CLIMATE CHANGE, NITROGEN AND YIELD RESPONSE OF WHEAT CROPS

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1 CLIMATE CHANGE, NITROGEN AND YIELD RESPONSE OF WHEAT CROPS J.H.J. Spiertz & A.H.C.M. Schapendonk WAGENINGEN UNIVERSITY AND RESEARCH CENTER, THE NETHERLANDS Abstract The response of crop growth and yield to enhanced CO 2 in the biosphere is known to depend on climatic conditions and is difficult to quantify due to complexity of the physiological and environmental processes involved. In multi-site and years wheat experiments average measured yield increase due to CO 2 -doubling was approx. 30 % (SD 22%). Average yield increases by simulation models amounted to approx. 25 and 40 %, respectively. The models predicted lower variation among sites and years than was measured. A considerable part of the variation in yield was not related to direct effects of light intensity, temperature, and CO 2 ; it is supposed that specific interactions cause this residual variation. Higher temperatures during the vegetative phase enhance the effect of elevated CO 2 levels; however, the duration of ear development is shortened and the number of grains reduced. Temperature stress during anthesis turned out to be an another factor influencing the number of grains per ear, and therefore the sink capacity. It is hypothesized, that the additive effect of elevated CO 2 concentrations and predicted temperature change will result in a 25 to 30 % increase of the current potential yield level, if new genotypes of wheat become available that are better adapted to the warmer conditions; especially, higher temperatures should not shorten the duration of ear development and the duration of post-anthesis crop growth. Furthermore, the availability and utilization of nitrogen and other nutrients have to be improved to meet the demands of a high-yielding crop and quality standards. Key words: CO 2 enrichment, climate change, wheat, crop growth and yield, nitrogen economy, grain quality Introduction The trend in wheat yields from 1900 onwards in the Netherlands and the United Kingdom show three phases (Spiertz et al., 1992): - from 1900 to 1950: an annual increase of approx. 1 %, which reflects the low input of fertilizers and the rather slow progress in breeding more productive cultivars. - from 1950 to 1970: an annual increase of approx. 2 % due to a higher input of nitrogen fertilizer and the introduction of cultivars with a higher harvest-index. - from 1970 to 1990: the annual increase in actual yields accelerated to % by utilizing the full genetic potential of high-yielding cultivars through an improved technology for growing wheat crops disease- and weed- free and with ample nitrogen supply. During the last decade yield improvements are leveling off due to lack of progress in yield improvement by breeding and environmental restrictions on the use of pesticides and of fertilizer nitrogen. A boost in genetic improvement of new wheat cultivars will be needed to match sustainable grain production with future demands for food and feed. Over the last century industrialization and urbanization has contributed to a major improvement in the living standards of people in well-endowed regions, it has also brought with it serious problems. Industrial and agricultural emissions of green house gases - carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide and others are resulting in an increase of these gases in the atmosphere; CO 2 concentration is currently increasing at the rate of about 1.5 PPM. per year. This increase in the socalled greenhouse gases is contributing to a gradual warming of the planet by retaining more heat within the earth s atmosphere, resulting in climate change. There exists still a large amount of uncertainty in what way the regional weather pattern will be influenced; likely effects are a higher air and soil temperature, more erratic rainfall distribution and a higher frequency of extreme weather IWC-paper04 1

2 conditions. According to the current predictions based on General Circulation Models (GCMs), a doubling of the current CO 2 level will bring about an increase in average global surface air temperatures of between 1.5 and 4 degree centigrade, with accompanying changes in precipitation patterns. Scheme of major global trends. a. Societal change - a fast growing population and simultaneously, a shift from rural to urban households. - industrialization and accelerated globalization leads to an increased use of fossil energy. b. Emissions and carbon balance - an increase in the use of fossil energy and intensification of agriculture and transportation leads to a rise in the emission of green-house gases; especially: carbon dioxide and NOx. - change in land use, e.g. deforestation, does have a negative