The world population

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Introduction

The importance of future agricultural responses

The world is facing a food crisis and as food prices soar more and more of the world population are vulnerable to food shortage. The World Bank projects that by 2030 worldwide demand for food will increase by 50% and for meat by 85% (Evans 2009). The combination of an increasing population size (9.2 billion by 2050) and issues with land use mean that solutions must be found if we are to meet the nutritional needs for future generations. Climate change will have a massive effect on future agriculture worldwide. The complex interactions between the different factors of climate change (CO2 rise, temperature increase, extreme climatic events and drought) and how plants respond to these are important to understand so that the best strategy for future food production can be found. This review will focus on the effect that the increasing CO2 levels associated with climate change will have on some of the world's most important crops. CO2 elevation is important especially to plants as it effects the responses to other stresses of climate change i.e. drought.

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It is important to understand how plant stomata respond to environmental change so that predictions can be made for the effect of future climatic changes. The rising level of carbon dioxide in the atmosphere has been well publicised and is now at its highest point in 26 million years with levels predicted to possibly double before the end of the century (Long et al., 2004). The acceleration of climate change is clear as for a thousand years prior to the

Industrial Revolution atmospheric carbon dioxide levels were constant at approximately 270 µmol mol-1. Evidence shows that levels have risen by 38% to approximately 372 µmol mol-1( Figure 1); this trend is predicted to continue and reach 550 µmol mol-1 by 2050 and rise to700 µmol mol-1 by 2100 (Prentice 2001).

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Climate change predictions show increasing incidences of extreme climatic events as summarised by the Intergovernmental Panel on Climate Change Fourth Assessment Report (2007). The IPPC reports that incidences of drought have increased since the 1950 due to slightly lower precipitation and higher evaporation over land areas. The IPPC predicts heavy precipitation events in some areas, an increase in tropical cyclones and a rise in sea levels resulting in contamination of fresh water sources and implications for the irrigation systems of crop plants.

Plants may benefit through the increase in atmospheric carbon dioxide as there is more carbon available for photosynthesis. Plants ability to cope with drought may be boosted by the accompanied rise of CO2 as if a plant can gain the carbon level it requires faster the stomata can then close reducing water lost and improving water use efficiency. This would have a large effect on agriculture as the world population increases the need to find more drought resistant crops are crucial.

Agriculturally important crops

The rice plant (Oryza sativa) provides the staple diet for large proportion of the world population especially in Asia, South America, West Indies and the Middle East. It is the most important cereal grain for human consumption. The top four producers of rice are China (26%), India (20%), Indonesia (9%) and Bangladesh (5%) with approximately 600 million tonnes of rice produced altogether (International Rice Research Institute 2004). Rice requires high levels of water either through rainfall or irrigation and therefore may be highly susceptible in some areas if incidences of drought increase.

Maize (Zea mays) or corn is a C4 plant grown all over the world particularly in the United States. Approximately 800 million tonnes were produced worldwide in 2007. Maize has a variety of uses including human consumption and animal fodder.

Wheat is the world's third most important crop and forms the staple diet of many populations as it is the basis for making flour and can be used as fodder for livestock. Like rice wheat uses C3 photosynthesis to fix Carbon. The growing of wheat is not confined to one particular area of the world but the countries making up the EU produce the most at 124 million tonnes.

Plant interaction with the environment via the stomata

The functional unit for gas exchange between plants and the atmosphere is known as the stomatal complex (Serna & Fenoll 2000). Stomata are pores in the epidermis that allow the movement of CO2, H2O and O2 in and out of the plant (Figure 2).

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They are flanked by two guard cells that through responses to turgor pressure provide control to open and close the pore. The density and conformational state of the stomata is important in balancing water loss and Carbon dioxide import (Figure 3).

http://plantandsoil.unl.edu/croptechnology2005/UserFiles/Image/siteImages/K+_opne&closed-LG.gif

The development of stomata and the waxy leaf cuticle are some of the primary adaptations that allowed the evolution of advanced terrestrial plants and enabled them to exist under a range of environmental conditions by having control over water content (Raven 2002). Stomata respond too many different signals in the environment such as light intensity/quality, humidity, concentration of carbon dioxide in the atmosphere and water availability, these stimuli influence stomatal aperture and development (Hetherington & Woodward 2003).

Stomata are important in maintaining the transport system in plants. They do this by allowing transpiration of water from the xylem into the outside environment creating upward movement of water due the negative pressures created. Stomata also provide a pathway for carbon dioxide to move into the leaves, supplying carbon for the photosynthetic reactions.

