A Comparison of the Effects of Enhanced UV-B Radiation on Some Crop Plants Exposed to Greenhouse and Field Conditions

Biochem. Physiol. Pflanzen iSO, 599-612 (1985)

A Comparison of the Effects of Enhanced UV-B Radiation on Some Crop Plants Exposed to Greenhouse and Field Conditions K. DUMPERT and T. KNACKER Battelle-Institut e.V., Frankfurt a. M., FRG Key Term Index: field conditions, greenhouse conditions, growth parameters, leaf components, UV-B radiation; Brassica oleracea, Hordeum vulgare, Lactuca sativa, Phaseolus vulgaris, Raphanus sativus, Spinacia oleracca, Triticum aestivum

Summary 21 economically important cultivars of 7 plant species were grown under greenhouse conditions for 7 weeks with an increased ultraviolet (UV)-B fluence rate that corresponded to a 25 % stratospheric ozone depletion. 6 weeks after precultivation in the greenhouse, 4 cultivars of these crops were grown for up to 11 weeks in the field where enhanced UV-B irradiance was adjusted to values expected from either a 10 % or a 25 % reduction of the stratospheric ozone layer. The growth parameters (plant height, leaf area, fresh weight, dry weight) and the leaf composition (chlorophyll, protein, flavonoid) were examined from plants exposed to both growth conditions. In the controlled environment, the responses of the monocotyledonous field crops Hordeum vulgare and Triticum aestivu1n to enhanced UV-B radiation were comparable. The reduced plant height of their cultivars was usually accompanied by decreased fresh weight and an increase in dry weight and protein content. On the other hand, the cultiyars of the dicotyledonous vegetables Brassica oleracea, Laduca sativa, Phaseolus vulgaris, R"aphanus sativus and Spinacia oleracea showed a wide range of different sensitivities to UV-B strpss. Partieularly, protein and flavonoid contents were in some cultivars enhanced and in others reduced. Under field conditions, the UV-B irradiance corresponding to a 25% ozone depletion caused greater physiological effeets than the radiation corresponding to a 10 % ozone depletion. Each cultivar exposed to UV-B stress in the field prespntcd a specific response pattern which was not identical with the effects exhibited in the greenhouse. However, statistically significant differences between plants grown in controlird and natrual environment were, on average, restricted to about one third of the examined parametrrs.

Introduction

Solar ultraviolet-B radiation (UV-B, 280-320 nm) on the earth's surface fluctuates with cloud cover, elevation above sea level, solar angle, atmospheric turbidity and total atmospheric ozone concentration. Due to absorption by the stratospheric ozone layer, no appreciable UV-radiation below 295 nm is received on the earth's surface (CALDWELL 1971). Nevertheless, solar UV-B radiation at wavelengths greater than 295 nm is present in sufficient quantities to produce a distinct environmental stress upon terrestrial plants (CALDWELL 1968; BOGENRIEDER and KLEIN 1977; CALDWELL et al. 1980; ROBBE RECHT et al. 1980). Moreover, it has been predicted that the release of man-made pollutants such as chlorofluormethanes and nitrogen oxides will partly destroy the stratospheric ozone layer which will increase UV-B irradiance on the earth's

600

K. DUMPERT and T. KNACKER

surface including shorter, more biologically damaging wavelengths (JoHNSTON 1971; CICERONE et al. 1974; CUT CHIS 1974; MOLINA et al. 1974). Recently, a reduction by 5-8 % has been assumed f
Plant materials and growth conditions 21 cultivars Qf 7 plant species were eXPQsed to. greenhQuse cQnditiQns and 4 cultivars Qf 3 species to. field cQnditiQns (Table 1). To. achieve cQmparable growth cQnditiQns, SQil from the field was used in the greenhQuse study. FQr bQth, greenhQuse and field experiments, mineral fertilizer and peat (sQil: peat ~ 3 : 1 v/v) were

UV-B Radiation Effects on Crop Plants

601

Table 1. Plant species and cultivars used to study growth alld composl:tion effects of enhanced UV-B radl:ation in the greenhouse and on the field Specips

Cultivar

Brassica oleracca var. gongylodes L. (kohlrabi)

Blaro D Blauer DelikateB D

Brassl:ca olera cca var. sabauda L. (savoy)

Marner Dau erwirsingD VorbotcO

Hordeum vulgare L. (spring barley)

Aramir" Georgic" Gunhild" Villa* Mondian D

Laduca sativa val. capitata L. (lettuce) Phaseolus vulgaris var. nanus L. (bush bean) Raphanus sat-ivus var. niger (radish)

Suzano Favorit D Maxidoro Saxa D

MILL.

