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Increased UV-B radiation reduces N2-fixation in tropical leguminous crops
Environmental Pollution, Vol. 95, No. 3, pp. 289-291, 1997 © 1997Publishedby ElsevierScienceLtd
~i!'/'t.._%i ELSEVIER
PII: S0269-7491
(96)00146-7
All rights reserved.Printed in Great Britain 0269-7491/97 $17.00+ 0.00
INCREASED UV-B RADIATION REDUCES N2-FIXATION IN TROPICAL L E G U M I N O U S CROPS Anupa Singh* Department of Botany, Banaras Hindu University, Varanasi-221 005, India (Received 1 May 1996; accepted 27 November 1996)
Abstract Net photosynthesis, leaf area, biomass, and number, size and activity of nodules were examined in three leguminous plants subjected under field conditions to supplemental UV-B radiation equivalent to a 15% ozone depletion at 25 ° N latitude. Enhanced U V-B radiation adversely affected the net photosynthetic rate, growth characteristics and nodule activity in all three species. Maximum reduction in net photosynthesis occurred in Phaseolus mungo cv. Pant U-30, whereas the greatest reduction in nitrogenase activity occurred in Vigna radiata. © 1997 Published by Elsevier Science Ltd
MATERIALS AND METHODS
Field experiments were conducted in the summer and rainy seasons in 1993, at the experimental botanical garden of the Banaras Hindu University at Varanasi (25°18 ' N latitude, and 83°1' E longitude at 76.19 m above the mean sea level). The soil was silty loam with an organic carbon content of 0.67%, pH 7.2 and cation exchange capacity of 16.6 meq%. The ambient temperature during summer ranged from 32--45°C and in the rainy season from 24-36°C. Annual rainfall was 750 mm, of which 80% occurred in the rainy season (Singh, 1995a). Leguminous crop species for the summer experiments were Vigna radiata L. cv P.S. 16 and Phaseolus mungo L. cv Pant U-30, and for the rainy season, Phaseolus mungo L. cv. Mash-48. These crops are commonly grown in the area. Seeds of each species were handplanted in rows spaced 38 cm apart, in 1.5 x 1.5 m plots. Six plots were maintained for each plant species (3 with supplemental UV-B and 3 without supplemental UV-B). After germination, plants were thinned in each row to one plant every 15 cm. Plants were irrigated with tap water as required. UV-B exposure was started from the date of sowing and continued until the end of the experiment. Supplemental UV-B radiation was supplied by Q-panel 313 UV-B lamps (Q panel, Cleveland, Ohio, USA) by suspending the lamps above each row of plants (three per plot). The lamps were covered by either a 0.13 mm cellulose diacetate filter (transmission down to 280 nm) for supplemental UV-B radiation, or a 0.13 mm polyester filter (absorbs all radiation below 320 nm) for the controls. Aluminium reflectance strips were used to avoid the scattering of UV-rays from the upper side of the lamps. The UV-B irradiance at plant height under the lamps was measured by a spectropower meter (Scienteck Inc., Boulder, Colorado, USA). The treated plants received 10.08 KJ m-2 UV-BBE on a daily basis under clear sky conditions, which was equivalent to an ozone depletion of 15% at 25°N (Green et al., 1980). Filters were changed weekly to avoid aging effects on the spectral transmission of UV-B and were presolarized before use. Further details of treatment are given elsewhere (Singh, 1996). Net photosynthesis was measured in the field on the central leaflet of attached, fully expanded trifoliate leaves from the second and third nodes (numbered
Keywords: Phaseolus mungo, Vigna radiata, nitrogenase activity, net photosynthesis. INTRODUCTION
