Hans-Joachim Weigel

Effects of elevated CO2 and drought on wheat: Testing crop simulation models for different experimental and climatic conditions

Effects of increasing carbon dioxide concentration [CO2] on wheat vary depending on water supply and climatic conditions, which are difficult to estimate. Crop simulation models are often used to predict the impact of global atmospheric changes on food production. However, models have rarely been tested for effects on crops of [CO2] and drought for different climatic conditions due to limited data available from field experiments. Simulations of the effects of elevated [CO2] and drought on spring wheat (Triticum aestivum L.) from three crop simulation models (LINTULCC2, AFRCWHEAT2, Sirius), which differ in structure and mechanistic detail, were compared with observations. These were from 2 years of free-air carbon dioxide enrichment (FACE) experiments in Maricopa, Arizona and 2 years of standardised (in crop management and soil conditions) open-top chamber (OTC) experiments in Braunschweig and Giessen, Germany. In a simulation exercise, models were used to assess the possible impact of increased [CO2] on wheat yields measured between 1987 and 1999 at one farm site in the drought prone region of Andalucia, south Spain. The models simulated well final biomass (BM), grain yield (GY), cumulative evapotranspiration (ET) and water use efficiency (WUE) of wheat grown in the FACE experiments but simulations were unsatisfactory for OTC experiments. Radiation use efficiency (RUE) and yield responses to [CO2] and drought were on average higher in OTC than in FACE experiments. However, there was large variation among OTC experiments. Plant growth in OTCs was probably modified by several factors related to plot size, the use (or not use) of border plants, airflow pattern, modification of radiation balance and/or restriction of rooting volume that were not included in the models. Variation in farm yields in south Spain was partly explained by the models, but sources of unexplained yield variation could not be identified and were most likely related to effects of pests and diseases that were not included in the models. Simulated GY in south Spain increased in the range between 30 and 65% due to doubling [CO2]. The simulated increase was larger when a [CO2]×drought interaction was assumed (LINTULCC2, AFRCWHEAT2) than when it was not (Sirius). It was concluded that crop simulation models are able to reproduce wheat growth and yield for different [CO2] and drought treatments in a field environment. However, there is still uncertainty about the combined effects of [CO2] and drought including the timing of drought stress and about relationships that determine yield variation at farm and larger scales that require further investigation including model testing.

Photosynthetic and growth responses of old and modern spring wheat cultivars to atmospheric CO2 enrichment

Abstract Cultivars of spring wheat (Triticum aestivum L.) introduced between 1890 and 1988 were cultivated in pots under optimal growth conditions and exposed during the whole growing season to normal (379 p.p.m.) and elevated CO2 concentrations (689 p.p.m.) in open-top chambers. CO2 effects were measured at anthesis on flag leaf composition (chlorophyll and protein) and photosynthetic parameters, and at maturity on plant growth and yield. CO2 enrichment did not affect light saturated rate of photosynthesis measured at 400 p.p.m. CO2 or protein, total chlorophyll and dry weight content per unit leaf area. However, single flag leaf area and fresh weight per leaf area were increased by CO2. This increase was possibly responsible for a significant decrease in the chlorophyll a/b ratio. Under normal atmospheric CO2 concentration, the total above-ground biomass, stem weight and height, and ear number were negatively correlated with the year of cultivar release. Despite no evidence of CO2 acclimation, i.e. changes in flag leaf composition, CO2 enrichment resulted in a greater growth stimulation of the older than the modern cultivars. This was due to a greater CO2 effect on those growth components that were altered during plant breeding of wheat in the past, i.e. stem weight and height, and ear number. The average CO2-related increase in biomass and grain yield amounted to ca 46% and 28% for the three old (1890–1943) and three modern cultivars (1965–1988), respectively. Differences in yield response to CO2 enrichment between old and modern cultivars could be mainly explained by changes in ear number.

Effects of season long CO2 enrichment on cereals. II. Nutrient concentrations and grain quality

Abstract Two cultivars each of spring wheat (Triticum aestivum L., cv. Star and cv. Turbo) and spring barley (Hordeum vulgare L., cv. Alexis and cv. Arena) were exposed season-long to ambient (384 p.p.m.) and above ambient CO2 concentrations (551, 718 p.p.m.) in open-top chambers. Plant samples were taken at the booting stage and at maturity. Concentrations (grams per gram dry weight) of macro (Ca, K, Mg, N, P, S) and micronutrients (Fe, Mn, Zn) were measured in stems, leaves, ears and grains, and the amino acid composition of the grain protein was determined. For most nutrients studied the sequence and size of the response of the four cereal plants to the CO2 enrichment was cv. Arena

Inhibition of photosynthetic reactions of isolated intact chloroplasts by cadmium