Raphanus sativus var. radl:cula Pers. (small radish)

Saxa Trcib"

Spinacia oleracea L. (spinach)

Hippie D Matadoro

Triticum aestivum L. (spring wheat)

Kolibri" Max D Selpek" Raile"

Secds purchased from: D Zwaan and Co. (Kleve, FRG); " Lochow-Petkin (Bergen, Kreis Celie, FRG); * J. Brenn (Herzogenaurach, FRG) addcd to the potting mixture. Befor c sowing the soil fertility was checked by measuring the pHvalues (6.8 in the greenhouse and field soil) and the contents of three major mineral nutrients (phosphorous, potassium, magnesium) in mg/100 g soil. The results (greenhouse: P 12.3, K 28.2, Mg 14.0; field : P 15.2, K 20.4, Mg 12.6) are means of 5 separate measurements carried out by the Hessische Landwirtschaftliche Vcrsuchsanstalt (Kassel, FRG). In the greenhouse study, a total of 400-600 seeds were placed for each cultivar in 20 plastic pots (volume: 1.11 ; diamet er: 14 em). Soon after germination the seedlings were thinned (10 to 15 plants per pot) to ensure uniformity of growth. It is known that the UV-B radiation sensitivity depends ou the level of the photosynthetically active radiation (400-700 nm) and varies from species to species (TE luMu RA 1980). Therefore, the natural sunlight in the grcenhouse was enhanced by Osram-Halogen lamps (HQI-T-200 WID) that provided a photosynthctically active radiation of 14,000 Ix on a light cycle of 14 and 10 h dark. The growth chamber was programmed for a 18/12 DC day/night t emperature regime and relative humidities averaged from 60 % (day-time) to 80% (night time). The plants were watered daily. The cultivars used in the fi eld study were precultivated under greenhouse conditions (lettuce cv. Mondian 6 weeks, the others 7 weeks). During the period of precultivation the UV-B treatment

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.. Time of sowing. A Time of harvest: A = lettuce "Mondian"; B "Vorbote" ; D = savoy "M. Dauerwirsing".

= kohlrabi "Blaro"; C = savoy

UV-B Radiation Effects on Crop Plants

603

was identical to that received by the plants of the greenhouse experiment. 10 d after sowing th e seedlings were transferred into peat-pots (volume: 0.3 I ; diam eter: 10 cm). When planting them into field the number of plants was reduced to one per pot. For consistency with the greenhouse study, the plants in the field were irradiated 14 h daily with supplemental UV-B. Considering the UV-B gradient, only those plants which had received at least 90 % of the desired UV-B dosage were analysed. Temperature and rainfall were recorded and are shown in Fig. 1. During periods of relative dryness the plants were watered. Since there was evidence that the quality of the soil differell from plot to plot, control plants were cultivated for both UV-B treatments, i.e., irradiance corresponding to 10 and 25 % ozone depl etion.