effect on the global carbon cycle. c. Atmospheric changes - a rise of the carbon dioxide concentration in the atmosphere. - more UV due to depletion of ozone layers. d. Climate change - an increase of air temperature. - change in cloudiness and rainfall pattern. e. Agriculture and food production - a net reduction of current yield levels in cereals. - on the long term a rise in the potential yield level of wheat in temperate regions. Agricultural productivity is of particular importance, as the world s food production resources are already under pressure from a rapidly increasing population. Both land use patterns and the productivity of crops are likely to be affected by climate change (Rosenzweig & Parry, 1994; Solomon & Leemans, 1990). Hence, it is vital, to obtain a good understanding of the effect of these changes on the growth and development of crops. The relationships between crop yields and long term climate variations have been studied by Chmielewsky & Potts (1995) using the yield data of the Broadbalk winter wheat experiment from 1854 to 1967 at Rothamsted, UK. They found that the main influence on grain yields was precipitation; a negative correlation between grain yield and rainfall for all months. The question is: whether this is a causal relation or the result of confounding effects of rainfall, reduced solar radiation or more disease infestation? Furthermore, they observed that the maximum air temperature was positively correlated with straw yield in all months except May, June and July. This may indicate that higher temperatures during winter and spring accelerate an early development of tillers and leaf formation. The temperature response is interesting in relation to the predicted rise in temperature due to climate change. With crop simulation modeling using GCM-generated weather Baethgen & Magrin (1995) found that wheat production was significantly more sensitive to temperature increase ( 0, 2, or 4 o C) than to the rainfall change ( -20, 0 or +20 %). Simulation studies suggested that crop yields under expected climate change conditions were lower, more variable and responded less to N fertilizer application than under current conditions. However, uncertainties arising from different crop modeling approaches also need to be evaluated and verified experimentally (Wolf et al., 1996) Response of wheat to elevated atmospheric CO 2 concentration and temperature During the last 40 years controlled-environment studies have added to our understanding of the effect of increased temperature and CO 2 on the productivity of many crops (Gifford, 1979; Kimball, 1983; Acock & Allen, 1985). During the last decade, a steady flow of information on the response of plants to increased atmospheric CO 2 concentration has become available. Studies have shifted from experiments in controlled-environment chambers with artificial lightning, to conditions more resembling field conditions. Wheat was grown at ambient and elevated CO 2 concentrations in Open Top Chambers (OTC s) and in field-tracking sun-lit climatized enclosures (Dijkstra et al.,1999). They found hardly any response to elevated CO 2 concentration during the early spring; however, elevated IWC-paper04 2

3 CO 2 concentration increased CO 2 exchange rate with 40 to 53 % during late spring, especially at midday. At the same time evapotranspiration rate was decreased with ca 15 % and water-useefficiency increased. Towards the end of the growing season the positive effect of elevated CO 2 level on carbon exchange rate was reduced to ca 25 %. At final harvest, grain yield and biomass were increased by ca 20 % under elevated CO 2 treatment. Therefore, the effect of elevated CO 2 -levels is more pronounced in spring than in winter wheat (Van de Geijn et al., 1995). The effects of increased temperatures will depend on the net result of the effect on photosynthetic rates of leaves and of the effect on the rate of crop development and senescence. Porter & Miglietta (1992) predicted, based on the results of simulations with AFRCWHEAT2, that in the vegetative phase leaf area would increase slightly with rises in mean daily temperature up to a maximum of 2 o C. With a higher rise in temperature leaf senescence became more important and less dry matter was produced. Lower dry matter harvest indices were predicted for annual crops for a 4,5 o C temperature rise. These results are confirmed by a report of the climate change experiment ESPACE-wheat which was performed in open-top chambers at nine sites throughout Western Europe between (Van Oijen, M & Ewert, F.,1999). In the MODEXCROP project the results of a large number of experiments with elevated CO 2 levels were analyzed. Some main results were: - averaged over all collected wheat experiments, total