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Effects of atmospheric CO2 enrichment on root: Shoot ratios of carrot, radish, cotton and soybean
Agriculture, Ecosystems and Environment, 21 (1988) 293-299
293
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Effects of A t m o s p h e r i c CO2 E n r i c h m e n t on Root: Shoot Ratios of Carrot, Radish, Cotton and Soybean* S.B. IDSO 1, B.A. KIMBALL 1, and J.R. MAUNEY 2
U.S. Water Conservation Laboratory, 4331 E. Broadway, Phoenix, AZ 85040 (U.S.A.) 2Western Cotton Research Laboratory, 4335 E. Broadway, Phoenix, AZ 85040 (U.S.A.) (Accepted for publication 9 May 1988 )
ABSTRACT Idso, S.B., Kimball, B.A. and Mauney, J.R., 1988. Effects of atmospheric C02 enrichment on root:shoot ratios of carrot, radish, cotton and soybean. Agric. Ecosystems Environ., 21: 293299. Detailed analyses of root:shoot ratios, determined at weekly intervals during a succession of cropping cycles, show that the responses of root crops, such as radish and carrot, differ from those of cotton and soybean. Whereas the root:shoot ratios of the latter crops were not affected by atmospheric CO2 enrichment, increasing the C02 concentration of the air from 340 (ambient) to 640 #mol C02 mol-1 air significantly increased the proportions of assimulates allocated to the roots of radish and carrot. This effect increased the root: shoot ratios of both root crops by approximately 36% at all stages of plant growth, suggesting a response to atmospheric C02 enrichment that is independent of plant size and not caused by a progressive reduction in nitrogen availability.
INTRODUCTION
Most plants grow better and agricultural crops produce greater yields in atmospheres with larger than ambient concentrations of COe (Idso, 1982; Kimball, 1983a,b; Lemon, 1983; Strain and Cure, 1985; Enoch and Kimball, 1986a, b). In general, all plant organs benefit in some way from this aerial "fertilizer effect" of atmospheric CO2 enrichment (Acock and Allen, 1985; Acock and Pasternak, 1986). What is not well known, however, is whether some parts of plants benefit more than others. Several studies have suggested that plant roots respond more to atmospheric *Contribution from the Agricultural Research Service, U.S. Department of Agriculture. Supported in part by the U.S. Department of Energy, Carbon Dioxide Research Division, Office of Energy Research, under Interagency Agreement No. DE-AII01-81ER-6001.
0167-8809/88/$03.50
© 1988 Elsevier Science Publishers B.V.
294 CO2 enrichment than plant shoots. Specifically, root:shoot ratios have been found to increase with increasing atmospheric CO2 concentration in the following plants: barley (Ford and Thorne, 1967), maize (Ford and Thorne, 1967; Rogers et al., 1983), grape (Kriedemann et al., 1976), kale (Ford and Thorne, 1967), loblolly pine (Rogers et al., 1983b), radish (Knecht, 1975; Sionit et al., 1982), rice (Imai and Murata, 1976), soybean (Rogers et al., 1983b), sugar beet (Ford and Thorne, 1967; Sionit et al., 1982), sweetgum (Rogers et al., 1983b), sweet potato (Bhattacharya et al., 1985), tomato (Tognoni et al., 1967), wheat (Sionit et al., 1981 ) and two C4 weeds (Potvin and Strain, 1985 ). Others have suggested that plant shoots are the main beneficiaries of atmospheric CO2 enrichment: Sionit (1983) in a study of soybean, and Gifford (1977) in a study of wheat. A third group has found no effect of atmospheric C02 enrichment on root:shoot ratios in maize (Imai and Murata, 1976; Sionit et al., 1982; Goudriaan and de Ruiter, 1983), soybean (Sionit, 1983), tomato (Hurd, 1968), chrysanthemum (Hughes and Cockshull, 1971), the water shrub Leea brunonia Clark (Kriedeman et al., 1976), Douglas fir (Leadem, 1979) and ponderosa pine and blue spruce (Tinus, 1972). Some of these differences may be attributable to differences in nitrogen availability to the plants (Brouwer, 1966; Radin et al., 1978; Chapin, 1980). Others are probably nothing more than random scatter, particularly those related to plants which have been reported to respond with increases and decreases, such as soybeans and wheat. As Acock and