Quantitative analyses Plant height, leaf area, fr esh and dry weight were chosen as growth param eters to determine the impact of enhanced UV-ll irradiance on the appearance of the food pla nts. Leaf composition was assessed by measuring the complex organic compounds chlorophyll, protein and flavonoid. To estimate the growth parameters and leaf composition plants from the greenhous e and the field were manipulated identically. In the greenhouse, plants were harvested after 7 weeks of growth. The analyses of the plants grown in the fi eld were carried out for each cultivar after different growth periods (Fig. 1). Plant height was either measured from the ground to the top of the shoot or, for so me cultivars, from the soil surface to the most distant leaf. After removing the oldest and youngest leaf of the plant, for each cultivar 1.5 g samples were cut into strips (1-2 em wide) and stored up to 4 weeks at - 85 ac. The samples were us ed to determine the chlorophyll, protein and fl avonoid contents. Chlorophyll was extracted under dim light conditions with chilled 80 % acetone. The procedure and spectrophotometrical measurement were as describ ed by ZIEGLER and EGLE (1965). Alter thorough homogenization in distilled water, boiling for 45 min and incubation with 2.5 % trichloroacetic acid over night at 4 ac, the fi ltered precipitate was used for th e determination of nitrogen aecording to Kjeldahl's method. The protein content was calculated by multiplying by a faetor of 6.0 (PIRI E 1955). For the estimation of th e flavonoid co nt ents the method describ ed by KRAUSE and REZNIK (1972) was used.

Results and Discussion

In Table 2 all data obtained in this study are summarised. It is shown that plants exposed to field conditions exhibit a large number of modifications. This is manifested by some clearly distinct but, in statistical terms, not significantly different mean values which were obtained when parameters of UV-treated and control plants were compared. In the greenhouse, the visible, though unquantified UV-B effects indicate that enhanced ultraviolet radiation caused increased susceptibility to diseases. Under field c(mditions, however, this observation was not confirmed. To develop an efficient, economic test for the effects of enhanced UV-B radiation on plants grown in the field this investigation was aimed to find preliminary criteria which would allow to predict these effects from the UV-B treatment of young plants grown under controlled greenhouse conditions. Therefore, in the greenhouse study a short duration of growth was chosen whereas in the field the growth period was extended until favourable horticultural usage of the plants was gained. As a result, the comparison of the data derived from plants grown in the greenhouse with those from plants grown in the field revealed drastical differences for most parameters (Table 2). In the field the fresh weight per plant was up to 250 times higher than in the greenhouse. On the

604

K. DUMPERT and T. KNAcKER

Table 2. The growth parameters and leaf composition of control and enhanced UV-B treated plants grown und! Plant

Growth condition

Plant height

Leaf areat per plant [cm2]

[cm]

Spring barley "Arimar" "Georgie" "Gunhild" "Villa" Spring wheat "Kolibri" "Max"

Bush bean

Spinach Radish

"Selpek "RaIle" "Favorit"l) "Maxidor" "Saxa" "Hippie"2) "Matador"2) "Rex"

Small radish "Saxa Treib"3) Kohlrabi

"Bl. DelikateB"4)

Lettuce Kohlrabi

"Suzan" "Blaro"5)

Lettuce

"Mondian"

Savoy

"M. Dauerwirsing"6)

Savoy

"Vorbote"6)

Fresh plant

C

T

G G G G G G G G G G G G G G

44.4 44.8 45.2 45.1 36.7 42.5 40.1 41.5 25.0 21.2 22.4 17.1 11.5 15.9

39.6 39.2 40.9 40.1 35.3 40.8 38.8 37.1 23.9 18.4 18.8 14.3 9.4 15.9

G

15.1

13.9

G G G F-10

21.0 20.4 13.0 13.0 19.4 20.6 24.9&) 27.0&)

F-25

23.5&) 23.1&) n.s.

1253.6 1638.3 n.s.

696.6

G F-10 F-25 G F-10 F-25

14.2 14.7 n.s. 14.3&) 13.2&) s* 12.3&) 12.6&) n.s.

138.3 176.4 n.s.

4.42 612.4 500.4 3.69 1144.1 732.9 3.60 1008.4 909.9

G F-10 F-25

17.4 48.2 C ) 47.3 18.1 28.9b ) 28.9b )

15.5 50.1 50.9 14.8 29.5b ) 29.3b )

P

C

T

P

C

s

s s* s s s* s s

s n.s.

107.4 160.6 n.s. 146.4 85.4 s* 143.9 77.5 s* 106.8 83.7 n.s. 87.3 51.1 s* 47.3 54.1 n.s. 96.2

n.s. n.s. n.s. n.s.

s* n.s. n.s. s n.s. n.s.

80.9 n.s.