biomass and grain yield increased per 100 PPM of CO 2 enrichment by 7 % of the yields under ambient atmospheric CO 2. - increased temperatures reduced wheat growth duration and yields: a 10 % yield reduction for 1 centigrade temperature rise. Increases of 1-2 centigrade during the total growth period and of 2-3 centigrade during the reproductive phase were sufficient to negate the grain yield increase due to doubling of ambient atmospheric CO 2. - the temperature effect on yield was less variable and more certain than the CO 2 -effect, as the standard deviation of the yield responses to one centigrade temperature rise was half the standard deviation of yield responses to CO 2 enrichment. Crop growth simulation studies predicted a much more stable CO 2 effect than was found in the experiments Although OTC s are more field-like than growth chambers, the growing conditions in OTC s still deviate in many ways from ambient conditions ( e.g. increased temperature, reduced light intensity, increased vapor pressure deficit, increased constancy of the wind speed pattern and increased spatial variability of rainfall). Van Oijen et al. (1998, 1999) carried out experiments in cooled open-top chambers with spring wheat. They found that a temperature difference of two centigrade affected crop development and morphology more strongly than CO 2 doubling; a temperature/ CO 2 interaction was absent. The higher growth response to elevated CO 2 in non-cooled vs. cooled OTC s shows that a cooling system may remove a bias towards overestimating crop growth response to CO 2 in open-top chambers. Both elevating CO 2 (from 350 to 700 PPM) and cooling ( centigrade) increased grain yield, by an average of 11 % and 23 % respectively. CO 2 and temperature stimulated yield via different mechanisms: CO 2 increased photosynthetic rate, but decreased crop light interception capacity (LAI), whereas cooling increased grain yield by increasing LAI and extending the growing season with 10 days. In general, a hypothetical doubling of the CO 2 concentration will enhance photosynthetic rates of leaves by %. This will stimulate the canopy leaf formation, thereby increasing light interception. This adds up to the increase in photosynthesis yielding an increase in plant productivity up to %. However, there are indications that photosynthesis acclimate to elevated CO 2 concentrations; thereby, decreasing the positive effects of CO 2 when the exposure is longer than 3 weeks. This acclimation, or photosynthetic down-regulation, increases with nitrogen shortage. Response of the wheat crop to water stress In wheat a doubling of the CO 2 level from 340 to 680 PPM is predicted to increase productivity and yields on average by ca 35 % under optimum conditions (Cure, 1985), mainly through the stimulation of photosynthetic processes in the plant and improvement in the water use efficiency (Goudriaan & van Laar, 1978). Water stress during the vegetative and reproductive development reduces tillering, IWC-paper04 3

4 leaf area development and grain set. Thus, the photosynthetic capacity as well as the potential sink capacity will be reduced to the degree and duration of the stress. During grain filling water stress affect mainly current assimilation through reductions in both photosynthetic area and activity (Fisher & Turner, 1978; Day et al., 1978). Higher temperatures are often associated with high evaporative demands. Under such conditions water stress not only increases the proportion of current assimilates translocated to the grains, but also may increase the contribution from assimilates stored prior to the beginning of rapid grain growth. Bidinger et al. (1977) found that the absolute contribution to grain yield of assimilate stored prior to anthesis was unaffected by water stress, but relative to grain yield, the contribution rose from 12 % with no water deficit to 22 % when there was a deficit during grain filling. The process of CO 2 assimilation inevitably leads to a loss of water from the crop to the atmosphere. Since the exchange of water vapor are governed by the same physical principles, a linear relation between the two processes is expected. A proportionality exists between dry matter yield and the total transpiration loss of the crop. The ratio between transpiration and assimilation is strongly influenced by stomatal behavior. When stomatal aperture is governed by CO 2 concentration inside the stomatal cavity as has been demonstrated for various plant species (Goudriaan & Van Laar, 1978), the ratio between assimilation and transpiration is around 100 kg H 2 O per kg CH 2 O for C 3 species under average conditions of radiation and humidity. In terms of dry matter this value then ranges between 125 and 150 kg H 2 O per kg dry matter. When, on