Pasternak (1986) noted with respect to the many experiments with soybeans "the results vary from one harvest to another and from one experiment to another" (Rogers et al., 1980; Allen et al., 1982; Acock et al., 1982 ). In many instances, however, there is reason to believe that atmospheric CO2 enrichment exerts a consistent positive influence on root:shoot ratios.To date, this ratio has generally been increased in root crops by increasing atmospheric CO2 concentrations. As noted by Knecht (1975), when considering the source/sink aspects of photosynthesis and photosynthate storage, it is logical to presume "that increased photosynthesis owing to added CO2 will be reflected by a positive response at the 'sinks' of the plants." Similarly, Bhattacharya et al. (1985) have suggested that such sinks "may be limited by source capacity under present ambient CO2 levels." Hence, considering the greater root:shoot ratios generally exhibited by root crops, as opposed to those of plants whose fruits are produced above ground, it is only natural to expect that their root: shoot ratios will increase in atmospheres with increased C02 concentrations. MATERIALSAND METHODS In an attempt to investigate this hypothesis, experiments have been done during the last few years with 4 different crop plants, two root crops, carrot (Daucus carota L. var. sativus cv. 'Red Cored Chantenay') and radish (Ra-
295
phanus sativa L. cv. 'Cherry Belle') and two crops producing harvestable fruits, cotton (Gossypium hirsutum L. cv. 'Deltaspine-61' ) and soybean (Glycine max (L.) Merr. cv. 'Bragg'). These crops were grown at Phoenix, Arizona in identical ambient (340 mol CO2 mo1-1 air) and CO2-enriched (640/lmol CO2 tool -1 air) clear-plastic-wall open-top chambers (Rogers et al., 1983a,b; Kimball et al., 1983). Destructive harvests, to determine mean per-plant root and shoot dry weights, were taken weekly over a 3-year period during which 11 complete crops of carrots and 17 complete crops of radish were grown (Idso et al., 1987). In the case of cotton, mean per-plant root and shoot dry weights were determined at the conclusion of each growing season for each of the various treatments and replications studied by Kimball et al. (1983, 1984, 1985, 1986). In addition, weekly data were obtained for two complete growth cycles in ambient and CO2enriched greenhouses and for one complete growth cycle in the open-top cham0.20
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ROOT : SHOOT RATIO AT 3 4 0 /.zrnol CO2 per tool air
Fig. 1. Effects of increasing the atmospheric CO2 concentration from 340 to 640/~mol C02 tool-1 air on root:shoot ratios calculated from weekly destructive samples of successive crops of cotton and soybean, including some end-of-season data for cotton. The solid diagonal line is the one-toone correspondence line.
296
bers. Weekly observations of soybean were made in similar circumstances to those for cotton. RESULTS AND DISCUSSION
Fig. i presents the results of the cotton and soybean studies. In the case of these two crops, root: shoot ratios are small and generally vary by no more than ___0.06 about a mean of 0.11, independent of plant age, as may be inferred from the distribution of the weekly and end-of-season cotton data. In addition, the root:shoot ratios of both crops appear to be totally unaffected by C02 enrichment. Fig. 2 depicts the results of the carrot and radish experiments. In the case of these crops, root: shoot ratios do increase with plant age. When plants of both species are very young, their root: shoot ratios are very small; and the root: shoot ratios of the two treatments are sometimes identical. As the ambient ratio rises above the range characteristic of cotton and soybeans, however, the C02-en3.6
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Fig. 2. Effects of increasing atmospheric C02 concentration from 340 to 640 ]~mol C02 mo1-1 air on root: shoot ratios calculated from weekly destructive samples of successive crops of carrots and radishes grown over a period of 3 years. The lower diagonal line is the one-to-one correspondence line.
