- Email: [email protected]
Growth promotion of tomato and radish plants by solar UV radiation reaching the Earth's surface
61
J. Photochem. Photobiol. B: Biol., 19 (1993) 61-66
Growth promotion of tomato and radish plants by solar UV radiation reaching the Earth’s surface Takafumi Tezuka+, Toshihiro Hottat+ and Ikuko Watanabe+++ Department of Agronomy, School of Agricultural Sciences, Nagoya University, Chikusa-Ku, Nagoya 464-01 (Japan) (Received February 1, 1993; accepted February 11, 1993)
Abstract The effect of natural solar UV radiation on the growth of tomato and radish plants was studied using polyvinylchloride films with different UV transmissions. The growth (fresh and dry weights) of tomato plants exposed to UVA (400-320 nm) radiation (UVC-320 film, transmission above 320 nm) was greater than that of plants exposed to no UV radiation (UVC-400 film, transmission above 400 nm) or to mainly near-UV radiation (400-300 nm; including UVA (400-320 nm) and the longer wavelength region within the UVB (320-280 nm); DE-290 film, transmission above 290 nm). Furthermore, the growth promotion of tomato and radish plants on exposure to UVA has associated with an increase in chlorophyll content and photosynthetic activity. Dark respiration of tomato and radish plants was also promoted by radiation (near-UV and UVA region) through UVC-290 and UVC-320 films. Radiation with UVA delayed the senescence of tomato leaves.
Keywords: Chlorophyll, sativzq UVA
Growth promotion,
Lycopekcon
1. Introduction
Since the spectral band now designated as the UV was discovered in 1801 by Johann Wilhelm Ritter, studies on the effects of UV on various organisms have been reported by many biologists [l-7]. The solar spectrum reaching the Earth’s surface does not extend below approximately 290 nm because ozone in the stratosphere effectively absorbs all radiation of shorter wavelengths [8, 93. Therefore the natural solar radiation of the UVB region at the Earth’s surface is between 290 and 320 nm. Many reports [S, 7,10-141 have shown that UVB radiation inhibits photosynthesis and the growth of plants. Hence, the cultivation of crops in a greenhouse covered with plastic film (polyvinylchloride which is opaque to UV) is used extensively. However, little is known about whether the use of opaque plastic film is appropriate for the cul‘Author to whom correspondence should be addressed. “Present adress: Seibu Extension Division of Shizuoka Agricultural Experiment Station, Hamamatsu 432, Japan. “‘Present address: Kawagoe Extension Division of Saitama Agricultural Experiment Station, Kawagoe 350, Japan.
loll-1344/93/$6.00
esculentum,
Near UV, Photosynthesis,
Raphanus
tivation of crops in all cases, and whether natural solar W radiation (400-290 nm), consisting of the UVA region and all UVB regions on the Earth’s surface, is also deleterious for the growth of plants. The purpose of this work is to investigate the effects of radiation with UV from the natural solar spectrum on the growth, photosynthesis and other physiological activities of plants.
2. Materials
and methods
2.1. Plant materials Seeds of tomato (Lycopersicon esculentum Mill. cv. Horyu) and radish (Raphanw sativw L. cv. Comet) plants were purchased from Takii Seed Co., Kyoto. Tomato seeds were sterilized with 0.1% Uspulun (Bayer Chem. Co., Germany) and radish seeds with 5% Antiformin (Wako Chem. Co., Japan), after which they were rinsed in running tap water. The seeds were sown under plastic film frames, in styrene foam boxes (30 cmx50 cmx 60 cm) containing a growth substratum composed of humus and paddy field soils (1:9, v/v), which was sieved through a 3 mm mesh. After emergence, 0 1993 - Elsevier Sequoia. All rights reserved
62
‘I7 Tezuka et al. I Growth promotion of tomato and radish by solar W radiation
two plants per box were transplanted for cultivation in other boxes (30 cm x 50 cm X 60 cm) containing the above growth substratum under plastic film frames in an experimental field at Nagoya University. Tomato plants were harvested for sampling at 25, 46 and 66 days from sowing and radish plants at 29 days. Water was supplied on alternate days.
the length of the various organs of the tomato plants, the area of the leaves was measured using an automatic area meter (AMM-5, Hayashi Denko Co. Ltd., Tokyo) for the expression of photosynthesis and respiration on a leaf area basis. Each organ was dried at 80 “C for 3 days for the measurement of dry weight. 2.4. Measurements
2.2. Frames Each plastic film frame (Quonset hut type; 3.8 mx5.1 rn~ 2.7 m) was covered with a layer of polyvinylchloride film (0.1 mm thick). To allow ventilation the portion of the frame which extended to 30 cm in height above ground level was not covered with film. Furthermore, the frame was equipped with a fan (50 cm X50 cm) connected to a thermostat which operates at 35 “C or more in the frame; the atmospheric temperature in the frame rises during the warm/hot season from early summer to early autumn. Films tested were UVC290, UVC-320 and UVC-400 (Mitsui Toatsu Co. Ltd., Nagoya, Japan). The transmittance curves of the films were measured using a spectrophotometer (Hitachi, type 200) and are shown in Fig. 1. Each styrene foam box for the cultivation of plants was placed on a base (80 cm high). 2.3. Measurements of fresh and dry weight, and root and stem length
Tomato plants at each sampling day after sowing were gently uprooted, the roots washed and the plants divided into leaves, stems and roots. Following the measurements of the fresh weight and
I
I
I
I
I
I
of photosynthesis
and
respiration
The gas exchange of the leaves was measured after 25 and 66 days (the third and tenth leaf) in tomato with an IR CO, gas analyser (URAS1, Hartman & Brawn Co.) as described elsewhere [15, 161. Net photosynthesis and dark respiration are expressed on a leaf area basis. 2.5. Measurement of chlorophyll content Ten leaf discs (1 cm in diameter) were taken from the third leaf of 25 day old and the tenth leaf of 66 day old tomato plants. These discs were homogenized with 80% acetone in a glass homogenizer as described previously [15, 161. The homogenate was centrifuged at 70% for 10 min, the pellet was suspended in 80% acetone and spun again at 7OOgfor 10 min. The supernatants were combined to measure the volume. The contents of (total) chlorophyll a and b were estimated according to Arnon’s method [17] and are expressed on the basis of leaf area. 2.6. Measurement of senescence The senescence of tomato plants was checked with the unaided eye 66 days after sowing, and senescence was quantified according to five grades of leaf colour; green, greenish yellow, yellow, yellowish brown and brown.