the other hand CO 2 -induced stomatal regulation is absent, so that stomata are fully open in the light and completely closed in the dark, the transpiration coefficient under identical conditions varies between 175 and 200 kg H 2 O per kg dry matter. These values show, that under water limited conditions, CO 2 -induced stomatal regulation is a desirable trait even though actual rates of CO 2 -exchange may be lower than without regulation during periods when sufficient moisture is available. An increase in water saving, due to partial closure of leaf stomata, is a beneficial effect that is expected when water resources are limiting. The productivity increase under elevated CO 2 will be more under water limiting conditions than under well watered conditions. However, the validity of this statement might be questioned when the stored water is more rapidly depleted due to higher growth rates and the greater leaf area for transpiration at elevated CO 2 levels. Response of the wheat crop to nutrients and elevated CO 2 Nitrogen availability stimulates biomass production through direct effects both of the source of carbohydrates, i.e. leaf photosynthesis and on the sink, i.e. leaf area expansion and formation of tillers. Therefore, interactive effects of N availability with the level of atmospheric CO 2 can be expected. High N supply helped to prevent photosynthetic acclimation to high CO 2 in wheat by stimulating tiller growth and partially eliminating a drop in tissue N concentration often observed at high CO 2 (Rogers et al., 1996). These findings led to the conclusion that acclimation to high CO 2 was primarily a response to N availability. The response of canopy photosynthesis to N availability can change under high atmospheric CO 2 if acclimation occurs, i.e. photosynthesis and the synthesis of photosynthetic enzymes decrease and N is reallocated within the photosynthetic machinery and within the plant. Under ambient CO 2 concentrations, canopy photosynthesis depends on the attenuation of light through the canopy and vertical leaf N distribution associated to it. The light-associated leaf N distribution changed dynamically during crop growth and was regulated by N availability but not by atmospheric CO 2 concentration (Dreccer, 2000). A higher concentration of soluble sugar and/or starch and a reduction of the N-concentration per unit dry weight as a result of elevated CO 2 concentration is often found in plants. The effects of increased atmospheric CO 2 on crop growth and dry matter allocation may change if nutrient supply becomes insufficient for maximal growth. It may also cause changes in minimum nutrient concentration of plant tissue and hence in the nutrient use efficiency or yield nutrient uptake ratios of crops. Wolf (1996) carried out pot experiments with spring wheat in glasshouses at ambient and doubled CO 2 concentration; plants were grown at different supplies of N, P or K. He found that doubling ambient CO 2 resulted in a large increase in total biomass and grain yield when the nutrient supply was optimum. With strong N and K limitations this CO 2 effect was about halved and with IWC-paper04 4

5 strong P limitation it became almost nil. Doubling of CO 2 resulted in a 10 % lower minimum N concentration in plant tissue and in no change in the minimum P concentration. Nitrogenous compounds for grain growth are mainly supplied by the vegetative parts and only to a smaller extent from post-anthesis uptake. At final harvest about 0.75 to 0.80 of the total above-ground nitrogen yield is located in the grains. Thus, the harvest index for nitrogen is considerable higher than for carbon; the latter ranges from 0.35 to 0.50 (Spiertz, et al., 1992). The ratio between the allocation of starch and proteins in the grains does influence the baking quality of the grain. Williams et al. (1995) found that temperature was by far the most influential growth factor affecting the quality of the grain lipids in wheat. They studied the effects of elevated temperature and atmospheric carbon dioxide concentration on the quality of grain lipids in wheat grown at two levels of nitrogen applications. Growth at elevated temperatures had the general effect of reducing the amounts of accumulated lipids, particularly non-polar lipids. It is suggested that the observed alterations in wheat lipids may affect the properties of any flours derived from grain grown under climate change conditions. Fangmeier et al. (1999) assessed nutrient element concentrations and grain quality in spring wheat grown under elevated CO 2 concentrations at different nitrogen supply rates at several European sites. They found that nitrogen acquisition by the crop did not match carbon acquisition under CO 2 enrichment. Correspondingly, grain