297
riched ratio is nearly always greater. And over the entire ambient ratio range, the linear regression analysis suggests that a + 300 Hmol CO2 mol-1 air increase in the CO2 content of the atmosphere produces an approximate 36% increase in the root:shoot ratio of both carrot and radish. Furthermore, the constancy of this enhancement with plant age and size is evidence for an adequate supply of nitrogen throughout the whole series of experiments; for if nitrogen had been limiting, as plant growth progressed and nitrogen stress increased, there would have been an ever-accelerating enhancement of the CO2enhanced root:shoot ratio relative to the ambient root:shoot ratio, i.e. there would have been a concave upwar d relationship in place of the linear relationship of Fig. 2. In the light of the consistent experimental findings for cotton and soybean, which give no indication of any CO2 effect on root:shoot ratio in those two species, and in light of the sometimes negative, sometimes positive, and sometimes inconclusive literature reports of CO2 effects on root:shoot ratios in other plant species whose primary yield component is produced above-ground, it appears unlikely that atmospheric CO2 enrichment has any significant effect on root: shoot ratios in such plants. For root crops, however, the literature is fairly consistent in suggesting a positive effect of atmospheric CO2-enrichment on this ratio; and the results for carrot and radish suggest that a + 300 Hmol CO2 mo1-1 air increase in the CO2 content of the atmosphere will increase the root: shoot ratio in these latter plants by about a third.
REFERENCES Acock, B. and Allen, L.H., Jr., 1985. Crop responses to elevated carbon dioxide concentrations. In: B.R. Strain and J.D. Cure (Editors), Direct Effects of Increasing Carbon Dioxide on Vegetation, DOE/ER-0238, U.S. Dept. Energy, Washington, DC, pp. 53-97. Acock, B. and Pasternak, D., 1986. Effects of C02 concentrations on composition, anatomy, and morphology of plants. In: H.Z. Enoch and B.A. Kimball (Editors), Carbon Dioxide Enrichment of Greenhouse Crops. Vol. II. Physiology, Yield, and Economics, CRC Press, Inc., Boca Raton, FL, pp. 41-52. Acock, B., Baker, D.N., Reddy, V.R., McKinion, J.M., Whisler, F.D., Del CastiUo, D. and Hodges, H.F., 1982. Soybean responses to carbon dioxide: measurement and simulation 1981. In: Response of Vegetation to Carbon Dioxide, U.S. Department of Energy Greenbook Series, 004, Washington, DC, 95 pp. Allen, L.H., Jr., Boote, K.J., Jones, J.W., Mishoe, J.W., Jones, P.H., Vu, C.V., Valle, R. and Campbell, W.J., 1982. Effects of increased carbon dioxide on photosynthesis and agricultural productivity of soybeans. In: Response of Vegetation to Carbon Dioxide. U.S. Dept. Energy Greenbook Series, 003, Washington, DC, 131 pp. Bhattacharya, N.C., Biswas, P.K., Bhattacharya, S., Sionit, N. and Strain, B.R., 1985. Growth and yield response of sweet potato (Ipomoea batatas) to atmospheric CO2 enrichment. Crop Sci., 25: 975-981. Brouwer, R., 1966. Root growth of grasses and cereals. In: F.L. Milthorpe and D.J. Ivins (Editors), The Growth of Cereals and Grasses. Butterworths, London, pp. 153-166.