Summary C0 2- and PGA-dependent O 2 -evolution of isolated intact chloroplasts from spinach ( Spinacia oleracea L.) were inhibited upon addition of CO 2 (0.1-1.0 mM). Inhibition was higher for C02- (50% inhibition at 0.5 mM Cd) than for PGA-reduction (10 % inhibition at 0.5 mM Cd) and was independent of the Pi concentration of the medium over a wide range (0.015 -1.0 mM P i ). Cd also shortened the lag-phase of C0 2 -dependent 0 2 -evolution. 14 CO 2 -reduction of intact chloroplasts in the presence of Cd resulted in an increase of the relative levels of 14 C-labeled glycolate, triosephosphate, pyruvate, FBP, and RuBP, and a decrease of PGA and malate. Measurements of fast chlorophyll a fluorescence induction and whole chain electron transport by means of methylviologen-dependent O 2 -consumption revealed that Cd did not affect electron transport reactions of intact chloroplasts. The light-induced proton gradient across the thylakoid membrane (Δ pH) and light/dark levels of ATP of intact chloroplasts were also not impaired by the metal. In isolated thylakoids Cd severely depressed cyclic photophosphorylation without inhibiting the Δ pH. It is suggested that in intact chloroplasts Cd affects photosynthesis by inhibition of different reaction steps of the Calvin cycle and not by interaction with photochemical reactions located on the thylakoid membranes.

Drought stress effects on wheat are mitigated by atmospheric CO2 enrichment

The atmospheric CO2 concentration is predicted to increase and to generate a rise in the global surface temperature, and change the seasonal precipitation pattern. This could aggravate the severity of summer drought conditions and affect crop yield. We studied the effect of the interaction of CO2 and water supply on seasonal absorption of photosynthetically active radiation and radiation-use efficiency of aboveground biomass production to understand the processes contributing to final yield. Wheat was grown over two years in open-top chambers at present or future (+280 ppmv) atmospheric CO2 concentration and under sufficient water supply or drought stress in lysimeters with a soil depth of 0.4 m (first year) or in the field with unrestricted root growth (second year). Drought stress was started after the first node stage by halving the water supply. Our results show that under sufficient watering, CO2 enrichment did not affect the green area index or seasonal radiation absorption. Drought stress always decreased the green area index and accelerated canopy senescence, which in the second year resulted in a decrease of 23% in the seasonal radiation absorption under the present atmospheric CO2 concentration. CO2 enrichment stimulated the green area index under drought stress in the second year and seasonal radiation absorption was only decreased by 16%. Radiation-use efficiency was reduced by drought and increased by CO2 elevation and the CO2 effect was higher under restricted (+60%) than under sufficient watering (+32%). This implies that CO2 enrichment enhanced final biomass and grain yield by less than 10% under well-watered conditions and by more than 44% under drought stress conditions, respectively. This study indicates that the increase in atmospheric CO2 concentration will attenuate the effects of summer drought on wheat grain yield.

Effect of CO2 enrichment on growth and daily radiation use efficiency of wheat in relation to temperature and growth stage

The objectives of the present study were to examine (i) the effect of whole season CO2 enrichment on seasonal radiation absorption and radiation use efficiency of above ground biomass production (RUE1) of wheat, (ii) the relationship between daily radiation use efficiency and temperature, and (iii) the effect of CO2 enrichment on this relationship in the period before and during grain filling when plant growth is assumed to be source and sink limited, respectively. During two consecutive years wheat (Triticum aestivum L. cv. Minaret) was grown in open-top chambers at different plot sizes (1 and 3 m2) at ambient and elevated CO2 concentrations (ca. +280 ppm above ambient) with sufficient water and nutrient supply and analysed for final biomass and grain yield. In the 2nd year also light absorption by the green canopy and above ground biomass production were measured during the whole season. Canopy CO2 exchange rates (CCER) were recorded on 50 days (2nd year) from stem elongation until canopy senescence with canopy chambers. CCER were used for the calculation of daily radiation use efficiency of the net CO2 flux (dRUE2) before and during grain filling. Daily net carbon assimilation was linearly related to absorbed photosynthetic active radiation. Mean seasonal RUE2, which was calculated from this relationship, was increased by CO2 enrichment. This corresponded to the findings for RUE1. Seasonal light absorption was unaffected by the CO2 treatment. Final biomass and grain yield were increased under CO2 elevation by <11 and <13% in the 1st and 2nd year, respectively. Regression analysis yielded a significant negative relationship between dRUE2 and temperature under ambient CO2 in the period before and during grain filling. CO2 enrichment mitigated this negative relationship in the period only before but not during grain filling. The present experimental findings support the theoretically expected decrease of RUE at ambient CO2 and the increase of the CO2 effect with temperature in the preanthesis phase. The results also indicate that the positive CO2 x temperature interaction on canopy assimilation disappears during grain filling, which might be responsible for the decrease of the CO2 effect on plant biomass between anthesis and grain maturity.

Growth responses and foliar sensitivities of native herbaceous species to ozone exposures

This paper reports on open-top chamber studies investigating the effects of different O3 exposures on native herbaceous plant species. Plants were grown up from seeds, potted into natural soils and exposed to near-ambient O3 levels during one growing season. A wide range of visible symptoms was apparent during the exposures. Species such as Rumex obtusifolius, Senecio vulgaris or Sonchus asper showed leaf colorations (e.g. reddish pigmentation) that probably indicate a non-specific stress response. In other species especially of the genera Malva and Cirsium the symptoms produced by O3 appeared to be similar to those characteristic for O3-specific foliar injury (stippling, flecking). In almost all species tested, O3 caused premature leaf senescence, which was sometimes associated with premature leaf abscission. However, earlier senescence did not necessarily result in changes in plant growth. Of all species tested, Malva sylvestris was found to be the most sensitive in terms of growth reduction and lower seed production.