I
3. Results
OL 250
300
350
400
450
Wavelength
500
550
600
650
(nm)
Fig. 1. Transmittance of the radiation-transmitting plastic films designated as UVC-290, UVC-320 and UVC-400. The transmittance of three plastic films was measured with a spectrophotometer (Hitachi, type 200).
Tomato plants developed the fourth and fifth and tenth and eleventh leaves 25 and 46 days after sowing respectively. Before anthesis (up to 25 days after sowing), the plant stems in the UVC400 (UV-non-transmitting) film frame were taller than those in the UVC-290 (UVA and UVBtransmitting) and WC-320 (WA-transmitting) film frames (Table 1). The plants at this age in the WC-320 film frame were the smallest compared with those in the other frames and seemed like a dwarf species. However, by 46 days, all the plants reached almost the same length. The roots of the plants in the three film frames showed almost the same length on both sampling dates (Table 1).
63
T. Tezuka et al. I Growth promotion of tomato and radish by solar UV radiation TABLE 1. Effects of solar W-transmitting UVC-290, UVC-320 and WC-400 plastic films on the length of stems and roots of 25 and 46 day old tomato plants (values are expressed as the mean of five replicates *standard deviation) Organ
Age (days)
Length
(cm) uvc-290
WC-290
UVC-320
WC-400
Stem
25 46
13.8f0.8 61.81t3.7
10.4 f 1.2 67.4 f 8.1
19.0f2.0 63.0*6.9
Root
25 46
10.8 i- 0.8 17.1 f 1.3
9.1 f 1.2 19.5 f 2.5
8.5 f 1.6 17.0f3.9
TABLE 2. Effects of solar W-transmitting WC-290, WC-320 and WC400 plastic films on the fresh and dry weights of leaves, stems and roots of 25 and 46 day old tomato plants (values are expressed as the mean of five replicates&standard deviation) Organ
Stem’ Leaves” Root’ Stemb Leavesb Rootb
Age (days)
25 46 25 46 25 46 25 46 25 46 25 46
TABLE 3. Effects of solar W-transmitting UVC-290, UVC-320 and UVC-400 plastic films on chlorophyll contents, net photosynthesis and dark respiration of the leaves of 25 and 66 day old tomato plants (values are expressed as the mean of five replicates f standard deviation)
Fresh or dry weight (g) WC-290
uvc-320
uvc400
2.4 f 0.4 38.4f6.2 2.3 f 0.7 34.2 f 10.6 0.5 * 0.2 3.5 f 1.2
1.1 f0.2 44.6 f 6.8 1.0*0.1 50.1 f 2.4 0.2fO.l 3.5 kO.8
3.OkO.2 27.Ok5.4 1.7 io.3 21.9i3.5 0.4*0.1 3.5 kO.8
0.11*0.02 1.77f0.28 0.24 f 0.08 2.67 f 0.83 0.05 f 0.02 0.38 kO.13
0.05 f 0.01 2.10*0.32 0.11 f 0.00 4.46 f 0.21 0.02 f 0.00 0.35 f 0.08
0.12f0.03 1.08 f 0.22 0.16kO.03 1.73 f 0.28 0.05 f 0.01 0.42f0.10
‘Fresh weight. bDry weight.
After 25 days, the fresh and dry weights of the stems, leaves and roots were lowest for plants under the UVC-320 film compared with those under the other films (Table 2). After 46 days, the fresh and dry weights of the stems and leaves were greatest in tomato plants under the UVC320 fihn, followed by the UVC-290 film and finally the UVC-400 film; the root showed no difference for plants under the three films (Table 2). In both 25 and 66 day old plants, the chlorophyll contents of the tomato leaves under the UVC290 and UVC-320 (+ UV) films were superior to those under the UVC-400 (-UV) film (Table 3). The net photosynthetic and dark respiratory activities of the leaves under the UVC-290 and UVC320 ( +UV) films were also greater than those under the UVC-400 ( -UV) film (Table 3). In the case of radish leaves (29 days old) in the UVC-320 fihn frame, chlorophyll contents and net photosynthetic and dark respiratory activities
Chlorophyll a +b (pg cm-*) 25 day old’ 24.22 f 0.87 66 day old’ 25.08 f 0.90
WC-320
uvc-400
25.70f0.88 26.58 f 0.29
22.44 f 0.29 22.72 f 0.30
Net photosynthesis (mg CO2 dm-* h-‘) 14.71 kO.83 15.38f0.64 25 day old 17.46 f 0.99 17.94 *0.75 66 day old Dark respiration 25 day old 66 day old
(mg CO2 dm-* h-‘) 0.93 f 0.03 1.01 f 0.03 1.43 f 0.08 1.09 f 0.07
12.82 f 0.87 15.34 f 0.77 0.78 f 0.06 0.84 f 0.03
‘The third leaf of 25 day old and tenth leaf of 66 day old plants were used for analyses. Leaf positions are numbered from the lower to the upper leaves. TABLE 4. Effects of solar W-transmitting UVC-290, UVC-320 and UVC-400 plastic films on chlorophyll contents, net photosynthesis and dark respiration of the third or fourth leaf of 29 day old radish plants (values are expressed as the mean of ten replicates f standard deviation)
Chlorophyll a + b (Pg cm-*) Net photosynthesis (mg CO2 dm-’ h-‘) Dark respiration (mg CO2 dm-’ h-l)
WC-290
UVC-320
uvc400
32.9 f 2.7
36.8& 1.1
31.3* 1.4
22.8 f 0.3
23.8 f 1.0
19.7*0.3
1.52rtO.08
1.61 kO.13
1.30*0.11
were also somewhat superior to those in the other film frames (Table 4). Solar UV radiation transmitted through the UVC-290 and UVC-320 ( +UV) films delayed the senescence of tomato leaves compared with radiation transmitted through the UVC-400 ( - UV) film (Table 5). 4. Discussion In order to determine whether solar UV radiation induces the inhibition of plant growth and photosynthesis [ll, 14, 181 compared to visible radiation only (without solar UV), research on tomato plants was carried out using a series of polyvinylchloride films. In 1919, Dorno stated that from late spring to mid-autumn, more solar W radiation reaches the Earth’s surface compared with other seasons (cited by Lundeg%rdh [19]). Therefore, in this study, tomato plants were cultured from late spring to