298 Chapin, F.S., 1980. The mineral nutrition of wild plants. Ann. Rev. Ecol. Syst., 11: 233-260. Enoch, H.Z. and Kimball, B.A. (Editors), 1986a. Carbon Dioxide Enrichment of Greenhouse Crops. Vol. I. Status and C02 Sources. CRC Press, Inc., Boca Raton, FL, 181 p. Enoch, H.Z. and Kimball, B.A. (Editors), 1986b. Carbon Dioxide Enrichment of Greenhouse Crops, Vol. II. Physiology, Yield, and Economics. CRC Press, Inc., Boca Raton, FL, 230 pp. Ford, M.A. and Thorne, G.N., 1967. Effect of C02 concentration on growth of sugar-beet, barley, kale and maize. Ann. Bot., 31: 629-644. Gifford, R.M., 1977. Growth pattern, carbon dioxide exchange and dry weight distribution in wheat growing under differing photosynthetic environments. Aust. J. Plant Physiol., 4: 99110. Gifford, R.M. 1979. Growth and yield of C02-enriched wheat under water-limited conditions. Aust. J. Plant. Physiol., 6: 367-378. Goudriaan, J. and de Ruiter, H.E., 1983. Plant growth in response to C02 enrichment, at two levels of nitrogen and phosphorus supply. 1. Dry matter, leaf area and development. Neth. J. Agric. Sci., 31: 157-169. Hughes, A.P. and Cockshull, K.E., 1971. The effects of light intensity and carbon dioxide concentration on the growth of Chrysanthemum morifolium cv. Bright Golden Anne. Ann. Bot., 35: 899-914. Hurd, R.G,, 1968. Effects of C02 enrichment on the growth of young tomato plants in low light. Ann. Bot., 32: 531-542. Idso, S.B., 1982. Carbon Dioxide: Friend or Foe? IBR Press, Tempe, AZ, 92 p. Idso, S.B., Kimball, B.A., Anderson, M.G. and Mauney, J.R., 1987. Effects of atmospheric C02 enrichment on plant growth: The interactive role of air temperature. Agric. Ecosys. Environ., 20: 1-10. Imai, K. and Murata, Y., 1976. Effect of carbon dioxide concentration on growth and dry matter production of crop plants. Proc. Crop Sci. Soc. Jpn., 45: 598-606. Kimball, B.A., 1983a. Carbon dioxide and agricultural yield: An assemblage and analysis of 430 prior observations. Agron. J., 75: 779-788. Kimball, B.A., 1983b. Carbon Dioxide and Agricultural Yield: An Assemblage and Analysis of 770 Prior Observations. WCL Report No. 114, U.S. Water Conservation Laboratory, Phoenix, AZ, 71 p. Kimball, B.A., Mauney, J.R., Guinn, G., Nakayama, F.S., Pinter, P.J., Jr., Clawson, K.L., Reginato, R.J. and Idso, S.B., 1983. Effects of increasing atmospheric C02 on the yield and water use of crops. In: Response of Vegetation to Carbon Dioxide. U.S. Dept. Energy Greenbook Series, 021, Washington, DC, 37 pp. Kimball, B.A., Mauney, J.R., Guinn, G., Nakayama, F.S., Pinter, P.J., Jr., Clawson, K.L., Idso, S.B., Butler, G.D. and Radin, J.R., 1984. Effects of increasing atmospheric C02 on the yield and water use of crops. In: Response of Vegetation to Carbon Dioxide. U.S. Dept. Energy Greenbook Series, 023, Washington, DC, 60 pp. Kimball, B.A., Mauney, J.R., Guinn, G., Nakayama, F.S., Idso, S.B., Radin, J.W., Hendrix, D.L., Butler, G.D., Zarembinski, T.I. and Nixon, P.E., II, 1985. Effects of increasing atmospheric C02 on the yield and water use of crops. In: Response of Vegetation to Carbon Dioxide. U.S. Dept. Energy Greenbook Series, 027, Washington, DC, 75 pp. Kimball, B.A., Mauney, J.R., Radin, J.W., Nakayama, F.S., Idso, S.B., Hendrix, D.L., Akey, D.H., Hartung, W., Allen, S.G. and Anderson, M.G., 1986. Effects of increasing atmospheric C02 on the yield and water use of crops. In: Response of Vegetation to Carbon Dioxide, U.S. Dept. Energy Greenbook Series, 039, Washington, DC, 125 pp. Knecht, G.S., 1975. Response of radish to high CO2. HortScience, 10: 274-275. Kriedemann, R.E., Sward, R.J. and Downton, W.J.S., 1976. Vine response to carbon dioxide enrichment during heat therapy. Aust. J. Plant Physiol., 3: 605-618.