Nitrogen fixation by nodulated legumes is a major mechanism for the entry of reduced nitrogen into agricultural lands (Quispel, 1974). Leguminous species have been shown to be sensitive to UV-B radiation (280-320 nm) in growth chamber and greenhouse studies, but most data have been obtained from temperate species and little is known for tropical plants (Singh, 1994). Depletion of stratospheric ozone will increase the intensity of UV-B radiation and plants are likely to be among those organisms most severely affected (Quaite et al., 1992). Global ozone measurements over the period 1979-1993 imply significant UV-B increases at high and mid-latitudes of both hemispheres but only small changes in the tropics (Madronich et al., 1995). However, due to a thinner ozone layer and a higher sun angle, the tropics already experience an intense UV climate (Caldwell et al., 1980). UV-B levels in the tropics are expected to exceed radiation levels that have been experienced on earth during the recent geological past (Finckh et al., 1995) and therefore could be particularly damaging to the leguminous crops in this region (Singh, 1995a). In the present study, net photosynthesis and nodule nitrogenase activity of three tropical leguminous crops were assessed under field conditions supplemented with UV-B radiation equivalent to a simulated 15% ozone depletion at 25°N latitude. *Present address: Department of Botany, Gujarat University, Ahmedabad-38009, India 289
290
A. Singh
acropetally) by the LI-COR model 6200 portable photosynthetic system (LI-COR, Lincoln, NE, USA), which also recorded photosynthetically active radiation (PAR). The PAR at the leaf level on cloud-free days at mid-day did not differ between control and treated plants and averaged 1244 lamol m -2 s-1 in summer and 1010.5 ~tmol m -2 s-l in the rainy season. Three plants were sampled randomly from each plot (one per row) at 40 days after planting. Leaf area (leaf area meter, Model 3000, LI-COR, USA), number and dimension of nodules, and shoot and root biomass (oven-dried at 80°C to constant weight) were recorded for each plant. Nitrogenase activity was estimated by acetylene reduction (Stewart, 1967) using a Gas Chromatograph (Chemito 8510, India). Data were analysed by ANOVA and the differences between means were tested by student's t-test (Snedecor & Cochran, 1967).
R E S U L T S AND D I S C U S S I O N Exposure to supplemental UV-B resulted in a significant decrease in net photosynthesis and leaf area in all the three legumes (Table 1). Maximum reduction occurred in P. mungo Pant U-30 (80% in net photosynthesis and 44% in leaf area) grown in summer and minimum in P. mungo Mash-48 (22% in net photosynthesis and 28% in leaf area) grown in the rainy season. Reductions in photosynthetic rate and leaf area have been reported for several species (Singh, 1994; Caldwell et al., 1995). For example, in 21 day old soybean plants, CO2 uptake rate declined by 21 and 25% at an elevated UV-B dose of 21.7 and 29.2 mW m -2, simulating 21 and 36% ozone depletion, respectively (Vu et al., 1982). A 36% reduction in leaf area of soybean occurred when plants were exposed to 5.1 KJ m -2 supplemental UV-B (Sullivan &
Teramura, 1990). UV-B predominantly attacks the PS II reaction centre (Iwanzik et al., 1983), and induces a change in the stoichiometry of P S I to PS II (Lingakumar & Kulandaivelu, 1993). Shoot dry wt, as well as root dry wt, decreased in all three legumes due to supplemental UV-B treatment. In V. radiata and P. mungo Pant U-30 reduction in root biomass (82 and 55%, respectively) was greater than in shoot biomass (45 and 52%, respectively), while in P. mungo Mash-48 the reverse was true (60% in shoot versus 13% in root) (Singh, 1995a,b, 1996). Thus interspecific and intervarietal responses differed in their pattern of biomass allocation, as observed in several other studies (Caldwell et al., 1995). Nodule number declined significantly in UV-B treated V. radiata (62%) and P. mungo Pant U-30 (57%), but not in P. mungo Mash-48 (Table 1). Nodule diameter also differed significantly between control and treated plants in V. radiata and P. mungo Pant U-30. Nitrogenase activity