T. Temka et al. / Growth promotion of tomato and radtih by solar W radiation
64
TABLE 5. Effects of solar UV-transmitting UVC-290, UVC-320 and UVC400 plastic films on the leaf senescence of 66 day old tomato leaves. Senescence was estimated by checking the grade” of leaf colour with the unaided eye (values are expressed as the mean of five replicates) Leaf positionb
uvc-290
UVC-320
uvc400
10 9 8 7 6 5 4 3 2 1
+++++a +++++ ++++ ++++ +++ ++ + +
+++++ +++++ +++++ +++++ ++++ ++++ +++ ++
+++++ +++++ ++++ +++ ++ + +
-I-
-
-
-
’ + + + + + , green; + + + + , greenish yellow; + + + , yellow; + +, yellowish brown; + , brown; - , fallen leaves. bLeaf positions are numbered from the lower to the upper leaves.
early summer and radish plants in autumn in order to make use of the natural solar UV radiation. Radish plants were not cultured at the same time as tomato plants as they produce flower buds under the long day conditions from spring to summer. In this experimental season, the maximum and minimum fluences of visible light (700-400 nm) were approximately 425 W m-* (clear sky) and 25 W m-* (cloudiness) respectively, and the fluences in the frames were approximately 90% of solar radiation (Fig. 1). According to the Nagoya Meteorological Observatory, during this experimental season, around the Nagoya area, the degree of cloudiness was 7.2/10 (tomato) and 7.1/10 (radish) on average; the sunshine duration, measured by a sunshine recorder, was 5.7 h (tomato) and 4.5 h (radish) per day on average; the number of cloudy days was 14 (April D-30), 19 (May) and 7 (June l-20) (tomato) and 17 (September 20 to October 20) (radish). The experimental field at Nagoya University is located at longitude 137” E and latitude 35” 11’ N. The average monthly temperatures in the plastic film frames during the experiments were 15 (April), 23 (May) and 35 “C (June), and those in the open air were 13 (April), 19 (May) ,and 23 “C (June). The temperatures of the film frames during cultivation showed little difference (1 “C or so at most). The solar UV radiation transmitted through the UVC-320 fihn seemed to delay the stem growth of tomato plants in the early growth stage but encouraged stem growth in the later stage compared with visible radiation only transmitted through the UVC-400 film (Table 1). In the early growth stage, the plants may have different sen-
sitivities to UVA (400-320 nm) and UVB (320-290 nm), within solar UV) radiation and this is now being investigated. This phenomenon was also observed for the growth of radish, Shanghai Pak Choi (a kind of Brussicu), eggplants, sweet peppers and rice (data not presented). These results suggest that solar near-UV radiation, especially UVA, can increase the metabolic activity related to the growth of plants in the later growth stage. Solar UV radiation (above approximately 300 nm in particular, near-UV); seems to contribute to the promotion of plant growth. Therefore near-UV radiation intensity not exceeding the intensity of natural solar UV radiation will promote the growth of plants on the Earth’s surface and intensities above this will inhibit growth. Indeed, the growth and nitrogen tixation by nodules of soybean plants were somewhat inhibited by supplementary nearUV radiation under field conditions (data not presented). If plants receive a radiation intensity stronger than that of solar UV radiation, they may be compelled to undergo abnormal metabolism in cells. Seibert et al. [20] have reported that UVA radiation at low light intensities stimulates growth, but at high light intensities inhibits the growth and shoot initiation, of tobacco callus (chlorophyllcontaining clone). The plants located under the UVC-290 film frame show good growth (Table 2). As can be seen in Tables 1 and 2, the thickening of 46 day old plant stems seems to be promoted by solar UV radiation passing through the UVC-290 and UVC-320 films. This increase in growth can be related to increases in chlorophyll content and photosynthetic activity in plants growth under the UVC-290 and UVC-320 films (Tables 3 and 4). The promotion of the net photosynthetic activity of tomato plants by natural solar UV radiation may result from alteration(s) in the structure and function of chloroplasts and/or chlorophyll content. Net photosynthesis and dark respiration on a dry weight basis showed different trends (UVC400> UVC-290> UVC-320; 66 day old tomato leaves) compared with results on a leaf area basis (data not presented). This seems to result from the increasing tomato leaf thickness and dry weight on exposure to natural solar UV radiation. According to Cen and Bornman [6], UVB radiation increased leaf thickness of bean plants by approximately 18%. Since the leaves were somewhat thicker under the UVC-290 and UVC-320 ( + UV radiation) film frames than under the UVC-400 ( - UV radiation) film frame, the size and/or number of palisade and spongy cells in the leaves may also be greater under the UVC-290 and UVC-
T. Tezuko et al 1 Growth promotion of tomato and radish by solar W radiation