Response of cellular antioxidants to ozone in wheat flag leaves at different stages of plant development

The effect of ozone (O3) on growth, yield and foliar antioxidants of spring wheat (Triticum aestivum L. cv. Turbo) was investigated in 1990 and 1991 in Braunschweig, Germany. Plants were grown full-season in pots in open-top chambers ventilated with charcoal-filtered (CF) air to which one or two levels of O3 were added. Mean 8 h day(-1) (10.00-18.00 h) O3 concentrations in the CF and CF + O3 treatments were 5.9, 61.2 and 92.5 nl litre(-1) in 1990, and 4.7 and 86.4 nl litre(-1) in 1991. Plants that received the high O3 level showed symptoms of premature senescence of the oldest leaves and yield reductions in both growing seasons. The contents of ascorbate and glutathione and the enzyme activities of ascorbate peroxidase and glutathione reductase were measured in symptomless flag leaves in weekly intervals before and after the beginning of anthesis. Leaf age had a significant effect on all antioxidants investigated. The O3 exposures of about 90 nl litre(-1) increased the activity of ascorbate peroxidase and the concentration of glutathione, but there were no pollutant effects on ascorbate content and glutathione reductase activity. Measurements of the antioxidant levels throughout one day showed no clear indications of diurnal changes in the antioxidative capacity in wheat flag leaves. The results are discussed in relation to the role of antioxidants in O3 detoxification.

Effects of season-long CO2 enrichment on cereals. I: Growth performance and yield

Abstract Two cultivars each of spring wheat (Triticum aestivum L., cultivars ‘Star’ and ‘Turbo’) and spring barley (Hordeum vulgare L., cultivars ‘Alexis’ and ‘Arena’) were exposed throughout the growing season to ambient (384 p.p.m.) and above ambient CO2 concentrations (471, 551, 624, 718 p.p.m.) in open-top chambers. Plant samples were taken four times during plant development and biomass partitioning into stem, leaves and ear was measured. Total above-ground biomass increased mainly in the CO2 concentration range between 400–550 p.p.m. for wheat, and between 400–700 p.p.m. for barley. Stimulation of biomass was largely due to an increase in tillering rate. At the tiller level CO2 enrichment revealed a decrease in leaf dry weight at anthesis stage, which was due to a reduction in leaf size (barley) and in leaf number (wheat). Specific leaf weight of the mature flag leaf was unaffected by CO2. Stem biomass per tiller was temporarily (‘Star’, ‘Alexis’) or during the whole growth period (‘Turbo’, ‘Arena’) increased by CO2 exposure, while ear dry weight was increased (barley) or even decreased (‘Star’). Except for the barley cultivar ‘Arena’, which showed a 84% increase in the number of grains per ear, the number of ears was almost entirely responsible for the increased grain yield among the CO2 treatments. At the highest CO2 concentration yield increase amounted to 19% and 27% for the two wheat cultivars, and 52% and 89% for the two barley cultivars in comparison with the ambient CO2 level. Among all cultivars there was an inverse relationship between the total shoot biomass produced at ambient CO2 conditions and the plants response to the CO2 enrichment. This indicates that the genetic potential of wheat unlike barley is highly adapted to present atmospheric CO2 conditions and thus responsible for the small CO2 effect on wheat.

Changes in atmospheric chemistry and crop health: A review

The concentrations of atmospheric compounds such as greenhouse gases, heavy metals and trace gas air pollutants have rapidly changed. Many of these compounds interact with agricultural systems and influence crop performance, both directly by affecting growth and quality or indirectly by altering the plant’s ability to cope with other abiotic and biotic stresses. Some atmospheric compounds have little or no discernible impact on the environment; others reach levels that exceed thresholds for damage to crops. In this review, we analyse the literature on airborne species that directly impact crop growth and health. In Europe and North America emissions of SO2, NOx and heavy metals have declined during the past decades and are currently not considered as a major threat to crops. By contrast, air pollutant emissions have been increasing in rapidly growing regions of Asia, Africa and Latin America. Ozone is the most phytotoxic of the common air pollutants. The widespread distribution of O3 already presents a risk to crop growth and health in many regions of the world. It is concluded that the continuous increase in background O3 concentrations will pose a critical threat to future world food security. Interactions with both biotic and abiotic factors must be taken into account in assessing risks of air pollutants in the field. There is evidence that these indirect effects could be more important under certain circumstances than the direct effects of air pollutants on plants. The parallel rapid increase in atmospheric CO2 concentrations accompanied by climate change has two major implications: (1) a possible benefit to crop growth by direct stimulation of photosynthesis and by mitigation of gaseous air pollutants and water stress; and (2) a threat to crop production due to an enhancement of crop quality losses.