Stomata open and close to maintain a balance between the amount of water lost via transpiration and the amount of carbon gained for photosynthesis. The dynamic ability of the stomata is crucial in plant responses to changing environmental conditions. In environments where water may be scarce plants can respond by closing there stomata to reduce water loss, this then leaves them with the problem of gaining carbon in the form of CO2 that is used in the Calvin cycle which it is fixed by the enzyme Ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO). The plant must find a balance between the two opposing factors, the rate at which a plant can fix carbon for the amount of water lost is the Water Use Efficiency (WUE).

Rubisco is used in the initial carbon fixing reaction and plants that do this are referred to as C3. Rubisco has an affinity for O2which under the low CO2 levels it adds to the Calvin Cycle this wastes energy and causes the product to split (Campbell & Reece 2005). Plants referred to as C4 use PEP carboxylase to fix CO2 to phosphoenolpyruvate which forms oxaloacetate. PEP is useful as it doesn't have an O2 binding site and therefore can fix CO2 when it's at a low partial pressure. Oxaloacetate is broken down releasing CO2 once inside the bundle-sheath cells where Rubisco is present. C4 plants can save up to 50% of the fixed carbon lost during photorespiration (Campbell & Reece 2005).

The sensing of CO2 by stomata

Stomata respond to increasing CO2 by closing the stomata therefore altering the rates of the transpiration of H2O and uptake of CO2. It is important to understand how guard cells sense CO2 levels as it will not only help predict the responses of plants to climate change but also provide ways of manipulating the response. Many of the ways guard cells regulate stomatal opening in response to the CO2 signals are unknown but cytosolic pH, malate levels, intercellular cytoplasmic Ca2+ levels, chloroplastic zeaxanthin levels and plasma-membrane anion channel regulation by apoplastic malate have been implicated (Assmann 1999). Raschke (1975) observed that there is an interaction between Abscisic acid (ABA) and CO2 sensing in some plants through experiments on Xanthium strumarium as their stomata did not close in response to high CO2 unless the plant had been treated with ABA. High CO2 causes an increase in calcium but there is no change in cytoplasmic pH however it does activate K+ channels through an unknown mechanism (Figure 4). The CO2 response may therefore be linked with the signalling of ABA as the downstream calcium dependant effects may be shared (Vavasseur &Raghavendra 2004).

Stomatal response to drought

Abscisic acid (ABA) plays an important role in the response of stomata to drought. ABA is present in unstressed plants in the form of ABAH. When a plant begins to experience a stress condition the pH of the xylem starts to become more acidic. During drought it is the roots that usually experience the first levels of stress and the lowered pH causes the ABAH form of abscisic acid to dissociate to the form ABA. ABA moves via the transpiration system of the plant to the leaves where it can then act on the stomata. ABA through its effect on second messengers such as calcium causes the opening of potassium (K+) channels in the plasma membrane of the guard cells (Campbell & Reece 2005). Due to the difference in concentration of K+ ions inside the cell compared to outside the cell there is a large efflux of K+ from the guard cell (Figure 3), this is followed by a movement of water as the loss of K+ solute has raised the water potential of the cell and water can now flow down its water potential gradient thus lowering the turgor pressure of the guard cell and causing the closing of the stomatal pore.

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If the environmental conditions that the plant is in change and the balance is shifted toward gaining CO2 then the stomata are opened. The opening mechanism works via a proton pump. Hydrogen ions are actively pumped out the guard cell this causes a change in membrane potential and the positively charged K+ ions move though transport proteins into the guard cell thus lowering its water potential causing water to move in and raise the turgor pressure. The orientation of the cellulose microfibrils that form the cytoskeleton of the guard cell cause it to bow outwards forming the stomata pore between a pair.

Chamber experiments on elevated CO2

The response of plants to higher levels of CO2 can be observed using enclosure studies and Free-Air Carbon Enrichment (FACE). Over the last century there has been extensive investigation looking into the effect of elevated CO2 on plants. Techniques such as open-top chambers (OTCs) and greenhouses were most widely used. There are obvious limitations posed by using these systems, size restrictions meaning that it is often the early stages of plants that are monitored. Concerns were also highlighted when plants where grown pots as other factors such as nutrient exhaustion can affect the response to elevated CO2 (Apr 1991). OTCs although exposing plants to the open environment modify it significantly via blocking wind, affecting rainfall patterns and through a combination of the two affecting the spread of pests and pathogens (Long et al., 2004).