320 film frames than under the UVC-400 film frame. Indeed, the size of palisade and spongy cells in leaves of cucumber plants was enhanced by near-UV radiation [21]. Therefore, chloroplasts and other cell organelles may increase in number on exposure to natural solar UV radiation compared with results obtained on exposure to natural solar radiation without UV. This is currently being investigated. The wavelength region above 400 nm (i.e. only visible light) may cause a kind of stress in plants. Radish plants on exposure to visible light (UVC400) show enhanced superoxide dismutase (SOD) activity [22]. This suggests that the onset of tomato leaf senescence may be accelerated by ion radicals, such as 02-, induce on exposure to visible light. As a result (Table 5), the radiation transmitted through the UVC-400 ( - UV) film accelerates leaf senescence, which is due to the inhibition of chlorophyll synthesis and/or the promotion of chlorophyll degradation. This study shows that the fluence rates of solar UV (in particular, near-UV) radiation on the Earth under existing circumstances do not cause any damage to the growth of plants. Indeed, crops in a frame covered with UV-non-transmitting plastic film were inferior to crops grown under solar UV radiation conditions. The growth of cucumber plants was also promoted by UV radiation above 300 nm [23]. Therefore it is premature to conclude that solar near-UV or UVA radiation causes damage to plant growth based on the inhibitory action of plant growth by UVC and the shorter wavelength region within UVB [l, 7, 11, 141. The mechanism of growth promotion by natural solar UV radiation has not yet been established. However, the functional sites of cell organelles, cell membranes and cytoplasm are regulated to promote photosynthetic, respiratory and other metabolic activities by natural solar UV radiation (including the UV region above approximately 300 nm). Solar W radiation (greater than 290 nm) seems to be an essential factor for plant growth not an inhibitory factor. Acknowledgments We thank S. Torii and K. Sakakibara for technical assistance. References 1 P. M. Klein, Plants and near-ultraviolet 44 (1978) 1-127.
radiation,
Bot. Rev.,
65
2 J. F. Bomman, Target sites of UV-B radiation in photosynthesis of higher plants, .I. Photochem. Photobiol. B: Biol., 4 (1989) 145-158. 3 Y. Takeuchi, M. Akizuki, H. Shimizu, N. Kondo and K. Sugahara, Effect of UV-B (290-320 nm) irradiation on growth and metabolism of cucumber cotyledons, Physiol. Plant., 76 (1989) 425-430. 4 A. H. Teramura, L. H. Ziska and A. E. Sztein, Changes in growth and photosynthetic capacity of rice with increased UV-B radiation, Physiol. Plant., 83 (1991) 373-380. 5 T. Hashimoto and M. Tajima, Effects of ultraviolet irradiation on growth and pigmentation in seedlings, PIant Cell Physiol., 21 (1980) 1559-1571. 6 Y. P. Cen and J. F. Bomman, The response of bean plants to UV-B radiation under different irradiances of background visible light, /. Qo. Bot., 41 (1990) 1481-1495. 7 A. H. Teramura, Effects of ultraviolet-B radiation on the growth and yield of crop plants, Physiol. Plant., 58 (1983) 415-427. 8 I. M. Campbell, Energy and the atmosphere. A Physical-Chemical Approach, Wiley, London, New York, Sydney, Toronto, 1980, Chapter 2. 9 T. Tezuka, Effect of ultraviolet radiation on plant growth, in T. Seiyama (ed.), Global Environment and Energy Issues (Proc. Fukuoka bat. Symp.90, Fukuoka, Japan, November 19-21, 1990), Electrochemical Society of Japan, Kyushu Branch, Fukuoka, Japan, 1990, pp. 267-270. 10 J. E. Ambler, D. T. Krizek and P. Semeniuk, Influence of W-B radiation in early cotyledons in cotton, Physiol. Plant., 34 (1975) 177-181. 11 J. R. Brandle, W. F. Campbell, W. B. Sisson and M. M. Caldwell, Net photosynthesis, electron transport capacity, and ultrastructure of Pisum sativum L. exposed to ultraviolet-B radiation, Plant Physiol., 60 (1977) 165-169. 12 P. Haldall, Ultraviolet action spectra of photosynthesis and photosynthetic inhibition in a green and red alga, Physiol. Plant., 17 (1964) 414-421. 13 W. B. Sisson and M. M. Caldwell, Photosynthesis, dark respiration, and growth of Rumor patentia L. exposed to ultraviolet irradiance (288-315 nanometers) simulating a reduced atmospheric ozone column, Plant Physiol., 58 (1976) 563-568. 14 T. K. Van, L. A. Garrard and S. H. West, Effects of UVB radiation on net photosynthesis of some crop plants, Crop Sci., I6 (1976) 715-718. 15 T. Tezuka, H. Sekiya and H. Ohno, Physiological studies on the action of CCC in Kyoho grapes, Plant Cell Physiol., 21 (1980) 969-977. 16 T. Tezuka, C. Takahara and Y. Yamamoto, Aspects regarding the action of CCC in hollyhock plants, J. Erp. Bot., 40 (1989) 689-692. 17 D. I. Amon, Copper enzymes in isolated chloroplasts. Polyphenoloxidase in Beta vulgati, Plant Physiol., 24 (1949) l-15. 18 T.K. Van, L. A. Garrard and S. H. West, Effects of 298~nm radiation on photosynthetic reactions of leaf discs and chloroplast preparations of some crop species, Environ. Exp Bot., 17 (1977) 107-112. 19 H. Lundegirdh, Klima und Boden in ihrer Wukung auf P&nzenleben, Gustave Fisher, Sweden, Kapitel 2, 3rd edn., 1957. 20 M. Seibert, P. J. Wetherbee and D. D. Job, The effects of light intensity and spectral quality on growth and shoot initiation in tobacco calus, Plant Physiol, 56 (1975) 130-139. 21 T. Tezuka and M. Mitani, unpublished data, 1992. 22 T. Tezuka and Y. Ando, unpublished data, 1991. 23 N. Kodno, personal communication, 1992.