was significantly less with supplemental UV-B in all three species (Table 1). This decline was greatest (78%) in IX. radiata, which also showed the greatest reduction in root biomass. Interestingly, P. mungo Pant U-30, which exhibited the greatest decline in photosynthesis, registered the smallest decrease in nitrogenase activity (16%) under supplemental UV-B radiation. Earlier results from pea and soybean (Mahon, 1982; Vance & Heichel, 1991) indicated no relationship between photosynthetic rate and nitrogenase activity in legume nodules. Unlike cyanobacteria and other photosynthetic bacteria which derive energy for N2 reduction directly from photosynthesis occurring in the same organism, the nodule rhizobia get their energy from reduced carbon deposited in the nodule by the legume (Thorneley, 1992). Evidently biomass allocation and hence metabolite transport, pool sizes and source-sink
Table 1. Effect of enhanced UV-B radiation on net photosynthesis, leaf area, number and size of nodules and nitrogenase activity in three tropical legumes under field conditions
Vigna radiata
Phaseolus mungo Pant U-30
Phaseolus mungo Mash 48
Control
Enhanced UV-B
Control
Enhanced UV-B
Control
Enhanced UV-B
19.41
13.5c
14.09
2.60c
11.85
9.22b
±0.12
+0.3
±0.9
+ 0.4
+ 0.35
+ 0.36
Leaf area (cm2 plant-1)
225.0 ±2.8
131.0a ± 1.7
210.7 ± 1.8
Nodule number plant-l
8.0 ± 1.0
3.0a ± 2.0
7.0 ± 2.0
Nodule diameter (mm)
6.2 ±6.0
4.0a ±0.3
8.0 ±0.3
Nitrogenase activity (~tmol C2H4 h-1 mg-l dry wt of nodule)
0.130
0.028~
0.243
117.7a ± 1.5 3.0a + 2.0 6.7a ±0.4 0.189~
1837.3 ±6.74 21.0 ± 4.0 7.3 ±0.9 0.174
1318.3c + 12.02 16.0Ms + 3.0 6.0Ns ±0.6 0.139c
± 0.0015
± 0.0007
± 0.0025
± 0.011
± 0.0045
± 0.0055
Net photosynthesis (larnol m-2 s-1)
Note: Values are means + SE. NS, not significant. aDifference between control and UV-B treated plants significant at p < 0.05. bDifference between control and UV-B treated plants significant at p < 0.01. CDifference between control and UV-B treated plants significant at p < 0.001.
UV-B radiation and N2-fixation
relations are more critical than photosynthetic rate for nitrogenase activity in the short-term (Vance & Heichel, 1991; Brown et al., 1995). The molecular basis for many of the observed changes, including suppression of nodulation and nitrogenase activity, following UV-B exposure remains unknown (CaldweU et al., 1995). Since the possibility of UV-B penetrating into the soil to affect nodulation and nodule activity directly is remote, the observed suppressive effect may result from altered assimilate transport which affects cortical cell turgor (Walsh, 1995) and changes in flavonoids which are implicated in rhizobial nod-gene induction (Hungria & Phillips, 1993). In conclusion, the enhanced UV-B radiation equivalent to 15% ozone depletion under field conditions affected not only net photosynthesis of the three legumes, it also depressed nitrogenase activity, particularly in V. radiata.
ACKNOWLEDGEMENTS Thanks are due to the Department of Science and the Technology, and the Council of Scientific & Industrial Research, New Delhi for funding support.
REFERENCES Brown, S. M., Oparka, K. J., Sprent, J. L. & Walsh, K. B. (1995). Symplastic transport in soybean root nodules. Soil Biol. Biochem., 27, 387-399. Caldwell, M. M., Robberecht, R. & Billings, W. D. (1980). A steep latitudinal gradient of solar ultraviolet-B radiation in the arctic-alpine life zone. Ecology, 61, 600~11. Caidwell, 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, 166-173. Finckh, M. R., Chavez, A., Dai, Q. & Teng, P. S. (1995). Effects of enhanced UV-B radiation on the growth of rice and its susceptibility to rice blast under glasshouse conditions. Agric. Ecosystems Environ, 52, 223-233. Green, A. E. S., Cross, K. R. & Smith, L. A. (1980). Improved analytical characterization of ultraviolet skylight. Photochem. Photobiol., 31, 59-65.