299 Leadem, C.L., 1979. Effects of carbon dioxide and day length on growth, development and hardiness of Douglas fir. Ph.D. Dissertation, Univ. Br. Columbia, Vancouver. Lemon, E.R. (Editor), 1983. CO2 and Plants. A.A.A.S. Selected Symp. 84, Westview Press, Inc., Boulder, CO, 280 pp. Potvin, C. and Strain, B.R., 1985. Effects of C02 enrichment and temperature on growth in two C4 weeds, Echinochloa crus-galli and Elevsine indica. Can. J. Bot., 63: 1495-1499. Radin, J.W., Parker, L.L. and Sell, C.R., 1978. Partitioning of sugar between growth and nitrate reduction in cotton roots. Plant Physiol., 62: 550-553. Rogers, H.H., Bingham, G.E., Cure, J.D., Heck, W.W., Heagle, A.S., Israel, D.W., Smith, J.M., Surano, K.A. and Thomas, J.F., 1980. Field studies of plant responses to elevated carbon dioxide levels. In: Response of Vegetation to Carbon Dioxide,. U.S. Dept. Energy Greenbook Series, 001, Washington, DC, 113 pp. Rogers. H.H., Heck, W.W. and Heagle, A.S., 1983a. A field technique for the study of plant responses to elevated carbon dioxide concentrations. Am. Poll. Control. Assoc. J., 33: 42-44. Rogers, H.H., Bingham, G.E., Cure, J.D., Smith, J.M. and Surano, K.A., 1983b. Responses of selected plant species to elevated carbon dioxide in the field. J. Environ. Qual., 12: 569-574. Sionit, N., 1983. Response of soybean to two levels of mineral nutrition in C02-enriched atmosphere. Crop Sci., 23: 329-333. Sionit, N., Strain, B.R. and Hellmers, H., 1981. Effects of different concentrations of atmospheric C02 on growth and yield components of wheat. J. Agric. Sci. Camb., 79: 335-339. Sionit, N., Hellmers, H. and Strain, B.R., 1982. Interaction of atmospheric CO2 enrichment and irradiance on plant growth. Agron. J., 74: 721-725. Strain, B.R. and Cure, J.D. (Editors), 1985. Direct Effects of Increasing Carbon Dioxide on Vegetation. U.S. Dept. Energy, Washington, DC, 286 p. Tinus, R.W., 1972. COs-enriched atmosphere speeds growth of ponderosa pine and blue spruce seedlings. Tree Plant. Notes, 23: 12-15. Tognoni, F., Havely, A.H. and Wittwer, S.H., 1967. Growth of bean and tomato plants as affected by root absorbed growth substances and atmospheric carbon dioxide. Planta, 72: 43-52.
293
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Effects of A t m o s p h e r i c CO2 E n r i c h m e n t on Root: Shoot Ratios of Carrot, Radish, Cotton and Soybean* S.B. IDSO 1, B.A. KIMBALL 1, and J.R. MAUNEY 2
U.S. Water Conservation Laboratory, 4331 E. Broadway, Phoenix, AZ 85040 (U.S.A.) 2Western Cotton Research Laboratory, 4335 E. Broadway, Phoenix, AZ 85040 (U.S.A.) (Accepted for publication 9 May 1988 )
ABSTRACT Idso, S.B., Kimball, B.A. and Mauney, J.R., 1988. Effects of atmospheric C02 enrichment on root:shoot ratios of carrot, radish, cotton and soybean. Agric. Ecosystems Environ., 21: 293299. Detailed analyses of root:shoot ratios, determined at weekly intervals during a succession of cropping cycles, show that the responses of root crops, such as radish and carrot, differ from those of cotton and soybean. Whereas the root:shoot ratios of the latter crops were not affected by atmospheric CO2 enrichment, increasing the C02 concentration of the air from 340 (ambient) to 640 #mol C02 mol-1 air significantly increased the proportions of assimulates allocated to the roots of radish and carrot. This effect increased the root: shoot ratios of both root crops by approximately 36% at all stages of plant growth, suggesting a response to atmospheric C02 enrichment that is independent of plant size and not caused by a progressive reduction in nitrogen availability.