J. Photochem. Photobiol. B: Biol., 19 (1993) 61-66
Growth promotion of tomato and radish plants by solar UV radiation reaching the Earth’s surface Takafumi Tezuka+, Toshihiro Hottat+ and Ikuko Watanabe+++ Department of Agronomy, School of Agricultural Sciences, Nagoya University, Chikusa-Ku, Nagoya 464-01 (Japan) (Received February 1, 1993; accepted February 11, 1993)
Abstract The effect of natural solar UV radiation on the growth of tomato and radish plants was studied using polyvinylchloride films with different UV transmissions. The growth (fresh and dry weights) of tomato plants exposed to UVA (400-320 nm) radiation (UVC-320 film, transmission above 320 nm) was greater than that of plants exposed to no UV radiation (UVC-400 film, transmission above 400 nm) or to mainly near-UV radiation (400-300 nm; including UVA (400-320 nm) and the longer wavelength region within the UVB (320-280 nm); DE-290 film, transmission above 290 nm). Furthermore, the growth promotion of tomato and radish plants on exposure to UVA has associated with an increase in chlorophyll content and photosynthetic activity. Dark respiration of tomato and radish plants was also promoted by radiation (near-UV and UVA region) through UVC-290 and UVC-320 films. Radiation with UVA delayed the senescence of tomato leaves.
Keywords: Chlorophyll, sativzq UVA
Growth promotion,
Lycopekcon
1. Introduction
Since the spectral band now designated as the UV was discovered in 1801 by Johann Wilhelm Ritter, studies on the effects of UV on various organisms have been reported by many biologists [l-7]. The solar spectrum reaching the Earth’s surface does not extend below approximately 290 nm because ozone in the stratosphere effectively absorbs all radiation of shorter wavelengths [8, 93. Therefore the natural solar radiation of the UVB region at the Earth’s surface is between 290 and 320 nm. Many reports [S, 7,10-141 have shown that UVB radiation inhibits photosynthesis and the growth of plants. Hence, the cultivation of crops in a greenhouse covered with plastic film (polyvinylchloride which is opaque to UV) is used extensively. However, little is known about whether the use of opaque plastic film is appropriate for the cul‘Author to whom correspondence should be addressed. “Present adress: Seibu Extension Division of Shizuoka Agricultural Experiment Station, Hamamatsu 432, Japan. “‘Present address: Kawagoe Extension Division of Saitama Agricultural Experiment Station, Kawagoe 350, Japan.
loll-1344/93/$6.00
esculentum,
Near UV, Photosynthesis,
Raphanus
tivation of crops in all cases, and whether natural solar W radiation (400-290 nm), consisting of the UVA region and all UVB regions on the Earth’s surface, is also deleterious for the growth of plants. The purpose of this work is to investigate the effects of radiation with UV from the natural solar spectrum on the growth, photosynthesis and other physiological activities of plants.
2. Materials
and methods
2.1. Plant materials Seeds of tomato (Lycopersicon esculentum Mill. cv. Horyu) and radish (Raphanw sativw L. cv. Comet) plants were purchased from Takii Seed Co., Kyoto. Tomato seeds were sterilized with 0.1% Uspulun (Bayer Chem. Co., Germany) and radish seeds with 5% Antiformin (Wako Chem. Co., Japan), after which they were rinsed in running tap water. The seeds were sown under plastic film frames, in styrene foam boxes (30 cmx50 cmx 60 cm) containing a growth substratum composed of humus and paddy field soils (1:9, v/v), which was sieved through a 3 mm mesh. After emergence, 0 1993 - Elsevier Sequoia. All rights reserved
62
‘I7 Tezuka et al. I Growth promotion of tomato and radish by solar W radiation
two plants per box were transplanted for cultivation in other boxes (30 cm x 50 cm X 60 cm) containing the above growth substratum under plastic film frames in an experimental field at Nagoya University. Tomato plants were harvested for sampling at 25, 46 and 66 days from sowing and radish plants at 29 days. Water was supplied on alternate days.
the length of the various organs of the tomato plants, the area of the leaves was measured using an automatic area meter (AMM-5, Hayashi Denko Co. Ltd., Tokyo) for the expression of photosynthesis and respiration on a leaf area basis. Each organ was dried at 80 “C for 3 days for the measurement of dry weight. 2.4. Measurements
2.2. Frames Each plastic film frame (Quonset hut type; 3.8 mx5.1 rn~ 2.7 m) was covered with a layer of polyvinylchloride film (0.1 mm thick). To allow ventilation the portion of the frame which extended to 30 cm in height above ground level was not covered with film. Furthermore, the frame was equipped with a fan (50 cm X50 cm) connected to a thermostat which operates at 35 “C or more in the frame; the atmospheric temperature in the frame rises during the warm/hot season from early summer to early autumn. Films tested were UVC290, UVC-320 and UVC-400 (Mitsui Toatsu Co. Ltd., Nagoya, Japan). The transmittance curves of the films were measured using a spectrophotometer (Hitachi, type 200) and are shown in Fig. 1. Each styrene foam box for the cultivation of plants was placed on a base (80 cm high). 2.3. Measurements of fresh and dry weight, and root and stem length
Tomato plants at each sampling day after sowing were gently uprooted, the roots washed and the plants divided into leaves, stems and roots. Following the measurements of the fresh weight and
I
I
I
I
I
I
of photosynthesis
and
respiration
The gas exchange of the leaves was measured after 25 and 66 days (the third and tenth leaf) in tomato with an IR CO, gas analyser (URAS1, Hartman & Brawn Co.) as described elsewhere [15, 161. Net photosynthesis and dark respiration are expressed on a leaf area basis. 2.5. Measurement of chlorophyll content Ten leaf discs (1 cm in diameter) were taken from the third leaf of 25 day old and the tenth leaf of 66 day old tomato plants. These discs were homogenized with 80% acetone in a glass homogenizer as described previously [15, 161. The homogenate was centrifuged at 70% for 10 min, the pellet was suspended in 80% acetone and spun again at 7OOgfor 10 min. The supernatants were combined to measure the volume. The contents of (total) chlorophyll a and b were estimated according to Arnon’s method [17] and are expressed on the basis of leaf area. 2.6. Measurement of senescence The senescence of tomato plants was checked with the unaided eye 66 days after sowing, and senescence was quantified according to five grades of leaf colour; green, greenish yellow, yellow, yellowish brown and brown.