54.0 48.9 74.6 90.0 66.5 72.0 1555.61187.9

60.5

n.s. n.s. n.s. n.s.

80.5 s*

167.5 103.5 s

1: tu: 1: tu:

3.23 10.48 3.57 3.22 2.72 2.82 2.84 2.80 3.39 3.48 3.46 3.16 2.15 3.11 6.04 8.91 5.62 3.18 2.84 2.78 85D.4

Abbreviations: C = control plants; T = plants treated with enhanced UV-B; G = greenhouse conditions; F-10 = field conditions (10% ozone depIction); F-25 = field conditions (25% ozone depletion); f.w. = fresh weight; 1 = leaf; tu = tuber; n.d. = not detected. Statistics: Results are expressed as means with n = 10 for plant heigt, n = 5 for leaf area, chlorophyll and protein, n = 3 for dry weight and flavonoid. The means for fresh weight are based on data derived from 100-150 plants grown in 10 pots for the greenhouse experiment and on 10-16 single plants for the field experiment. Dry weight was determined from mixed fresh weight samples

605

UV-B Radiation Effects on Crop Plants greenhouse and field conditions weight per

Dry weight

Chlorophyll

Protein

Flavonoid

[g]

[% of f. w.]

[mg/g f.w.]

[mg/g f.w.]

[mg 10-2 /g f.w.]

T

P

C

T

s n.s. s*

15.2 10.4 13.8 18.1 16.4 18.3 16.9 18.6

17.6 13.6 16.4 18.9 19.4 18.9 18.9 16.5

18.6 16.0 14.3 18.1 15.0

16.9 17.0 15.7 16.1 14.4

12.5 10.8

2.86 10.90 3.20 2.73 2.58 2.71 2.47 2.57 3.20 2.78 2.72 2.34 1.62 3.00 5.15

n.s. n.s.

3.16 4.00

s*

2.76

s*

n.s. n.s.

n.s.

s s

P

C

P

C

T

n.s.

22.2 16.0 21.8 16.4

32.2 22.5 20.2 21.8 20.8 23.5 24.5 21.6 22.6 25.5 21.7 24.2 23.6

P

C

T

P

5.3 8.5 8.5 10.8 11.6 19.2 15.0 11.0

10.4 6.5 10.9 9.8 11.3 20.8 18.0 15.8

n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s.

34.8 25.0 27.1 19.8 21.5

s* n.s. s n.s. n.s.

8.1 7.3 6.9 7.3

8.1 8.0 7.9 8.4

n.s. n.s.

10.6 10.7 10.8 8.2

10.9 11.4 10.9 10.5

n.s. n.s. n.s. s*

8.9 7.0 7.5 5.2 6.1

s* n.s. n.s.

n.s.

7.4 7.4 8.0 5.1 5.8

n.s.

21.0 19.8 23.5 21.1 23.7 22.8 21.8 15.5 20.7

n.s.

50.5 27.0 50.4 18.6 18.3

11.6

n.s.

2.8

3.2

n.s.

12.5

13.7

n.s.

15.3

12.9

n.s.

11.0

n.s.

3.9

3.7

n.s.

14.5

12.9

s*

22.7

27.9

s*

19.1

20.9

n.s.

3.2

4.4

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11. 7

15.6

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25.0

27.9

n.s.

16.4

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1.0 3.8 6.3

1.5 3.8 7.3

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s* s* n.s.

n.s.

11.0

18.4

1.7 2.3 2.1 2.3 5.7 5.3

s* n.s.

s* s* s* n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s.

19.1 29.7 15.9

6.2

10.6 15.0 29.2 5.4 20.8 7.3 12.3 11.3 12.6 9.2 18.7 21.3

25.2 22.5 13.3

6.7

8.5 11.9 I: 18.9 tu: 6.2 I: 23.8 tu: 7.4 10.1 10.7 14.5 10.8 18.3 20.7

43.1 4.3 5.4 26.3 1.2 17.1

3.9 6.6 7.1

9.2 24.8 22.5

9.6 23.5 27.8

n.s. n.s. n.s.