291
Hungria, M. & Phillips, D. A. (1993). Effects of a seed color mutation on rhizobial nod-Gene-Inducing flavonoids and nodulation in common bean. Molec. Plant-Microbe Interae., 6, 418-422. Iwanzik, W., Tevini, M., Dohnt, G., Voss, M., Weiss, W., Graber, P. & Renger, G. (1983). Action of the UV-B radiation on photosynthetic primary reactions in spinach chloroplasts. Physiol. Plant, 58, 401-407. Lingakumar, K. & Kulandaivelu, G. (1993). Changes induced by ultraviolet-B radiation in vegetative growth, foliar characteristics and photosynthetic activities in Vigna unuiculata. Aust. J. Plant Physiol., 20, 299-308. Madronich, S., Mckenzie, R. L., Caldwell, M. M. & Bjorn, L. O. (1995). Changes in ultraviolet radiation reaching the earth's surface. Ambio, 24, 143-152. Mahon, J. D. (1982). Field evaluation of growth and nitrogen fixation in peas selected for high and low photosynthetic CO2 exchange. Can. ,I. Plant Sci., 62, 5-17. Quaite, F. E., Sutherland, B. M. & Sutherland, J. C. (1992). Action spectrum for DNA damage in alfalfa lowers predicted impact of ozone depletion. Nature, 358, 576--578. Quispel, A. (1974). The Biology of Nitroen Fixation. North Holland Publishing Co., Amsterdam, Netherlands. Singh, A. (1994). Effects of enhanced UV-B radiation on higher plants. Trop. Ecol., 35, 164-189. Singh, A. (1995a). Effect of UV-B radiation on growth and yield of Glycine max and Phaseolus mungo. Ind. J. Exp. Biol., 33, 872-875. Singh, A. (1995b). Influence of enhanced UV-B radiation on tropical legumes. Trop. Ecol., 36, 249-252. Singh, A. (1996). Growth, physiological, and biochemical responses of three tropical legumes to enhanced UV-B radiation. Can. J. Bot., 74, 135-139. Snedecor, G. W. & Cochran, W. G. (1967). Statistical Method. The Iowa State University Press, Ames, Iowa, U.S.A. Sullivan, J. H. & Teramura, A. H. (1990). A field study of the interaction between UV-B radiation and drought on photosynthesis and growth in soybean. Plant Physiol, 92, 141-146. Stewart, W. D. P., Fitzgerald, G. P. & Burris, R. H. (1967). In situ studies on N2 fixation using the acytelene reduction technique. Proc. Natl. Acad. Sci. USA, 58, 2071-2078 Thorneley, R. N. F. (1992). New light on nitrogenase. Nature, 360, 532-533. Vance, C. P. & Heichel, G. H. (1991). Carbon in N2 fixation: Limitation or exquisite adaptation? Ann. Rev. Plant Physiol. Plant Mol. Biol., 42, 373-392. Vu, C. V., Allen, L. H., Jr & Garrard, L. A. (1982). Effects of UV-B radiation (280--320 nm) on photosynthetic constituents and processes in expanding leaves of soybean (Glycine max L. Mers.). Environ. Exp. Bot., 22, 465-473. Walsh K. B., (1995). Physiology of the legume nodule and its response to stress. Soil Biol. Biochem., 27, 637-655.
~i!'/'t.._%i ELSEVIER
PII: S0269-7491
(96)00146-7
All rights reserved.Printed in Great Britain 0269-7491/97 $17.00+ 0.00
INCREASED UV-B RADIATION REDUCES N2-FIXATION IN TROPICAL L E G U M I N O U S CROPS Anupa Singh* Department of Botany, Banaras Hindu University, Varanasi-221 005, India (Received 1 May 1996; accepted 27 November 1996)
Abstract Net photosynthesis, leaf area, biomass, and number, size and activity of nodules were examined in three leguminous plants subjected under field conditions to supplemental UV-B radiation equivalent to a 15% ozone depletion at 25 ° N latitude. Enhanced U V-B radiation adversely affected the net photosynthetic rate, growth characteristics and nodule activity in all three species. Maximum reduction in net photosynthesis occurred in Phaseolus mungo cv. Pant U-30, whereas the greatest reduction in nitrogenase activity occurred in Vigna radiata. © 1997 Published by Elsevier Science Ltd
MATERIALS AND METHODS
