In contrast, plants with C4 pho- tosynthesis will respond little to rising atmospheric CO2 because a mechanism to increase the concentration of CO2 in leaves ...
Crop Responses to Elevated Carbon Dioxide Leon Hartwell Allen, Jr. USDA, Gainesville, Florida, U.S.A.
P. V. Vara Prasad University of Florida, Gainesville, Florida, U.S.A.
INTRODUCTION Atmospheric carbon dioxide (CO2) concentration has increased from 280 ppm (parts per million, mole fraction basis) in preindustrial times to 370 ppm today. As concentrations of CO2 and other greenhouse gases rise, global temperature is anticipated to increase.[1] Elevated CO2 will improve crop yields due to increased photosynthesis. However, at above-optimum temperatures for reproductive growth processes, the benefits of elevated CO2 could be overwhelmed by negative effects of high temperature, leading to lower seed yield. The extent of growth and yield responses of plants to elevated CO2 depends on the photosynthetic pathway. Crops with C3 photosynthesis will respond markedly to increasing CO2 concentrations. Common C3 crops are small grain cereals (wheat, rice, barley, oat, and rye); grain legumes or pulses (soybean, peanut, various beans and peas); root and tuber crops (potato, cassava, sweet potato, sugar beet, yams); most oil, fruit, nut, vegetable, and fiber crops; and temperate-zone (cool-climate) forage and grassland species. In contrast, plants with C4 photosynthesis will respond little to rising atmospheric CO2 because a mechanism to increase the concentration of CO2 in leaves causes CO2 saturation of photosynthesis at current ambient concentrations. Common C4 crops are maize (corn), sugarcane, sorghum, millet, and many tropical and subtropical zone (warm-climate) grass species. This article focuses on responses to elevated CO2 and increased temperature of C3 crops. Response patterns are similar, but not the same, across a broad range of species and conditions.[2]
EFFECTS OF CO2 AND TEMPERATURE Photosynthesis and Respiration Doubling of CO2 concentration will increase photosynthesis of C3 crop species by 30–50%.[2–4] The primary enzyme in leaf photosynthesis of C3 plants, ribulose 1,5bisphosphate carboxylase/oxygenase (Rubisco), can bind 346
to either CO2 or O2. An increase in the concentration of CO2 enables this molecule to better compete with dissolved O2 for binding sites on the Rubisco protein, thus leading to an increase of photosynthesis of C3 species. The CO2 concentrating mechanism of C4 plants is mediated by the enzyme phosphoenolpyruvate carboxylase (PEPcase). The contrasting effect of CO2 on photosynthesis of C3 and C4 plants is illustrated in Fig. 1. Response curves of photosynthesis versus CO2 are nonlinear, and little benefit will accrue above 700 ppm. The hypothesis that elevated CO2 has a direct, immediate effect in decreasing the respiration rate of plants seems to have little basis. However, the indirect, long-term effect of elevated CO2 can cause an increase in respiration via an increase in the amount of living biomass. Rice plants grown in CO2 ranging from 160 to 900 ppm had respiration rates directly proportional to the total nitrogen content (protein content) of the plant.[5] However, elevated temperatures can increase plant dark respiration rates regardless of CO2 concentration. Furthermore, elevated temperature decreases solubility of CO2 relative to O2 in the cytosol, thereby decreasing photosynthesis, but this solubility effect on photosynthesis is usually offset more in high CO2 than in ambient CO2. Stomatal Conductance, Transpiration, and Water Use Increasing CO2 causes partial closure of stomata, the small pores (formed by slits between two flexible guard cells) on leaves that govern photosynthetic CO2 uptake and transpiration (water vapor loss). Stomatal conductance for water vapor decreases about 40% for a doubling of CO2. Decreased stomatal conductance decreases transpiration of leaves, but not in direct proportion to the decrease of stomatal conductance because leaf temperature increases by 1–2°C in doubled CO2 due to decreased evaporational cooling. In turn, vapor pressure of water inside leaves increases and causes a greater leaf-toair vapor pressure difference, which is the driving force for transpiration. This effect partially offsets decreased stomatal conductance, and thus whole-crop transpiration Encyclopedia of Plant and Crop Science DOI: 10.1081/E-EPCS 120005566 Copyright D 2004 by Marcel Dekker, Inc. All rights reserved.