34.4 4.3 3.4 27.5 0.8 7.6 28.6 3.0 3.6

s n.s. n.s. n.s. n.s. n.s. n.s.

2.83 737.3

n.s. n.s.

18.6 12.4

18.6 20.9 13.6

727.7

n.s.

10.5

13.5

5.50 635.3 562.8 3.86 991.7 1006.1

n.s. n.s. n.s. n.s. n.s. s*

19.8 5.3 7.7 21.8 13.9 12.9

16.5 4.5 5.5 18.4 14.0 12.5

n.s. n.s. n.s. n.s.

1.3 1.8 3.6 2.4 5,2 4.6

3.31 960.1 983.3

n.s. n.s. n.s.

22.3 11.2 12.2

21.5 10.7 11.1

n.s. n.s. n.s.

3.6 7.5 6.7

4.49

T

s n.s.

n.S.

n.s. n.s. n.s. n.s. n.s. n.s.

n.s. n.s. s* n.s. n.s. s* n.s. n.s.

36.1 5.4

s* n.s. s* n.s. n.s. n.s. s* n.s.

9.8

of approximately 1 g. Statistically significant differences between control plants and plants treated with enhanced UV-B were calculated by the Students's T-test: s = significant (P < 0.01), s* = significant (P < 0.05), n.s. = not significant. Visible, however, not quantified UV-B effects on plants grown in the greenhouse:

1) Leaves begin to curl, few brown spots scattered on the lamina. 2) Leaves curled, yellow spots on upper leaf surface, foliage overspread with boils. '0 Biochem. Physiol. Pflanzen, Bd.180

606

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3) Approximately 20 % of leaf surface covered with red spots which turn yellow during the 4. and o. week of growth; plants were harvested with the beginning of flowering after 0 weeks. 4) Bluish spots start to develop near the main leaf vein. 5) Parts of leaf edges curled. 6) Large leaf areas covered with brown spots, outer leaves which bend their lower surface towards light produce blue-violet pigments. Accomplishment of plant height: a) After 3 weeks of growth. b) After 4 weeks of growth. C) After 6 weeks of growth in the field.

other hand, the dry weight as percentage of fresh weight and the flavonoid content were reduced while the chlorophyll and protein contents were increased under field conditions. In Fig. 2 the statistically significant, UV-B dependent variations of plants exposed to the greenhouse conditions are shown. The responses of the monocotyledonous plants, barley and wheat, were similar. Moreover, the results obtained by TEVINI et al. (1981) for seedlings of H. vulgare cv. Villa are in agreement with the data presented in this study. All cultivars of both species exhibited reduced plant height which, in most cases, was accompanied by decreased fresh weight, increased dry weight and enhanced protein content. The UV-B dependent alterations of the barley cultivars were more pronounced. This was expected since wheat has been described as a UV-B resistant species (HART et al. 1975; BIGGS and KOSSUTH 1978). The UV-B effects found in dicotyledons, on the other hand, varied considerably (Fig. 2). Even the responses of cultivars from the same species differed to remarkable extenso Compared to controls beans and spinach were characterized by reduced plant height, leaf area, fresh and dry weight. Furthermore, some bean cultivars exhibited enhanced chlorophyll and reduced flavonoid contents. In accordance with data produced by TEVINI et al. (1981 and 1983), radish cv. Saxa Treib exhibited reduced fresh weights (leaves and tubers) and an increased amount of flavonoid. The reduced protein content, however, found by TEVINI et al. (1981) for the cv. Saxa Treib remained unconfirmed in this study. Under UV-B stress kohlrabi cv. Blaro produced increased amounts of all leaf components except chlorophyll, whereas cv. Blauer DelikateB displayed reduced fresh weight. The only plant distinguished by an UV-B dependent increase of fresh weight was lettuce cv. Suzan. With regard to the variable sensitivity of plants to UV-B, the most

Fig. 2. Statistically significant effects of UV-B radiation on growth parameters and leaf composition of plants grown under greenhouse conditions. Data expressed as relative increases or decreases in comparison with the control plants. Abbreviations: PH = plant height, LA = leaf area, FW = fresh weight, DW = dry weight, Chi = ehlorophyll, P = protein, F = flavonoid, L = leaf, Tu = tuber.

