radiation on yield components of spring wheat crops in 1996 and 1997
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L. Yuan et al.r Field Crops Research 57 (1998) 253–263
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Table 4 The effects of enhanced UV-B radiation on area of the last leaf, leaf area index ŽLAI. and specific leaf weight ŽSLW. of spring wheat at various developmental stages in 1996 UV-B ŽkJ my2 .
0 2.54 4.25 5.31
SLW Žg my2 .
Area of the last leaf Žm2 my2 .
LAI
Tillering
Flowering
Early tillering
Tillering
Booting
Flowering
Early tillering
Tillering
Booting
1.6a 1.3b 1.3b 1.1c
2.3a 2.2a 1.9b 1.6c
3.6a 3.3ab 3.1b 2.0c
4.2a 3.8b 3.6b 2.1c
6.2a 5.6b 5.4b 4.4c
5.1a 4.5b 4.3b 3.2c
23.5c 24.1bc 25.1b 28.1a
29.6c 30.0bc 31.2b 38.9a
46.9b 52.1a 51.7a 54.4a
Means in each column followed by the same letter are not significantly different at P - 0.05 based on Duncan’s Multiple Range Test Ž n s 6.. Area of the last leaf is expressed on a ground area basis.
model of Madronich et al. Ž1995.. Total daily photosynthetic photon fluence ŽPPF between 400–700 nm. under lamp fixtures was 90% of that above the lamps. 2.3. Measurements and statistical analyses The phenological calendar Ždays after planting. was defined as completion of each developmental stage ŽTable 1., respectively, by 50 of the main shoots. We used 20 main shoots per plot for observing plant height. Plant height was expressed as the distance from soil surface to the shoot tips from three-leaf stage to milk stage, and to spike tips at ripening stage. Plants in two subplots of 0.5 = 0.5 m2 each were harvested from each plot to determine crop structure parameters ŽTable 2. at each developmental stage and yield components at maturity ŽTable 3.. Shoots
in each subplot were separated into leaves, stems, spikes and grains, and roots were carefully extracted by washing away the mineral matter from soil monoliths 0.5 = 0.5 = 0.3 m3 at maturity. All plant samples were oven-dried at 688C for 68 h and weighed. Harvest index ŽHI, 100 = grain yield divided by shoot mass Ž%.. was calculated. Total leaves, the last leaf Žthe uppermost fully expanded leaf. and a subsample of 20 leaves per subplot were collected at each developmental stage ŽTable 4., leaf area ŽLA. of this subsample was measured with a Li-Cor 3100 ŽLi-Cor, Lincoln, NE, USA. area meter, then all leaves were oven dried at 688C for 68 h and weighed. A regression relationship was developed between leaf weight and leaf area Ž r s 0.5813, P - 0.01.. This linear regression was used to determine total LA and area of the last leaf of crops, then LAI and SLW Žthe ratio of leaf mass to area. were determined.
Table 5 The regression analyses between spring wheat crop characteristics Ž y . and enhanced UV-B radiation Ž x, kJ my2 ., and the predicted percent reductions of the effectiveness of UV-B radiation at a simulated 10 or 20% ozone depletion at ripening stage Characteristics
Regression models
r
F-value
Mean error Ž%.
PPR 10 Ž%.
PPR 20 Ž%.
Grain yield Žg my2 . Total biomass Žg my2 . Leaf biomass Žg my2 . Total shoots Žno. my2 . Spikes Žno. my2 . Plant height Žcm.
y s 438.91 y 41.38 x y s 1387.83 y 89.19 x y s 144.11ey0.0589 x y s 746.80 ey0.0869 x y s 644.86 ey0.0693 x y s 111.34 ey0 .0310 x
y0.96 y0.96 y0.98 y0.98 y0.96 y0.99
18.9 19.6 88.5 177.3 18.8 109.7
9.3 4.6 1.9 3.0 3.6 1.0
17.4 12.6 11.9 16.9 15.5 6.0
38.1 26.4 22.3 31.0 27.1 12.0
All correlation coefficient Ž r . and F-values are significant at P - 0.05 Ž n s 4.. All characteristics except plant height are expressed on a ground area basis. PPR 10 or PPR 20 : the predicted percent reductions of the effectiveness of UV-B radiation at a simulated 10 or 20% ozone depletion.