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CO2 than seed or forage crops. Increased photosynthesis also favors symbiotic nitrogen fixation in legumes. Since legumes can supply nitrogen via symbiotic nitrogen fixation, crop legumes (both seed and forage crops) might respond relatively more to a rise in CO2 concentration than non-legumes. Seed Yield and Quality
Fig. 1 Typical leaf photosynthetic rate responses of C3 and C4 plants to CO2 concentration when measured in non-limiting (high light) conditions.
is maintained only slightly lower (10%) than would exist at ambient CO2.[6] Although crop transpiration might decrease slightly in elevated CO2, water use will increase if temperatures rise. Fig. 2 shows the increase of average daily transpiration of a soybean crop with increasing temperature at two levels of CO2. The reduction in water use by doubled CO2 was about 9% at the mean temperature of 23°C. Crop water use might increase about four-fold over the average daily temperature range of 20–40°C. Therefore, small increases in temperatures would more than offset the water-saving effect of CO2 via reduced stomatal conductance.[6] Shoot and Root Growth Crops exposed to elevated CO2 generally grow larger.[2] Plants such as soybean have a higher percentage of total biomass in stems to support leaves and seed pods. Crops such as rice and wheat produce a larger number of tillers, which leads to greater yield because of the greater number of seed heads per plant. Leaves may be larger or thicker and accumulate more starch, especially for plants like soybean. Elevated temperatures may either increase or decrease the vegetative biomass production of crops. Vegetative biomass of warm-climate species or cultivars of forages, sugarcane, soybean, and peanut may increase slightly with temperature increases, whereas vegetative biomass of cool-climate cultivars tends to decrease with increasing temperature. Elevated CO2 generally increases biomass, volume, and length of roots, as well as increasing biomass allocation to roots (increased root-shoot ratio). Root and tuber crops tend to have a greater yield response to elevated
Seed yields generally increase nonlinearly in response to increasing CO2, but this increase is not quite as much as the increase in photosynthesis.[2] Part of the additional carbon fixed goes into producing more plant vegetative biomass. Increases in seed yields of many C3 crops range between 20% and 35%,[3] whereas increases for C4 crops are only about 10% to 15%. Elevated CO2 may cause higher carbohydrate and lower nitrogen content of small cereal grains, but no changes tend to occur in grain legumes.[7] Although wheat and barley showed increases in seed numbers (about + 15%) in elevated CO2, seed N concentration was even more strongly reduced (about 20%). Under limiting water or nutrient conditions, relative yield responses to elevated CO2 may increase, although absolute yields will decrease. Increasing temperature is detrimental to seed production, as illustrated for tropical lowland rice and kidney bean in Fig. 3.[4,8] The quantitative responses of seed yield reduction to increasing temperature vary among species and crop cultivars, but the pattern is the same. Each crop has an optimum temperature for reproductive growth processes. Seed yields decline about 10% per °C to zero at about 10°C above the optimum temperature. Seed yields decline to zero at about 32°C for a cool-climate cultivar of kidney bean, 36°C for tropical lowland rice, and 40°C for warm-climate cultivars of peanut and soybean. Fig. 3
Fig. 2 Typical average daily whole-crop transpiration of C3 plants when grown at two levels of CO2 and across a mean daily temperature range of 20–40°C. (Adapted from Ref. 6.)
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ARTICLES OF FURTHER INTEREST
Fig. 3 Typical seed yields of rice and kidney bean at two CO2 concentrations (ambient and double ambient) and across a range of temperatures. (Adapted from Refs. 4 and 8.)
shows that elevated CO2 does not offset the decline of seed yield with increasing temperature. Crops are especially sensitive to elevated temperature from a few days before pollen maturation through fertilization of the ovule.[9] Important processes during this period are viable pollen production, pollen shedding, pollen tube growth, and fertilization. Crops may also be sensitive to temperature during seed-filling processes, the time when the seeds load up with proteins, carbohydrates, oils, and other nutrients.[7] Soybean and kidney bean seeds increasingly fail to fill properly as temperatures increase and form smaller, shriveled seeds with reduced seed germination capability and nutritional quality than under optimum temperatures.