Field experiments were conducted in the summer and rainy seasons in 1993, at the experimental botanical garden of the Banaras Hindu University at Varanasi (25°18 ' N latitude, and 83°1' E longitude at 76.19 m above the mean sea level). The soil was silty loam with an organic carbon content of 0.67%, pH 7.2 and cation exchange capacity of 16.6 meq%. The ambient temperature during summer ranged from 32--45°C and in the rainy season from 24-36°C. Annual rainfall was 750 mm, of which 80% occurred in the rainy season (Singh, 1995a). Leguminous crop species for the summer experiments were Vigna radiata L. cv P.S. 16 and Phaseolus mungo L. cv Pant U-30, and for the rainy season, Phaseolus mungo L. cv. Mash-48. These crops are commonly grown in the area. Seeds of each species were handplanted in rows spaced 38 cm apart, in 1.5 x 1.5 m plots. Six plots were maintained for each plant species (3 with supplemental UV-B and 3 without supplemental UV-B). After germination, plants were thinned in each row to one plant every 15 cm. Plants were irrigated with tap water as required. UV-B exposure was started from the date of sowing and continued until the end of the experiment. Supplemental UV-B radiation was supplied by Q-panel 313 UV-B lamps (Q panel, Cleveland, Ohio, USA) by suspending the lamps above each row of plants (three per plot). The lamps were covered by either a 0.13 mm cellulose diacetate filter (transmission down to 280 nm) for supplemental UV-B radiation, or a 0.13 mm polyester filter (absorbs all radiation below 320 nm) for the controls. Aluminium reflectance strips were used to avoid the scattering of UV-rays from the upper side of the lamps. The UV-B irradiance at plant height under the lamps was measured by a spectropower meter (Scienteck Inc., Boulder, Colorado, USA). The treated plants received 10.08 KJ m-2 UV-BBE on a daily basis under clear sky conditions, which was equivalent to an ozone depletion of 15% at 25°N (Green et al., 1980). Filters were changed weekly to avoid aging effects on the spectral transmission of UV-B and were presolarized before use. Further details of treatment are given elsewhere (Singh, 1996). Net photosynthesis was measured in the field on the central leaflet of attached, fully expanded trifoliate leaves from the second and third nodes (numbered
Keywords: Phaseolus mungo, Vigna radiata, nitrogenase activity, net photosynthesis. INTRODUCTION
Nitrogen fixation by nodulated legumes is a major mechanism for the entry of reduced nitrogen into agricultural lands (Quispel, 1974). Leguminous species have been shown to be sensitive to UV-B radiation (280-320 nm) in growth chamber and greenhouse studies, but most data have been obtained from temperate species and little is known for tropical plants (Singh, 1994). Depletion of stratospheric ozone will increase the intensity of UV-B radiation and plants are likely to be among those organisms most severely affected (Quaite et al., 1992). Global ozone measurements over the period 1979-1993 imply significant UV-B increases at high and mid-latitudes of both hemispheres but only small changes in the tropics (Madronich et al., 1995). However, due to a thinner ozone layer and a higher sun angle, the tropics already experience an intense UV climate (Caldwell et al., 1980). UV-B levels in the tropics are expected to exceed radiation levels that have been experienced on earth during the recent geological past (Finckh et al., 1995) and therefore could be particularly damaging to the leguminous crops in this region (Singh, 1995a). In the present study, net photosynthesis and nodule nitrogenase activity of three tropical leguminous crops were assessed under field conditions supplemented with UV-B radiation equivalent to a simulated 15% ozone depletion at 25°N latitude. *Present address: Department of Botany, Gujarat University, Ahmedabad-38009, India 289
290
A. Singh
acropetally) by the LI-COR model 6200 portable photosynthetic system (LI-COR, Lincoln, NE, USA), which also recorded photosynthetically active radiation (PAR). The PAR at the leaf level on cloud-free days at mid-day did not differ between control and treated plants and averaged 1244 lamol m -2 s-1 in summer and 1010.5 ~tmol m -2 s-l in the rainy season. Three plants were sampled randomly from each plot (one per row) at 40 days after planting. Leaf area (leaf area meter, Model 3000, LI-COR, USA), number and dimension of nodules, and shoot and root biomass (oven-dried at 80°C to constant weight) were recorded for each plant. Nitrogenase activity was estimated by acetylene reduction (Stewart, 1967) using a Gas Chromatograph (Chemito 8510, India). Data were analysed by ANOVA and the differences between means were tested by student's t-test (Snedecor & Cochran, 1967).