L. Yuan et al.r Field Crops Research 57 (1998) 253–263
Statistical differences between absolute treatment means of any measured parameter were determined by Duncan’s Multiple Range Test at the P - 0.05 level. Regression models were used to predict the effectiveness of supplemental UV-B radiation in altering each measured parameter ŽTable 5.. The Fvalue and correlation coefficient were utilized to test the significance of the regression models.
3. Results 3.1. DeÕelopmental stages Each developmental stage was shorter in 1996 than that in 1997, with 0 and 5.31 kJ my2 UV-B radiation, respectively. From early tillering to ripening in 1996 and 1997, developmental stages were delayed by supplemental UV-B radiation, depending strongly on UV-B radiation levels. The delay was expressed more after flowering ŽTable 1.. With treatment of 2.54, 4.25 or 5.31 kJ my2 UV-B radiation, developmental stages were postponed by 1, 3 or 4 to 7 days, respectively from early tillering to flowering, and were delayed by 2, 4 or 7 to 8 days respectively after flowering. 3.2. Plant height Spring wheat plants were significantly dwarfed by UV-B radiation in 1996 and 1997 ŽTable 1.. There was no significant difference in plant height between treatments when supplemental UV-B radiation was provided at the three-leaf stage, but it was significantly decreased from early tillering to ripening. Reductions in plant height were closely related to developmental stage, and were more obvious after stem elongation. Reductions in plant height were the greatest with highest UV-B radiation Ž5.31 kJ my2 . after stem elongation. For spring wheat exposed to 2.54, 4.25 or 5.31 kJ my2 UV-B radiation by ripening, plant height decreased by a 7.2, 10.6 or 16.2%, respectively, and differences in plant height were the most obvious, because plant height was measured at the tips of spikes, including the length of spike and spike stalk, which was decreased a 8.5, 9.4 or 32.3%, respectively, and significant differences Žbased on
259
Duncan’s Multiple Range Test, P - 0.05, n s 20. were observed between treatments. 3.3. Crop structure The main shoot number of spring wheat was 600 my2 in all plots at tillering. Enhanced UV-B radiation affected crop structure in 1996 and 1997 ŽTable 2.. UV-B radiation significantly decreased the total number of tillers produced, this resulted in reductions in total shoot number at tillering stage. Increases in dead shoot number and the percentage of dead shoots were observed, and dead shoot number was more than the total number of tillers produced with enhanced UV-B radiation levels. In fact, a few main shoots died, and the number of dead main shoot was increased by enhanced UV-B radiation. Most dead shoots occurred during the tillering and elongation stages. Those changes reduced the number of head-bearing and nonhead-bearing shoots. Differences in crop structure except nonhead-bearing shoot number were not observed between 2.54 and 4.25 kJ my2 UV-B radiation. Changes in crop structure were greatest at 5.31 kJ my2 UV-B radiation. 3.4. Leaf area dynamics and specific leaf weight The experiment showed a significant decline in area of the last leaf at tillering and flowering and LAI at early tillering, tillering, booting and flowering ŽTable 4.. Both the area of last leaf and LAI were least at 5.31 kJ my2 UV-B radiation. At early tillering, tillering and booting, SLW was increased by enhanced UV-B radiation, although the differences were not significant between 2.54, 4.25 and 5.31 kJ my2 UV-B radiation at booting ŽTable 4.. SLW was not sensitive to UV-B radiation levels at booting. 3.5. Crop biomass and partitioning patterns Because most dead shoots occurred during tillering and stem elongation, the biomass of dead tissue was small, and was included in total biomass. UV-B radiation was effective in reducing crop total biomass and the biomass of leaves, stems, roots and spikes as well as in producing large shifts in biomass partition-
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Table 6 The effects of enhanced UV-B radiation on biomass and partitioning patterns of spring wheat crops at ripening stage in 1996 and 1997 UV-B ŽkJ my2 .
Total biomass Žg my2 .
Leaves
Stems
Roots
Biomass Žg my2 .
%
Biomass Žg my2 .
%
Biomass Žg my2 .
1996 0 2.54 4.25 5.31 1997 0 5.31
Spikes %
Biomass Žg my2 .