I
3. Results
OL 250
300
350
400
450
Wavelength
500
550
600
650
(nm)
Fig. 1. Transmittance of the radiation-transmitting plastic films designated as UVC-290, UVC-320 and UVC-400. The transmittance of three plastic films was measured with a spectrophotometer (Hitachi, type 200).
Tomato plants developed the fourth and fifth and tenth and eleventh leaves 25 and 46 days after sowing respectively. Before anthesis (up to 25 days after sowing), the plant stems in the UVC400 (UV-non-transmitting) film frame were taller than those in the UVC-290 (UVA and UVBtransmitting) and WC-320 (WA-transmitting) film frames (Table 1). The plants at this age in the WC-320 film frame were the smallest compared with those in the other frames and seemed like a dwarf species. However, by 46 days, all the plants reached almost the same length. The roots of the plants in the three film frames showed almost the same length on both sampling dates (Table 1).
63
T. Tezuka et al. I Growth promotion of tomato and radish by solar UV radiation TABLE 1. Effects of solar W-transmitting UVC-290, UVC-320 and WC-400 plastic films on the length of stems and roots of 25 and 46 day old tomato plants (values are expressed as the mean of five replicates *standard deviation) Organ
Age (days)
Length
(cm) uvc-290
WC-290
UVC-320
WC-400
Stem
25 46
13.8f0.8 61.81t3.7
10.4 f 1.2 67.4 f 8.1
19.0f2.0 63.0*6.9
Root
25 46
10.8 i- 0.8 17.1 f 1.3
9.1 f 1.2 19.5 f 2.5
8.5 f 1.6 17.0f3.9
TABLE 2. Effects of solar W-transmitting WC-290, WC-320 and WC400 plastic films on the fresh and dry weights of leaves, stems and roots of 25 and 46 day old tomato plants (values are expressed as the mean of five replicates&standard deviation) Organ
Stem’ Leaves” Root’ Stemb Leavesb Rootb
Age (days)
25 46 25 46 25 46 25 46 25 46 25 46
TABLE 3. Effects of solar W-transmitting UVC-290, UVC-320 and UVC-400 plastic films on chlorophyll contents, net photosynthesis and dark respiration of the leaves of 25 and 66 day old tomato plants (values are expressed as the mean of five replicates f standard deviation)
Fresh or dry weight (g) WC-290
uvc-320
uvc400
2.4 f 0.4 38.4f6.2 2.3 f 0.7 34.2 f 10.6 0.5 * 0.2 3.5 f 1.2
1.1 f0.2 44.6 f 6.8 1.0*0.1 50.1 f 2.4 0.2fO.l 3.5 kO.8
3.OkO.2 27.Ok5.4 1.7 io.3 21.9i3.5 0.4*0.1 3.5 kO.8
0.11*0.02 1.77f0.28 0.24 f 0.08 2.67 f 0.83 0.05 f 0.02 0.38 kO.13
0.05 f 0.01 2.10*0.32 0.11 f 0.00 4.46 f 0.21 0.02 f 0.00 0.35 f 0.08
0.12f0.03 1.08 f 0.22 0.16kO.03 1.73 f 0.28 0.05 f 0.01 0.42f0.10
‘Fresh weight. bDry weight.
After 25 days, the fresh and dry weights of the stems, leaves and roots were lowest for plants under the UVC-320 film compared with those under the other films (Table 2). After 46 days, the fresh and dry weights of the stems and leaves were greatest in tomato plants under the UVC320 fihn, followed by the UVC-290 film and finally the UVC-400 film; the root showed no difference for plants under the three films (Table 2). In both 25 and 66 day old plants, the chlorophyll contents of the tomato leaves under the UVC290 and UVC-320 (+ UV) films were superior to those under the UVC-400 (-UV) film (Table 3). The net photosynthetic and dark respiratory activities of the leaves under the UVC-290 and UVC320 ( +UV) films were also greater than those under the UVC-400 ( -UV) film (Table 3). In the case of radish leaves (29 days old) in the UVC-320 fihn frame, chlorophyll contents and net photosynthetic and dark respiratory activities
Chlorophyll a +b (pg cm-*) 25 day old’ 24.22 f 0.87 66 day old’ 25.08 f 0.90
WC-320
uvc-400
25.70f0.88 26.58 f 0.29
22.44 f 0.29 22.72 f 0.30
Net photosynthesis (mg CO2 dm-* h-‘) 14.71 kO.83 15.38f0.64 25 day old 17.46 f 0.99 17.94 *0.75 66 day old Dark respiration 25 day old 66 day old
(mg CO2 dm-* h-‘) 0.93 f 0.03 1.01 f 0.03 1.43 f 0.08 1.09 f 0.07
12.82 f 0.87 15.34 f 0.77 0.78 f 0.06 0.84 f 0.03
‘The third leaf of 25 day old and tenth leaf of 66 day old plants were used for analyses. Leaf positions are numbered from the lower to the upper leaves. TABLE 4. Effects of solar W-transmitting UVC-290, UVC-320 and UVC-400 plastic films on chlorophyll contents, net photosynthesis and dark respiration of the third or fourth leaf of 29 day old radish plants (values are expressed as the mean of ten replicates f standard deviation)
Chlorophyll a + b (Pg cm-*) Net photosynthesis (mg CO2 dm-’ h-‘) Dark respiration (mg CO2 dm-’ h-l)
WC-290
UVC-320
uvc400
32.9 f 2.7
36.8& 1.1
31.3* 1.4
22.8 f 0.3
23.8 f 1.0
19.7*0.3
1.52rtO.08
1.61 kO.13
1.30*0.11
were also somewhat superior to those in the other film frames (Table 4). Solar UV radiation transmitted through the UVC-290 and UVC-320 ( +UV) films delayed the senescence of tomato leaves compared with radiation transmitted through the UVC-400 ( - UV) film (Table 5). 4. Discussion In order to determine whether solar UV radiation induces the inhibition of plant growth and photosynthesis [ll, 14, 181 compared to visible radiation only (without solar UV), research on tomato plants was carried out using a series of polyvinylchloride films. In 1919, Dorno stated that from late spring to mid-autumn, more solar W radiation reaches the Earth’s surface compared with other seasons (cited by Lundeg%rdh [19]). Therefore, in this study, tomato plants were cultured from late spring to