R E S U L T S AND D I S C U S S I O N Exposure to supplemental UV-B resulted in a significant decrease in net photosynthesis and leaf area in all the three legumes (Table 1). Maximum reduction occurred in P. mungo Pant U-30 (80% in net photosynthesis and 44% in leaf area) grown in summer and minimum in P. mungo Mash-48 (22% in net photosynthesis and 28% in leaf area) grown in the rainy season. Reductions in photosynthetic rate and leaf area have been reported for several species (Singh, 1994; Caldwell et al., 1995). For example, in 21 day old soybean plants, CO2 uptake rate declined by 21 and 25% at an elevated UV-B dose of 21.7 and 29.2 mW m -2, simulating 21 and 36% ozone depletion, respectively (Vu et al., 1982). A 36% reduction in leaf area of soybean occurred when plants were exposed to 5.1 KJ m -2 supplemental UV-B (Sullivan &
Teramura, 1990). UV-B predominantly attacks the PS II reaction centre (Iwanzik et al., 1983), and induces a change in the stoichiometry of P S I to PS II (Lingakumar & Kulandaivelu, 1993). Shoot dry wt, as well as root dry wt, decreased in all three legumes due to supplemental UV-B treatment. In V. radiata and P. mungo Pant U-30 reduction in root biomass (82 and 55%, respectively) was greater than in shoot biomass (45 and 52%, respectively), while in P. mungo Mash-48 the reverse was true (60% in shoot versus 13% in root) (Singh, 1995a,b, 1996). Thus interspecific and intervarietal responses differed in their pattern of biomass allocation, as observed in several other studies (Caldwell et al., 1995). Nodule number declined significantly in UV-B treated V. radiata (62%) and P. mungo Pant U-30 (57%), but not in P. mungo Mash-48 (Table 1). Nodule diameter also differed significantly between control and treated plants in V. radiata and P. mungo Pant U-30. Nitrogenase activity was significantly less with supplemental UV-B in all three species (Table 1). This decline was greatest (78%) in IX. radiata, which also showed the greatest reduction in root biomass. Interestingly, P. mungo Pant U-30, which exhibited the greatest decline in photosynthesis, registered the smallest decrease in nitrogenase activity (16%) under supplemental UV-B radiation. Earlier results from pea and soybean (Mahon, 1982; Vance & Heichel, 1991) indicated no relationship between photosynthetic rate and nitrogenase activity in legume nodules. Unlike cyanobacteria and other photosynthetic bacteria which derive energy for N2 reduction directly from photosynthesis occurring in the same organism, the nodule rhizobia get their energy from reduced carbon deposited in the nodule by the legume (Thorneley, 1992). Evidently biomass allocation and hence metabolite transport, pool sizes and source-sink
Table 1. Effect of enhanced UV-B radiation on net photosynthesis, leaf area, number and size of nodules and nitrogenase activity in three tropical legumes under field conditions
Vigna radiata
Phaseolus mungo Pant U-30
Phaseolus mungo Mash 48
Control
Enhanced UV-B
Control
Enhanced UV-B
Control
Enhanced UV-B
19.41
13.5c
14.09
2.60c
11.85
9.22b
±0.12
+0.3
±0.9
+ 0.4
+ 0.35
+ 0.36
Leaf area (cm2 plant-1)
225.0 ±2.8
131.0a ± 1.7
210.7 ± 1.8
Nodule number plant-l
8.0 ± 1.0
3.0a ± 2.0
7.0 ± 2.0
Nodule diameter (mm)
6.2 ±6.0
4.0a ±0.3
8.0 ±0.3
Nitrogenase activity (~tmol C2H4 h-1 mg-l dry wt of nodule)
0.130
0.028~
0.243
117.7a ± 1.5 3.0a + 2.0 6.7a ±0.4 0.189~
1837.3 ±6.74 21.0 ± 4.0 7.3 ±0.9 0.174
1318.3c + 12.02 16.0Ms + 3.0 6.0Ns ±0.6 0.139c
± 0.0015
± 0.0007
± 0.0025
± 0.011
± 0.0045
± 0.0055
Net photosynthesis (larnol m-2 s-1)
Note: Values are means + SE. NS, not significant. aDifference between control and UV-B treated plants significant at p < 0.05. bDifference between control and UV-B treated plants significant at p < 0.01. CDifference between control and UV-B treated plants significant at p < 0.001.
UV-B radiation and N2-fixation
relations are more critical than photosynthetic rate for nitrogenase activity in the short-term (Vance & Heichel, 1991; Brown et al., 1995). The molecular basis for many of the observed changes, including suppression of nodulation and nitrogenase activity, following UV-B exposure remains unknown (CaldweU et al., 1995). Since the possibility of UV-B penetrating into the soil to affect nodulation and nodule activity directly is remote, the observed suppressive effect may result from altered assimilate transport which affects cortical cell turgor (Walsh, 1995) and changes in flavonoids which are implicated in rhizobial nod-gene induction (Hungria & Phillips, 1993). In conclusion, the enhanced UV-B radiation equivalent to 15% ozone depletion under field conditions affected not only net photosynthesis of the three legumes, it also depressed nitrogenase activity, particularly in V. radiata.