%
1350.8a 1163.8ab 1087.5b 850.0c
144.4a 122.0b 116.1c 103.4c
10.5 10.5 10.7 12.2
520.8a 448.4b 424.8b 340.0c
38.0 38.5 39.1 40.0
113.0ab 129.0a 98.4b 80.2c
9.7 11.1 9.1 9.4
572.6a 464.4b 448.2b 326.4c
41.8 39.9 41.4 38.4
1417.7a 848.5b
155.9a 118.7b
11.0 14.0
544.7a 340.4b
38.4 40.1
128.4a 73.5b
8.7 8.7
588.7a 315.9b
39.8 37.3
Means in each column for each year followed by the same letter are not significantly different at P - 0.05, based on Duncan’s Multiple Range Test Ž n s 6.. Biomass is expressed on a ground area basis, and biomass partitioning patterns are expressed as a percentage of total biomass.
ing at ripening stage in 1996 and 1997 ŽTable 6.. A 37.1% and 40.2% decrease in total biomass was observed with 5.31 kJ my2 UV-B radiation in 1996 and 1997, respectively. The treatment of 5.31 kJ my2 UV-B radiation resulted in obvious shifts in biomass partitioning patterns, a larger proportion of biomass was in leaves and stems and less in spikes, whereas the proportion of roots did not show marked change. 3.6. Yield components Enhanced UV-B radiation was shown to affect yield components in 1996 and 1997 ŽTable 3.. Significant reductions in crop spike number, crop grain yield, grain number per spike, spike length, grain length and thousand grain weight were observed, grain number per spike and grain length were unaffected with 2.54 kJ my2 UV-B radiation. In addition, UV-B radiation significantly decreased spike biomass ŽTable 6.. HI did not change with 2.54 or 4.25 kJ my2 UV-B radiation, but it decreased obviously at 5.31 kJ my2 UV-B radiation. Reductions in yield components were the greatest with 5.31 kJ my2 UV-B radiation. 3.7. Regression models and prediction Based on experimental results in 1996, regression models were used to predict the effectiveness of
supplemental UV-B radiation in altering spring wheat crop characteristics at ripening. The most appropriate predictor models for UV-B effectiveness differed for each parameter ŽTable 5., both correlation coefficient Ž r . and F-value of all predictor models were significant Ž P - 0.05., so the predictor models were useable. The effectiveness of UV-B radiation in simulating a 10 or 20% stratospheric ozone depletion on grain yield, total biomass, leaf biomass, total shoot number, spike number and plant height at ripening were predicted ŽTable 5.. Ozone depletion effected the greatest decrease in grain yield and the least reduction in plant height.
4. Discussion This is the first report to suggest that UV-B radiation may postpone developmental stages of spring wheat in the field. This is supported by earlier finding that UV-B radiation may delay developmental stages of alpine plants and the onset of flowering of other plants ŽCaldwell, 1968; Ziska et al., 1992; Musil, 1995; Mark et al., 1996.. UV-B radiation may directly affect cell division and some intrinsic growth characteristics, and delay the rate of plant development, this general growth delay has been recognized as one means of protection from UV-B radiation ŽBeggs et al., 1985.. It may be associated with many
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of the changes observed following UV-B exposure, such as the changes in plant height, grain yield, total biomass, tiller and leaf area dynamics. The delay in flowering was the most important, it resulted in yield reduction in maize ŽMark et al., 1996.. In the case that high UV-B radiation delays flowering and thus harvestable yield, this may have severe economic and ecological consequences for many crops and natural ecosystems as well. In many areas, delay in harvest of crops might not be possible due to changing climate conditions. On the other hand, in natural ecosystems pollinators may not be available when flowers are in their final reproductive phase. This may lead to changes in biodiversity. The reduction in plant height has often been the index used to assess the degree of UV-B radiation sensitivity ŽBiggs and Kossuth, 1978.. Although UV-B radiation had no consistent effect on wheat height in a greenhouse experiment ŽTeramura, 1980., in this study, UV-B radiation significantly dwarfed spring wheat, primarily due to shorter internodes rather than node number, because there were six nodes in plants in all treatments. This can be due to a photo-oxidative destruction of the phytohormone indole acetic acid followed by reduced cell wall extensibility as demonstrated in sunflower seedlings ŽRos and Tevini, 1995.. The levels of ethylene, which promotes radial growth and reduces elongation, are increased after irradiation with UV-B ŽCaldwell et al., 1995.. Total biomass accumulation is a good indicator of UV-B radiation effects on growth ŽTeramura, 1983.. Decreases in total biomass in this study are similar to those found in greenhouses ŽTeramura, 1980, 1983; Barnes et al., 1990.. Other greenhouse experiments have reported either increase or no change in biomass of wheat with enhanced UV-B radiation ŽTeramura, 1983.. This study suggests that reduction in total biomass might be often accompanied by substantial modification in the partitioning of biomass into component plant organs. A similar conclusion in wheat has been reported ŽTeramura, 1980.. A larger proportion of biomass was invested as leaves and stems, and less as spikes, these changes might be related to the translocation of dry matter from leaves and stems into spikes. The larger proportion of biomass partitioned to leaves might be a result of increases in SLW. Increasing SLW, leaf thickening ŽSullivan and