CONCLUSION Photosynthesis and growth of C3 crops are increased when grown at high CO2; however, the extent of stimulation varies with temperature among species and cultivars. The potential decrease in transpiration caused by partial closure of stomata in elevated CO2 is largely negated by the energy balance between the crop and environment, which results in similar total water use in similar climatic conditions. Seed yields are increased by elevated CO2 under optimal temperature. However, at supra-optimal temperature, seed yields are decreased under both ambient and elevated CO2.[2,5,8] If increases in temperature accompany increases in CO2 concentration, seed yields will decrease in regions where temperatures are at or above optimum. Future research should be directed toward identifying high-temperature tolerant cultivars that can produce more seeds under harsh climatic conditions.
Air Pollutants: Interactions with Elevated Carbon Dioxide, p. 17 Crops and Environmental Change, p. 370 Drought and Drought Resistance, p. 386 Ecophysiology, p. 410 Leaves and Canopies: Physical Environment, p. 646 Leaves and the Effects of Elevated Carbon Dioxide Levels, p. 648 Osmotic Adjustment and Osmoregulation, p. 850 Photosynthate Partitioning and Transport, p. 897 Photosynthesis and Stress, p. 901 Seed Vigor, p. 1139 Water Use Efficiency Including Carbon Isotope Discrimination, p. 1288
REFERENCES 1.
Rosenzweig, C.; Hillel, D. Climate Change and the Global Harvest; Oxford University Press: New York, 1998. 2. Climate Change and Global Crop Productivity; Reddy, K.R., Hodges, H.F., Eds.; CABI Publishing: Oxon, UK, 2000. 3. Ainsworth, E.A.; Davey, P.A.; Bernacchi, C.J.; Dermody, O.C.; Heaton, E.A.; Moore, D.J.; Morgan, P.B.; Naidu, S.L.; Ra, H.S.Y.; Zhu, X.G.; Curtis, P.S.; Long, S.P. A metaanalysis of elevated CO2 effects on soybean (Glycine max) physiology, growth and yield. Glob. Chang. Biol. 2002, 8 (8), 695 – 709. 4. Baker, J.T.; Allen, L.H. Contrasting Crop Species Responses to CO2 and Temperature: Rice, Soybean and Citrus. In CO2 and Biosphere; Rozema, J., Lambers, H., van de Geijn, S.C., Cambridge, M.L., Eds.; Kluwer Academic Publishers: Dordrecht, 1993; 239 – 260. 5. Baker, J.T.; Laugel, F.; Boote, K.J.; Allen, L.H. Effects of daytime carbon dioxide concentration on dark respiration of rice. Plant Cell Environ. 1992, 15 (2), 231 – 239. 6. Allen, L.H.; Pan, D.; Boote, K.J.; Pickering, N.B.; Jones, J.W. Carbon dioxide and temperature effects on evapotranspiration and water-use efficiency of soybean. Agron. J. 2003, 95 (4), 1071 – 1081. 7. Jablonski, L.M.; Wang, X.Z.; Curtis, P.S. Plant reproduction under elevated CO2 conditions: A meta-analysis of reports on 79 crop and wild species. New Phytol. 2002, 156 (1), 9 – 26. 8. Prasad, P.V.V.; Boote, K.J.; Allen, L.H.; Thomas, J.M.G. Effects of elevated temperature and carbon dioxide concentration on seed-set and yield of kidney bean (Phaseolus vulgaris L.). Glob. Chang. Biol. 2002, 8 (8), 710 – 721. 9. Prasad, P.V.V.; Craufurd, P.Q.; Kakani, V.G.; Wheeler, T.R.; Boote, K.J. Influence of high temperature during preand post-anthesis stages of floral development on fruit-set and pollen germination in peanut. Aust. J. Plant Physiol. 2001, 28 (3), 233 – 240.
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