Free-Air Carbon Enrichment

FACE is designed to remove as many of the artificial situations that enclosure studies create. FACE allows the analysis of plants in a virtually undisturbed ecosystem allowing plants natural interaction with environmental conditions (wind, rain, temperatures, insects and pathogens) (Brookhaven FACE research 2008).

FACE usually consists of a roughly circular area with pipes running around the perimeter (Figure 5). The pipes are used to release either CO2 or CO2 enriched air into an area that can range from just above the soil level to above the growing canopy. FACE uses a computer relay system to monitor wind direction and respond by releasing CO2 from the pipes that stand in the upwind direction. If wind speed is very low then CO2 is released alternately from adjacent release points (McLeod & Long 1999).

The FACE system can be adapted to many specifications e.g. for plants that grow only a meter few vertical release points are needed compared to plants that grow tall where many release points are require to maintain a stable CO2 level. The greater size of FACE areas compared to enclosure studies allows adequate sampling of plant material without making an impact of the outcome of the experiment and for the analysis of vegetation to canopy closure.

Methods

Web of science was used to gain the majority of the information. It was chosen as it allows for easy manipulation of search terms and addition of many combined search categories. A modification of the suggested search methods was required due to the large amount of information available around the subject area of elevated CO2.

The first method used was to search key words combined with Boolean logic on web of science. AND was used to combine keywords; crop, FACE and Agriculture and then more specific search using elevated CO2 AND maize, wheat and rice (figure 5). It became clear that this alone was not very successful at refining the number of hits as it took to many specific field searches, although adding the relevance restriction condensed the most suited papers onto the first few pages enabling the titles and abstracts of appropriate papers to be read.

In order to find the right original papers and ensure important papers were identified another refining method was used. An appropriate title was identified by the previous method using keywords (figure 5) and the abstract read. If the abstract was applicable then the 'related articles' feature on web of science allowed for a more specific subject area search.

A review on the subject area or a related area was also used to find original papers via the references option on web of science. This allowed identification of the original papers that the review was based on. These could then be searched for on web of science using either the authors name and date of publication or the name of the paper found in the references.

Analysis of key papers

Will photosynthesis of maize (Zea mays) in the US Corn Belt increase in future [CO2] rich atmospheres? An analysis of diurnal courses of CO2 uptake under free-air concentration enrichment (FACE)

Leakey, A.D.B et al. (2004) Global Change Biology 10:951-962.

Why this paper was chosen

By the middle of this century maize is predicted to become the world most important food crop, therefore experimental evidence on the effects of elevated CO2 are crucial in understanding future yields, agricultural processes and water use of maize. This paper was chosen as it offered an insight into the effects that atmospheric CO2 rises may have on crops that use C4 photosynthesis. It was also the first to compare Zea mays in a FACE system and record photosynthesis at many points during the day throughout the plants crop cycle.

Methods

This study was undertaken at the SOYbean FACE facility in Champaign. The growing conditions are highly productive and typical of the US Corn Belt. Zea mays was planted on 30th of May and grown according to typical agricultural practices. The crop was harvested on the 10th of October. In order to control variations in soil and topographic area the planted Zea mays was planted in 4 separate sections in the field each with two circular plots one at ambient (354 µmol mol-1) and another at elevated CO2 (549 µmol mol-1). In the elevated CO2 condition the plants were exposed during the day from planting till harvest. The FACE system pipe structure was also placed around the ambient crop plots to act as a control and each plot separated by 100 m to minimise contamination.

The youngest leaf was assessed on five occasions over the growing season corresponding to different stages of Zea mays. To do this a 6 cm2 leaf chamber was used with water vapour analysers and gas exchange system monitors. Measurements were recorded in two hour intervals during the day and gas exchange systems were alternated between plots and ambient and elevated CO2 conditions. Intercellular CO2, stomatal conductance and leaf net assimilation (A) were calculated using von Caemmerer & Farquhar (1981) equations.

Key findings

Leaf net assimilation rate increased by up to 41% under elevated CO2 at certain times (figure 6) and 10% for season average. There was however no significant effect of elevated CO2 by the end of the season.

It was found that on July 11th & 22nd the high assimilation rate at elevated CO2 correlated with low rainfall (Figure 7) compared to August 9th & 21st where no enhancement of CO2 was found after high rainfall.