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Teramura, 1990. may be an adaptation to UV-B radiation, with upper leaf tissue acting as an anatomical screen or filter to decrease UV-B transmission into sensitive underlying tissue ŽTeramura, 1983.. Very little is known about UV-B effects on grain yield of wheat ŽTeramura, 1983., only an 8% decrease has been reported in a greenhouse study ŽTeramura et al., 1990b.. In the present study, grain yield was positively correlated with crop spike number Ž r s 0.9728, P - 0.05., grain number per spike Ž r s 0.9517, P - 0.05. and thousand grain weight Ž r s 0.9784, P - 0.05.. Crop spike number was the result of the change of crop structure. Grain number per spike and thousand grain weight might be related to an integration of all phenological and physiological presses that affect growth ŽTeramura, 1980, 1983.. Although UV-B radiation increased secondary tiller number of isolated wheat plants in the greenhouse ŽTeramura, 1980; Barnes et al., 1990., in this experiment, it significantly decreased crop tiller number under field conditions. The same effect has been observed in 14 of 16 rice cultivars ŽTeramura et al., 1991.. On the other hand, UV-B radiation exacerbated intraspecific competition and self-thinning of a spring wheat crop, increasing dead shoot number. In this way, head-bearing shoot number was decreased at ripening stage, reducing total biomass, grain yield, LAI and area of the last leaf. Total shoot number at ripening was positively correlated with total biomass Ž r s 0.9771, P - 0.05. and grain yield Ž r s 0.9640, P - 0.05.. Additionally, reduced photosynthetic rates due to reduction in enzyme activity ŽJordan et al., 1992., in the efficiency of photosystem II ŽStrid et al., 1990. and in stomatal conductance ŽNegash and Bjorn, 1986. might contribute to the decrease in the grain yield and total biomass. Previous studies concerning the effects of UV-B radiation on wheat have been conducted with isolated plants as short-term experiments in greenhouse, the responses of plant height and tiller number to UV-B radiation ŽTeramura, 1980; Barnes et al., 1990. do not coincide with those of this study with crops under field conditions. Because plants respond differently to UV-B in each environment ŽTeramura and Murali, 1985., such as the differences between greenhouses and field conditions ŽCaldwell et al., 1995.. In addition to the genetic differences inherent
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in different cultivars, 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 ŽTeramura et al., 1991.. Besides, intraspecific competition and self-thinning in spring wheat crops have been observed in this study ŽTable 2., so the responses of spring wheat crops to UV-B radiation may be more complicated than those of plants in isolation.
5. Conclusion In conclusion, enhanced UV-B radiation had significant effects on spring wheat crops under field conditions. The effects on developmental stages and crop structure were the most important, they might change growth, biomass partitioning and grain yield. The data presented here show that spring wheat is a potentially UV-B sensitive species, although wheat has been previously shown to be UV-B resistant ŽBiggs and Kossuth, 1978.. UV-B sensitivity might be associated with UV-B radiation fluences and long-term accumulation ŽTable 1.. Spring wheat is an economically important crop. Its response to UV-B radiation under field conditions is not clearly understood and will be studied intensively. Since the effects of UV-B radiation on plants are related to other environmental factors, including PPF, CO 2 , drought, phosphorus nutrition, temperature, ozone fumigation and heavy metal ŽTeramura et al., 1990a; Caldwell et al., 1995., the field studies and an understanding of the relationship between UV-B radiation effectiveness and other environmental variables would greatly enhance our ability to more realistically assess the impacts of increased levels of solar UV-B radiation resulting from stratospheric ozone depletion.
Acknowledgements This work was supported by the National Natural Science Foundation of China. We wish to thank Elisabeth Kessler, the editor-in-chief of Ambio, for providing Ambio Vols. 24 No. 2 and 3, 1995.
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