T. Temka et al. / Growth promotion of tomato and radtih by solar W radiation
64
TABLE 5. Effects of solar UV-transmitting UVC-290, UVC-320 and UVC400 plastic films on the leaf senescence of 66 day old tomato leaves. Senescence was estimated by checking the grade” of leaf colour with the unaided eye (values are expressed as the mean of five replicates) Leaf positionb
uvc-290
UVC-320
uvc400
10 9 8 7 6 5 4 3 2 1
+++++a +++++ ++++ ++++ +++ ++ + +
+++++ +++++ +++++ +++++ ++++ ++++ +++ ++
+++++ +++++ ++++ +++ ++ + +
-I-
-
-
-
’ + + + + + , green; + + + + , greenish yellow; + + + , yellow; + +, yellowish brown; + , brown; - , fallen leaves. bLeaf positions are numbered from the lower to the upper leaves.
early summer and radish plants in autumn in order to make use of the natural solar UV radiation. Radish plants were not cultured at the same time as tomato plants as they produce flower buds under the long day conditions from spring to summer. In this experimental season, the maximum and minimum fluences of visible light (700-400 nm) were approximately 425 W m-* (clear sky) and 25 W m-* (cloudiness) respectively, and the fluences in the frames were approximately 90% of solar radiation (Fig. 1). According to the Nagoya Meteorological Observatory, during this experimental season, around the Nagoya area, the degree of cloudiness was 7.2/10 (tomato) and 7.1/10 (radish) on average; the sunshine duration, measured by a sunshine recorder, was 5.7 h (tomato) and 4.5 h (radish) per day on average; the number of cloudy days was 14 (April D-30), 19 (May) and 7 (June l-20) (tomato) and 17 (September 20 to October 20) (radish). The experimental field at Nagoya University is located at longitude 137” E and latitude 35” 11’ N. The average monthly temperatures in the plastic film frames during the experiments were 15 (April), 23 (May) and 35 “C (June), and those in the open air were 13 (April), 19 (May) ,and 23 “C (June). The temperatures of the film frames during cultivation showed little difference (1 “C or so at most). The solar UV radiation transmitted through the UVC-320 fihn seemed to delay the stem growth of tomato plants in the early growth stage but encouraged stem growth in the later stage compared with visible radiation only transmitted through the UVC-400 film (Table 1). In the early growth stage, the plants may have different sen-
sitivities to UVA (400-320 nm) and UVB (320-290 nm), within solar UV) radiation and this is now being investigated. This phenomenon was also observed for the growth of radish, Shanghai Pak Choi (a kind of Brussicu), eggplants, sweet peppers and rice (data not presented). These results suggest that solar near-UV radiation, especially UVA, can increase the metabolic activity related to the growth of plants in the later growth stage. Solar UV radiation (above approximately 300 nm in particular, near-UV); seems to contribute to the promotion of plant growth. Therefore near-UV radiation intensity not exceeding the intensity of natural solar UV radiation will promote the growth of plants on the Earth’s surface and intensities above this will inhibit growth. Indeed, the growth and nitrogen tixation by nodules of soybean plants were somewhat inhibited by supplementary nearUV radiation under field conditions (data not presented). If plants receive a radiation intensity stronger than that of solar UV radiation, they may be compelled to undergo abnormal metabolism in cells. Seibert et al. [20] have reported that UVA radiation at low light intensities stimulates growth, but at high light intensities inhibits the growth and shoot initiation, of tobacco callus (chlorophyllcontaining clone). The plants located under the UVC-290 film frame show good growth (Table 2). As can be seen in Tables 1 and 2, the thickening of 46 day old plant stems seems to be promoted by solar UV radiation passing through the UVC-290 and UVC-320 films. This increase in growth can be related to increases in chlorophyll content and photosynthetic activity in plants growth under the UVC-290 and UVC-320 films (Tables 3 and 4). The promotion of the net photosynthetic activity of tomato plants by natural solar UV radiation may result from alteration(s) in the structure and function of chloroplasts and/or chlorophyll content. Net photosynthesis and dark respiration on a dry weight basis showed different trends (UVC400> UVC-290> UVC-320; 66 day old tomato leaves) compared with results on a leaf area basis (data not presented). This seems to result from the increasing tomato leaf thickness and dry weight on exposure to natural solar UV radiation. According to Cen and Bornman [6], UVB radiation increased leaf thickness of bean plants by approximately 18%. Since the leaves were somewhat thicker under the UVC-290 and UVC-320 ( + UV radiation) film frames than under the UVC-400 ( - UV radiation) film frame, the size and/or number of palisade and spongy cells in the leaves may also be greater under the UVC-290 and UVC-
T. Tezuko et al 1 Growth promotion of tomato and radish by solar W radiation