INTRODUCTION
Most plants grow better and agricultural crops produce greater yields in atmospheres with larger than ambient concentrations of COe (Idso, 1982; Kimball, 1983a,b; Lemon, 1983; Strain and Cure, 1985; Enoch and Kimball, 1986a, b). In general, all plant organs benefit in some way from this aerial "fertilizer effect" of atmospheric CO2 enrichment (Acock and Allen, 1985; Acock and Pasternak, 1986). What is not well known, however, is whether some parts of plants benefit more than others. Several studies have suggested that plant roots respond more to atmospheric *Contribution from the Agricultural Research Service, U.S. Department of Agriculture. Supported in part by the U.S. Department of Energy, Carbon Dioxide Research Division, Office of Energy Research, under Interagency Agreement No. DE-AII01-81ER-6001.
0167-8809/88/$03.50
© 1988 Elsevier Science Publishers B.V.
294 CO2 enrichment than plant shoots. Specifically, root:shoot ratios have been found to increase with increasing atmospheric CO2 concentration in the following plants: barley (Ford and Thorne, 1967), maize (Ford and Thorne, 1967; Rogers et al., 1983), grape (Kriedemann et al., 1976), kale (Ford and Thorne, 1967), loblolly pine (Rogers et al., 1983b), radish (Knecht, 1975; Sionit et al., 1982), rice (Imai and Murata, 1976), soybean (Rogers et al., 1983b), sugar beet (Ford and Thorne, 1967; Sionit et al., 1982), sweetgum (Rogers et al., 1983b), sweet potato (Bhattacharya et al., 1985), tomato (Tognoni et al., 1967), wheat (Sionit et al., 1981 ) and two C4 weeds (Potvin and Strain, 1985 ). Others have suggested that plant shoots are the main beneficiaries of atmospheric CO2 enrichment: Sionit (1983) in a study of soybean, and Gifford (1977) in a study of wheat. A third group has found no effect of atmospheric C02 enrichment on root:shoot ratios in maize (Imai and Murata, 1976; Sionit et al., 1982; Goudriaan and de Ruiter, 1983), soybean (Sionit, 1983), tomato (Hurd, 1968), chrysanthemum (Hughes and Cockshull, 1971), the water shrub Leea brunonia Clark (Kriedeman et al., 1976), Douglas fir (Leadem, 1979) and ponderosa pine and blue spruce (Tinus, 1972). Some of these differences may be attributable to differences in nitrogen availability to the plants (Brouwer, 1966; Radin et al., 1978; Chapin, 1980). Others are probably nothing more than random scatter, particularly those related to plants which have been reported to respond with increases and decreases, such as soybeans and wheat. As Acock and Pasternak (1986) noted with respect to the many experiments with soybeans "the results vary from one harvest to another and from one experiment to another" (Rogers et al., 1980; Allen et al., 1982; Acock et al., 1982 ). In many instances, however, there is reason to believe that atmospheric CO2 enrichment exerts a consistent positive influence on root:shoot ratios.To date, this ratio has generally been increased in root crops by increasing atmospheric CO2 concentrations. As noted by Knecht (1975), when considering the source/sink aspects of photosynthesis and photosynthate storage, it is logical to presume "that increased photosynthesis owing to added CO2 will be reflected by a positive response at the 'sinks' of the plants." Similarly, Bhattacharya et al. (1985) have suggested that such sinks "may be limited by source capacity under present ambient CO2 levels." Hence, considering the greater root:shoot ratios generally exhibited by root crops, as opposed to those of plants whose fruits are produced above ground, it is only natural to expect that their root: shoot ratios will increase in atmospheres with increased C02 concentrations. MATERIALSAND METHODS In an attempt to investigate this hypothesis, experiments have been done during the last few years with 4 different crop plants, two root crops, carrot (Daucus carota L. var. sativus cv. 'Red Cored Chantenay') and radish (Ra-
295