nitrogen concentrations decreased on average by 15 % when CO 2 concentrations almost doubled and as consequence grain quality was reduced; i.e. the Zeleny value and Hagberg value. It was concluded, that despite the beneficial effect of CO 2 enrichment on growth and yield of C 3 cereal crops, declines in flour quality due to reduced nitrogen content are likely in a future CO 2 -rich world. Carbon and nitrogen economy of wheat Growth and yield of a wheat crop is the result of the interactive response of the plants to weather and soil factors. Assuming optimal crop protection, crop growth is governed by environmental conditions and availability of water and nutrients. Potential grain yields of wheat have often been calculated from the photosynthetic potentials. However, grain yields depend also strongly on the storage capacity of the ear, and as a consequence on the allocation of assimilates during the grain filling period. Crop development in wheat does exert a great influence on the initiation and formation of the ear and as a consequence on the potential number of grains, hence, the sink capacity during the grain filling stage. The carbon and nitrogen economy of the wheat plant effects rate and duration of grain growth. The phenology, development and growth of the wheat shoot apex is described in great detail by Master (1997). The primary source during grain filling is photosynthesis by leaves; however, sources and sinks change over the course of grain filling. Peduncle and lower internodes often increase in dry weight and soluble sugar content up to about 2 weeks after anthesis, particular under conditions of high assimilation and retarded grain development (Spiertz, 1977). The rate of grain growth is determined by temperature: nitrogen allocation to the grains is more enhanced by increased temperatures than carbohydrate. So, under elevated temperatures during grain filling the reallocation of nitrogen from the vegetative parts to the grains is enhanced. The earlier senescence of the green organs of the wheat plant at high temperatures can partly be offset by late nitrogen dressings. Duration of grain growth is inversely related to temperature and genetically determined. Under warm conditions the demand of the fast growing grains for assimilates, ranging from 200 to 350 kg per ha per day, can not be met by daily photosynthesis. So, reallocation from reserves in the vegetative parts is essential to maintain the growth of the grains. Concluding remarks The results of experimental research and simulation modeling of the effects of elevated CO 2 and temperature on wheat growth and yields give clear trends, although emphasis is mainly given to the response of conventional cultivars selected for the current climatic conditions. In Table 2 the relative effects are quantitatively summarized. IWC-paper04 5

6 The qualitative effects of elevated CO 2 and temperature on physiological processes, growth and yield of wheat can be described as follows: HIGHER CARBONDIOXIDE HIGHER TEMPERATURE - increased rate of photosynthesis, especially - enhanced development rate of the crop, during midday, resulting in a higher biomass shorter development phases and growth yield duration - a higher water use efficiency, due to stomatal - accelerated grain growth and a shorter regulation in C 3 plants duration of grain filling In the future more emphasis should be given to define new ideotypes that are adapted to milder temperatures during winter and spring and more temperature extremes during flowering and grain filling. Extending the early development phases, especially from ear initiation to flowering may contribute to increase the number grains the sink capacity ; this is a prerequisite to make optimal use of the higher supply of photosynthates under elevated CO 2 levels. From the experiments under controlled conditions it may be concluded that an increase of the potential grain yield level with 25 to 30 % will be possible. Potential grain yields of about 15 tons per ha require a stand of at least 600 heads per m2 with a minimum of 50 grains per head. The nitrogen demand of such a high yielding crop can only partially be covered by reallocation of nitrogen from the vegetative parts. A substantially uptake of nitrogen should take place during the post-flowering period. Acknowledgements We thank J. Goudriaan and J. Wolf ( WU Plant Production Systems) for their valuable comments. References Acock, B. & Allen, L.H. (1985): Crop responses to elevated carbon dioxide concentrations. In: Direct effects of increasing carbon dioxide on vegetation (B.R.Strain &J.D.Cure, eds.). United States Department of Energy DOE/ER 0238, pp Baethgen, W.E.