Stomatal conductance was found to be lower (23%) in the elevated CO2 condition thus proving Zea mays with lower shoot water stress when there was high evaporative demand and a lower soil water extraction. This would help reduce the chance of drought after low rain fall.

Conclusions

The findings from this study where not what was originally hypothesised as Zea mays did so an increase in leaf photosynthesis. The increase in leaf photosynthetic rate in C4 plants is thought to be down to improved water status under drought conditions yet the yearly rain fall average for 2002 was marginally above the 50 year average suggesting that Zea mays did not experience conditions of water deficit.

This study whilst recording the net photosynthetic CO2 assimilation did not correlate this with responses of the yield which may be more important in terms of the responses of Maize to future CO2 concentrations.

Photosynthesis, Productivity, and Yield of Maize Are Not Affected by Open-Air Elevation of CO2 Concentration in the Absence of Drought

Leakey et al. (2006), Plant Physiology 140:779-790

Reasons for choice of paper

40 % of Maize production is based in the US Corn Belt (USDA 2005) and therefore the importance of understanding the responses of maize to future rises in CO2 is clear particularly regarding yield.

This study investigates the effect of elevated CO2 on yield of maize using the FACE methodology. The paper was identified as being particularly important as it builds on and expands the results found in Leakey et al. (2004) to give a more complete picture to the responses of Zea mays to elevated CO2. This paper measures the effect on yield biomass as well as stomatal conductance and the activity of key photosynthetic enzymes in vivo and vitro.

The fact that this experiment looks at Maize at 5 developmental stages and over many intervals during the day means that it is a more reliable prediction of future responses than past chamber studies that may normally measure at one point in the crop life cycle in a system that does not allow for normal atmospheric coupling.

The paper also is of high importance to future estimations of yield production for Zea mays in regards to growing under adequate water availability and drought.

Methods

The experiment was conducted soyFACE facility in Champaign (US Corn belt). Zea mays was grown at ambient (354 µmol mol-1) and elevated CO2 (549 µmol mol-1) using same set up as explained in Leakey et al. (2004).

In situ gas exchange and chlorophyll florescence were measured from the youngest fully expanded leaf at five different developmental stages over the growing season. Intercellular CO2, stomatal conductance and leaf net assimilation (A) were calculated using von Caemmerer & Farquhar (1981) equations.

Leaf discs of 1.2 cm2 were cut and analysed for amino acids, carbohydrates, proteins and chlorophyll. 2.4 cm2 leaf discs were used to measure water potential and 3.6 cm2 leaf discs were dried and weight for calculation of specific leaf area (SLA). The activities of Rubisco, PEP and PPDK were measure indirectly in vitro by measuring the oxidation of NADH in a dual-beam spectrophotometer.

Crop biomass and yield were measured by sampling four plants from each plot, heating these to 76oC and recording the weight and mass of the grain and stover.

Key findings

This paper found that there is no direct effect of elevated CO2 on the photosynthesis of Zea mays (figure 8). A possible reason for no increase in net CO2 assimilation rate (A) in situ, was thought to be due to the acclimatisation of maize to elevated CO2 resulting in a reduction in the capacity or regeneration of PEP, PEPc, PPDK and Rubisco, however this study found no evidence for this theory as no chance in the key photosynthetic enzymes was observed in vivo or vitro under elevated CO2.

Stomatal conductance reduced significantly by 34% under elevated CO2 (figure 8) and resulted in improved water availability in the soil although the overall plant water status was unchanged. Zea mays would only benefit from more water in the soil if drought occurred and the conditions in the 2004 growing season were ideal with no incidences of drought.

No effects were observed on protein, amino acid and SLA under CO2 enrichment. Biomass of the stover and grain were unchanged and kernel number, weight, overall leaf area and silking date showed no significant difference between ambient and elevated CO2 plots.

Conclusions

This study shows that under future atmospheric CO2 concentrations Zea mays will not increase in productivity unless the crop is in an area where water deficit occurs such as tropical latitudes. When compared to the previous experiment by Leakey et al. (2004) on the same site, the incidences of drought that occur ever 2-3 years may provide times when elevated CO­2 would improve crop production.

Prediction of future crop yield is difficult and it appears that elevated CO­2 will not increase Maize production as much as first thought, therefore future analysis must include the interaction with water stresses in that environment.