phanus sativa L. cv. 'Cherry Belle') and two crops producing harvestable fruits, cotton (Gossypium hirsutum L. cv. 'Deltaspine-61' ) and soybean (Glycine max (L.) Merr. cv. 'Bragg'). These crops were grown at Phoenix, Arizona in identical ambient (340 mol CO2 mo1-1 air) and CO2-enriched (640/lmol CO2 tool -1 air) clear-plastic-wall open-top chambers (Rogers et al., 1983a,b; Kimball et al., 1983). Destructive harvests, to determine mean per-plant root and shoot dry weights, were taken weekly over a 3-year period during which 11 complete crops of carrots and 17 complete crops of radish were grown (Idso et al., 1987). In the case of cotton, mean per-plant root and shoot dry weights were determined at the conclusion of each growing season for each of the various treatments and replications studied by Kimball et al. (1983, 1984, 1985, 1986). In addition, weekly data were obtained for two complete growth cycles in ambient and CO2enriched greenhouses and for one complete growth cycle in the open-top cham0.20
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ROOT : SHOOT RATIO AT 3 4 0 /.zrnol CO2 per tool air
Fig. 1. Effects of increasing the atmospheric CO2 concentration from 340 to 640/~mol C02 tool-1 air on root:shoot ratios calculated from weekly destructive samples of successive crops of cotton and soybean, including some end-of-season data for cotton. The solid diagonal line is the one-toone correspondence line.
296
bers. Weekly observations of soybean were made in similar circumstances to those for cotton. RESULTS AND DISCUSSION
Fig. i presents the results of the cotton and soybean studies. In the case of these two crops, root: shoot ratios are small and generally vary by no more than ___0.06 about a mean of 0.11, independent of plant age, as may be inferred from the distribution of the weekly and end-of-season cotton data. In addition, the root:shoot ratios of both crops appear to be totally unaffected by C02 enrichment. Fig. 2 depicts the results of the carrot and radish experiments. In the case of these crops, root: shoot ratios do increase with plant age. When plants of both species are very young, their root: shoot ratios are very small; and the root: shoot ratios of the two treatments are sometimes identical. As the ambient ratio rises above the range characteristic of cotton and soybeans, however, the C02-en3.6
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Fig. 2. Effects of increasing atmospheric C02 concentration from 340 to 640 ]~mol C02 mo1-1 air on root: shoot ratios calculated from weekly destructive samples of successive crops of carrots and radishes grown over a period of 3 years. The lower diagonal line is the one-to-one correspondence line.
297
riched ratio is nearly always greater. And over the entire ambient ratio range, the linear regression analysis suggests that a + 300 Hmol CO2 mol-1 air increase in the CO2 content of the atmosphere produces an approximate 36% increase in the root:shoot ratio of both carrot and radish. Furthermore, the constancy of this enhancement with plant age and size is evidence for an adequate supply of nitrogen throughout the whole series of experiments; for if nitrogen had been limiting, as plant growth progressed and nitrogen stress increased, there would have been an ever-accelerating enhancement of the CO2enhanced root:shoot ratio relative to the ambient root:shoot ratio, i.e. there would have been a concave upwar d relationship in place of the linear relationship of Fig. 2. In the light of the consistent experimental findings for cotton and soybean, which give no indication of any CO2 effect on root:shoot ratio in those two species, and in light of the sometimes negative, sometimes positive, and sometimes inconclusive literature reports of CO2 effects on root:shoot ratios in other plant species whose primary yield component is produced above-ground, it appears unlikely that atmospheric CO2 enrichment has any significant effect on root: shoot ratios in such plants. For root crops, however, the literature is fairly consistent in suggesting a positive effect of atmospheric CO2-enrichment on this ratio; and the results for carrot and radish suggest that a + 300 Hmol CO2 mo1-1 air increase in the CO2 content of the atmosphere will increase the root: shoot ratio in these latter plants by about a third.
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