& Magrin, G.O. (!995): Assessing the impacts of climate change on winter crop production in Uruguay and Argentina using crop simulation models. In: Climate change and agriculture: analysis of potential international impacts. ASA-Special_Publication, 5, Bidinger, F, Musgrave R.B. & Fischer, R.A. (1977). Contribution of stored pre-anthesis assimilate to grain yield in wheat and barley. Nature (London), 270, Chmielewski, F.M. & Potts, J.M.(1995): The relationship between crop yields from an experiment in Southern England and long term climate variations. Agricultural & Forest Meteorology, 73, Cure, J.D. (1985): Carbon dioxide doubling responses: a crop survey. In: Direct effects on increasing carbon dioxide on vegetation. (Eds. B.R. Strain and J.D. Cure, eds.). US Department of Energy DOE ER 0238, pp Dreccer, Fernanda M. (2000): Radiation and nitrogen use in wheat and oilseed rape crops. Thesis Wageningen University, 133 pp. Dijkstra, P., Schapendonk, A.H.C.M., Groenwold, K., Jansen, M., Van de Geijn, S.C. (1999): Seasonal change in the response of winter wheat to elevated atmospheric CO2 concentration grown in Open-Top Chambers and field tracking enclosures. Global Change Biology,5, Fangmeier, A., De Temperman, L., Mortensen, L., Kemp, K., Burke, J., Mitchell, R., Van Oijen, M. & Weigel, H.-J. (1999): Effects on nutrients and on grain quality in spring wheat crops grown under elevated CO2 concentrations and stress conditions in the European, multi-site experiment ESPACE-Wheat. European J. of Agronomy 10, Fischer, R.A.& Turner, N.C. (1978). Plant productivity in the arid and semi-arid zones. Ann. Rev. Plant Physiol. 29, Geijn van de S.C., Dijkstra, P., Grashoff, C., Schapendonk, A.H.C.M. & Nonhebel, S. (1995): Effects of climate change on wheat productivity: model evaluation of observed year-to-year variability of the CO 2 response. J. Experimental Botany 46, Supplement: P1.27 IWC-paper04 6

7 Goudriaan, J. & Van Laar, H.H. (1978). Relations between leaf resistance, CO2 concentration and CO2 assimilation in maize, beans, lalang grass and sunflower. Photosynthetica 12, Gifford, R.M. (1979): Growth and yield of carbon-dioxide enriched wheat under water-limited conditions. Aust. J. Plant Physiol.,6, Kimball, B.A. (1983): Carbon dioxide and agricultural yield. An assemblage and analysis of 430 prior observations. Agron. J.,75, McMaster, Gregory S. (1997): Phenology, development, and growth of the wheat (Triticum aestivum L.) shoot apex: a review. Advances in Agronomy,59, Oijen van M., Schapendonk, A.H.C.M., Jansen, M.J.H., Pot, C.S., Kleef van J., Goudriaan, J. (1998): Effects of elevated CO2 on development and morfology of spring wheat grown in cooled and non-cooled open-top chambers. Aust. J. Plant Physiol., 25, Oijen van M., Schapendonk, A.H.C.M., Jansen, M.J.H., Pot, C.S., Maciorowwski, R. (1999): Do open-top chambers overestimate the effects of rising CO2 on plants? An analysis using spring wheat. Global Change Biology, 5, Oijen van M.& Ewert, F. (1999): The effects of climatic variation in Europe on the yield response of spring wheat cv. Minnaret to elevated CO2 and O3; an analysis of open-top chamber experiments by means of two crop growth simulation models. European J. of Agronomy, 10, Porter, J.R. & Miglietta, F. ( 1992): Modelling the effects of CO2-induced climatic change on cereal crops. In: Impact of global climatic changes on photosynthesis and plant productivity ( Abrol, Y.P. et al., eds.). Proceedings of the Indo-US workshop. Oxford and IBH Publishing Co., New Delhi, India. Rosenzweig, Cynthia & Parry, Martin, L. (1994): Potential impact of climate change on world food supply. Nature, Vol 367, Solomon, A.M., Leemans, R. (1990): Climatic change and landscape ecological response: issue and analysis. In: Landscape Ecological Impact of Climatic Change (Boer, M.M. & Groot, de R.S., eds.). IOS Press, Amsterdam, pp Spiertz, J.H.J. (1977). The influence of temperature and light intensity on grain growth in relation to the carbohydrate economy of the wheat plant. Neth. J. agric. Sci., 25, Spiertz, J.H.J., Heemst van H.D.J., Keulen van H. (1992). Field- crop systems in North-Western Europe. In: Ecosystems of the world; field crop ecosystems (Ed.: C.J.Pearson). Elsevier, Amsterdam, pp Williams, M., Shewry, P.R., Lawlor, D.W. & Harwood, J.L. (1995): The effects of elevated temperature and atmospheric carbon dioxide concentration on the quality of grain lipids in wheat (Triticum aestivum L.) grown at two levels of nitrogen applications. Plant, Cell and Environment, 18, Wolf, J. (1996): Effects of nutrient supply (NPK) on spring wheat response to elevated atmosferic CO2. Plant & Soil 185, Wolf, J., Evans, L.G., Semenov, M.A., Eckersten, H. & Iglesias, A. (1996): Comparison of wheat simulation models under climate change. I Model calibration and sensitivity analyses. Climate Research 7, IWC-paper04 7