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striking results were opposing physiological reactions of different cultivars which belong to the same species; e.g., enlarged/diminished leaf area (savoy) and increased/decreased flavonoid contents (lettuce). In general, most plants examined in this report exhibited reduced plant height, leaf area and fresh weight when exposed to enhanced UV-B irradiance. These results are in agreement with data obtained from other plants cultivated under different growth conditions (e.g. BRANDLE et al. 1977; SEMENIUK and STEWART 1979; TERAMURA et al. 1980; TEVINI et al. 1981; Vu et al. 1982; TENINI et al. 1983). For some plants the loss of fresh weight was correlated to unaltered, or even increased, dry matter production (Fig. 2). This indicates a lowered diffusive resistance to water vapour in the leaf. The resulting water stress may cause reduced growth rates (TERAMURA et al. 1983). Flavonoids are accumulated in the vacuoles of epidermal cells and, in combination with waxes (CLARK and LISTER 1975), absorb the majority of incident UV-radiation (ROBBERECHT and CALDWELL 1978). In some reports (ARTHUR 1936; LINDOO and CALDWELL 1978; ROBBERECHT and CALDWELL 1983) it has been discussed that UV-B stress could enhance the synthesis of flavonoids to protect plants from detrimental impacts. The results presented in this paper, however, demonstrate that there is no correlation between UV-B stress and flavonoid contents. The conflicting results on the estimations of protein and chlorophyll in leaves after UV-B radiation (ANDERSEN and KAPERBAUER 1973; BASIOUNY et al. 1978; KLEIN 1978; TEVINI et al. 1981; Vu et al. 1982; TEVINI et al. 1983) agree with the inconsistent changes of these leaf components in the cultivars investigated in this report. In Fig. 3 the UV-B effects of four eultivars exposed to a controlled and a natural environment were compared. The only effects caused by the UV-B irradiance corresponding to a 10% ozone depletion were the decreased plant height of lettuce and the increased protein content of kohlrabi. All the other UV-B dependent changes in the field caused by the UV-B stress corresponding to a 25 % ozone depletion. The flavonoid contents of all cultivars were considerably enhanced under field conditions. Seme UV-B effects expressed under greenhouse conditions did not occur in the field, e.g. reduced plant height, increased/decreased leaf area (savoy) and decreased dry weight (lettuce). On the other hand, the reduced plant height of lettuce as well as the increased fresh weight of savoy cv. M. Dauerw. and the increased dry weight of kohlrabi were detectable in the field but not in the greenhouse.

Fig. 3. The comparison of statistically significant UV-B effects on plants grown under greenhouse conditions with plants grown under field conditions (10, 25 % ozone depletion). Data shown as relative increases or decreases in relation to the control plants. For abbreviations see FI:g. 2.

610

K. DUMPERT and T. KNACKER

A notable UV-B effect was revealed by the lettuce cv. Mondian where the chI oro phyll content was increased in the greenhouse and decreased in the field. Howeverthe majority of parameters (65%) of the 4 cultivars exposed to UV-B stress were either, unaffected or varied similarly in both growth conditions. The comparison between plants grown in a controlled and a natural environment indicates clearly that, with respect to the examined parameters, variable climatic factors as well as different levels of photosynthetically active radiation (TERAMURA 1980) alter the UV-B radiation sensitivity of plants. The degree of change varies from cultivar to cuItivar. However, qualitative similarities exist between the responses of plants when exposed to enhanced UV-B irradiance for different durations of growth. Aknowledgements This work was supported by Bundesministerium fiir Forschung und Technologie (KBF 54). The authors thank GUDRUN SCHONECKER, Botanisches Institut II, Universitat Karlsruhe for the quantitative estimation of the flavonoids.

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Received April 4, 1985; accepted May 14, 1985 Authors' address: Dr. K. DUMPERT and Dr. T. KNACKER, BatteIIe-Institut e.V., Am Romerhof 35, Postfach 900160, D - 6000 Frankfurt a. M. 90.