320 film frames than under the UVC-400 film frame. Indeed, the size of palisade and spongy cells in leaves of cucumber plants was enhanced by near-UV radiation [21]. Therefore, chloroplasts and other cell organelles may increase in number on exposure to natural solar UV radiation compared with results obtained on exposure to natural solar radiation without UV. This is currently being investigated. The wavelength region above 400 nm (i.e. only visible light) may cause a kind of stress in plants. Radish plants on exposure to visible light (UVC400) show enhanced superoxide dismutase (SOD) activity [22]. This suggests that the onset of tomato leaf senescence may be accelerated by ion radicals, such as 02-, induce on exposure to visible light. As a result (Table 5), the radiation transmitted through the UVC-400 ( - UV) film accelerates leaf senescence, which is due to the inhibition of chlorophyll synthesis and/or the promotion of chlorophyll degradation. This study shows that the fluence rates of solar UV (in particular, near-UV) radiation on the Earth under existing circumstances do not cause any damage to the growth of plants. Indeed, crops in a frame covered with UV-non-transmitting plastic film were inferior to crops grown under solar UV radiation conditions. The growth of cucumber plants was also promoted by UV radiation above 300 nm [23]. Therefore it is premature to conclude that solar near-UV or UVA radiation causes damage to plant growth based on the inhibitory action of plant growth by UVC and the shorter wavelength region within UVB [l, 7, 11, 141. The mechanism of growth promotion by natural solar UV radiation has not yet been established. However, the functional sites of cell organelles, cell membranes and cytoplasm are regulated to promote photosynthetic, respiratory and other metabolic activities by natural solar UV radiation (including the UV region above approximately 300 nm). Solar W radiation (greater than 290 nm) seems to be an essential factor for plant growth not an inhibitory factor. Acknowledgments We thank S. Torii and K. Sakakibara for technical assistance. References 1 P. M. Klein, Plants and near-ultraviolet 44 (1978) 1-127.
radiation,
Bot. Rev.,
65
2 J. F. Bomman, Target sites of UV-B radiation in photosynthesis of higher plants, .I. Photochem. Photobiol. B: Biol., 4 (1989) 145-158. 3 Y. Takeuchi, M. Akizuki, H. Shimizu, N. Kondo and K. Sugahara, Effect of UV-B (290-320 nm) irradiation on growth and metabolism of cucumber cotyledons, Physiol. Plant., 76 (1989) 425-430. 4 A. H. Teramura, L. H. Ziska and A. E. Sztein, Changes in growth and photosynthetic capacity of rice with increased UV-B radiation, Physiol. Plant., 83 (1991) 373-380. 5 T. Hashimoto and M. Tajima, Effects of ultraviolet irradiation on growth and pigmentation in seedlings, PIant Cell Physiol., 21 (1980) 1559-1571. 6 Y. P. Cen and J. F. Bomman, The response of bean plants to UV-B radiation under different irradiances of background visible light, /. Qo. Bot., 41 (1990) 1481-1495. 7 A. H. Teramura, Effects of ultraviolet-B radiation on the growth and yield of crop plants, Physiol. Plant., 58 (1983) 415-427. 8 I. M. Campbell, Energy and the atmosphere. A Physical-Chemical Approach, Wiley, London, New York, Sydney, Toronto, 1980, Chapter 2. 9 T. Tezuka, Effect of ultraviolet radiation on plant growth, in T. Seiyama (ed.), Global Environment and Energy Issues (Proc. Fukuoka bat. Symp.90, Fukuoka, Japan, November 19-21, 1990), Electrochemical Society of Japan, Kyushu Branch, Fukuoka, Japan, 1990, pp. 267-270. 10 J. E. Ambler, D. T. Krizek and P. Semeniuk, Influence of W-B radiation in early cotyledons in cotton, Physiol. Plant., 34 (1975) 177-181. 11 J. R. Brandle, W. F. Campbell, W. B. Sisson and M. M. Caldwell, Net photosynthesis, electron transport capacity, and ultrastructure of Pisum sativum L. exposed to ultraviolet-B radiation, Plant Physiol., 60 (1977) 165-169. 12 P. Haldall, Ultraviolet action spectra of photosynthesis and photosynthetic inhibition in a green and red alga, Physiol. Plant., 17 (1964) 414-421. 13 W. B. Sisson and M. M. Caldwell, Photosynthesis, dark respiration, and growth of Rumor patentia L. exposed to ultraviolet irradiance (288-315 nanometers) simulating a reduced atmospheric ozone column, Plant Physiol., 58 (1976) 563-568. 14 T. K. Van, L. A. Garrard and S. H. West, Effects of UVB radiation on net photosynthesis of some crop plants, Crop Sci., I6 (1976) 715-718. 15 T. Tezuka, H. Sekiya and H. Ohno, Physiological studies on the action of CCC in Kyoho grapes, Plant Cell Physiol., 21 (1980) 969-977. 16 T. Tezuka, C. Takahara and Y. Yamamoto, Aspects regarding the action of CCC in hollyhock plants, J. Erp. Bot., 40 (1989) 689-692. 17 D. I. Amon, Copper enzymes in isolated chloroplasts. Polyphenoloxidase in Beta vulgati, Plant Physiol., 24 (1949) l-15. 18 T.K. Van, L. A. Garrard and S. H. West, Effects of 298~nm radiation on photosynthetic reactions of leaf discs and chloroplast preparations of some crop species, Environ. Exp Bot., 17 (1977) 107-112. 19 H. Lundegirdh, Klima und Boden in ihrer Wukung auf P&nzenleben, Gustave Fisher, Sweden, Kapitel 2, 3rd edn., 1957. 20 M. Seibert, P. J. Wetherbee and D. D. Job, The effects of light intensity and spectral quality on growth and shoot initiation in tobacco calus, Plant Physiol, 56 (1975) 130-139. 21 T. Tezuka and M. Mitani, unpublished data, 1992. 22 T. Tezuka and Y. Ando, unpublished data, 1991. 23 N. Kodno, personal communication, 1992.