ACKNOWLEDGEMENTS Thanks are due to the Department of Science and the Technology, and the Council of Scientific & Industrial Research, New Delhi for funding support.
REFERENCES Brown, S. M., Oparka, K. J., Sprent, J. L. & Walsh, K. B. (1995). Symplastic transport in soybean root nodules. Soil Biol. Biochem., 27, 387-399. Caldwell, M. M., Robberecht, R. & Billings, W. D. (1980). A steep latitudinal gradient of solar ultraviolet-B radiation in the arctic-alpine life zone. Ecology, 61, 600~11. Caidwell, 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, 166-173. Finckh, M. R., Chavez, A., Dai, Q. & Teng, P. S. (1995). Effects of enhanced UV-B radiation on the growth of rice and its susceptibility to rice blast under glasshouse conditions. Agric. Ecosystems Environ, 52, 223-233. Green, A. E. S., Cross, K. R. & Smith, L. A. (1980). Improved analytical characterization of ultraviolet skylight. Photochem. Photobiol., 31, 59-65.
291
Hungria, M. & Phillips, D. A. (1993). Effects of a seed color mutation on rhizobial nod-Gene-Inducing flavonoids and nodulation in common bean. Molec. Plant-Microbe Interae., 6, 418-422. Iwanzik, W., Tevini, M., Dohnt, G., Voss, M., Weiss, W., Graber, P. & Renger, G. (1983). Action of the UV-B radiation on photosynthetic primary reactions in spinach chloroplasts. Physiol. Plant, 58, 401-407. Lingakumar, K. & Kulandaivelu, G. (1993). Changes induced by ultraviolet-B radiation in vegetative growth, foliar characteristics and photosynthetic activities in Vigna unuiculata. Aust. J. Plant Physiol., 20, 299-308. Madronich, S., Mckenzie, R. L., Caldwell, M. M. & Bjorn, L. O. (1995). Changes in ultraviolet radiation reaching the earth's surface. Ambio, 24, 143-152. Mahon, J. D. (1982). Field evaluation of growth and nitrogen fixation in peas selected for high and low photosynthetic CO2 exchange. Can. ,I. Plant Sci., 62, 5-17. Quaite, F. E., Sutherland, B. M. & Sutherland, J. C. (1992). Action spectrum for DNA damage in alfalfa lowers predicted impact of ozone depletion. Nature, 358, 576--578. Quispel, A. (1974). The Biology of Nitroen Fixation. North Holland Publishing Co., Amsterdam, Netherlands. Singh, A. (1994). Effects of enhanced UV-B radiation on higher plants. Trop. Ecol., 35, 164-189. Singh, A. (1995a). Effect of UV-B radiation on growth and yield of Glycine max and Phaseolus mungo. Ind. J. Exp. Biol., 33, 872-875. Singh, A. (1995b). Influence of enhanced UV-B radiation on tropical legumes. Trop. Ecol., 36, 249-252. Singh, A. (1996). Growth, physiological, and biochemical responses of three tropical legumes to enhanced UV-B radiation. Can. J. Bot., 74, 135-139. Snedecor, G. W. & Cochran, W. G. (1967). Statistical Method. The Iowa State University Press, Ames, Iowa, U.S.A. Sullivan, J. H. & Teramura, A. H. (1990). A field study of the interaction between UV-B radiation and drought on photosynthesis and growth in soybean. Plant Physiol, 92, 141-146. Stewart, W. D. P., Fitzgerald, G. P. & Burris, R. H. (1967). In situ studies on N2 fixation using the acytelene reduction technique. Proc. Natl. Acad. Sci. USA, 58, 2071-2078 Thorneley, R. N. F. (1992). New light on nitrogenase. Nature, 360, 532-533. Vance, C. P. & Heichel, G. H. (1991). Carbon in N2 fixation: Limitation or exquisite adaptation? Ann. Rev. Plant Physiol. Plant Mol. Biol., 42, 373-392. Vu, C. V., Allen, L. H., Jr & Garrard, L. A. (1982). Effects of UV-B radiation (280--320 nm) on photosynthetic constituents and processes in expanding leaves of soybean (Glycine max L. Mers.). Environ. Exp. Bot., 22, 465-473. Walsh K. B., (1995). Physiology of the legume nodule and its response to stress. Soil Biol. Biochem., 27, 637-655.