Effects of elevated CO2 on grain yield and quality of wheat:

results from a 3-year free-air CO2 enrichment experiment

Hoegy, P. et al. Plant Biology 11:60-69

Why the paper was chosen

This paper was chosen as it is a long term study of wheat under elevated CO2 using the FACE system and therefore offers a more reliable prediction of the future impact on wheat crops under a more natural ecosystem environment. It is also the most recent paper cited in this review and shows good comparison with previous investigation in the area whilst highlighting contradictory points (particularly with chamber studies).

The paper focused on wheat grain quality and is the only study that took a closer look at the implications that changes in quality may have on industrial processing, marketing and consumer nutrition and health. Studies on the mineral composition changes of wheat under elevated CO2 are limited so this study offers a unique insight into the compositional changes wheat experiences under elevated CO2.

Methods

Plant cultivation (wheat Triticum aestivum cv. TRISO) was set up in a three year study using a mini-FACE system along with 13 associated weed species in Stuttgart Germany. Fertilizers were applied uniformly based on the agronomic practice recommended for wheat. Plots were manually irrigated and all plots had the same amount of water applied.

Three conditions were set up for wheat each containing five replicates; Control plot with ambient CO2 levels, Ambient within frame system and FACE plots at 150 µl/l-1 above ambient CO2 levels. Mature plants were harvested and stems, leaves and ears collected.

The grain was obtained by threshing and the minerals, non-structural carbohydrates, lipids and amino acids were analysed. Data from all three years was pooled and the control plot data excluded as it did not differ significantly from the ambient condition.

Key findings

This study found that CO2 did act as a C 'fertiliser' as expected, with total above ground biomas increasing by 11.8%. Grain yield also increased up to 10.4% under elevated CO2 and it observed that grain size pattern had shifted in elevated CO2 to a smaller grain size.

Negative effects were observed on the wholegrain chemical quality. Protein concentration decreased by 7.4% in elevated CO2.

Levels of macro and micro elements in the grain were also altered. K and Pb were increased significantly under higher CO2 whereas Fe, Cd, Mg, Si decreased (figure 7).

Starch is the main carbohydrate in wheat grains and was unaffected in elevated CO2 although other non-structural carbohydrates (fructose) did increase significantly due to the sink organs having more Carbon available.

Conclusion

The changes in grain quality observed in this study have wide implication for the production of healthy food, industrial processing and market value.

The lower protein concentrations found at elevated CO2 have particular implications for the process industry as protein content as strong correlation with bread volume. In less economically countries lower levels of the micro-element iron (Fe) may increase the incidences of malnutrition thats already effects 3.5 billion people (Hogy 2009).

Effects of elevated CO2 concentration on growth, water use, yield and grain quality of wheat under two soil water levels.

Wu. D.X et al. (2004) Agriculture Ecosystems and Environment

Why the paper was chosen

This paper was chosen as it looks at many of the impacts that CO2 enrichment can have on the agriculturally important crop of wheat. It is important to know not only the effect that CO2 has on growth but also how this alters the composition of plant tissues, grain yield and the changes it can have on transpiration rates and WUE. The paper investigates all of these issues and also provides an insight into the wider implications of CO2 elevation in the future e.g. possible increase in malnutrition due to decreasing nutritional values of grain.

Methods

The experiment took place in China at Lanzhou University where wheat (Triticum aestivum L.) was grown in two growth chambers, one at elevated CO2 of 700 µl/l and the other at ambient levels (350 µl/l). In each condition there were 12 pots, 6 under 80% FWC and 6 under 40% FWC were set up. Three pots were randomly selected to asses root growth via periodical destructive growth analysis. Plants were harvested after three months and the overall grain, shoot and thousand-seed dry weight along with grain number per plant was calculated. Using the recordings of pot weight and plants per pot the water consumption of each plant was calculated. Shoot/grain dry weight was divided by water consumption enabling water use efficiency (WUEs/WUEg) to be found.

The grain was analysed chemistry in regards to; starch using polarimeter, nitrogen (N) & phosphorus (P) using Spectrophotometer, zinc (Zn) & potassium (K) using spectrometer and lysine via dye binding lysine method. Protein (Pr) content was calculated by nitrogen content times by constant 5.7. Multiplication of these qualities of the grain by estimated grain yield per hectare was used to gain the nutritive values of the grain per hectare. The concentration values of each nutrient where obtained from mean of three pots replicates.

The effects of CO2 concentration and water availability on shoot dry weight, shoot height, grain yield and water consumption data was analysed by a Two-way ANOVA and T-test for under the same soil water content. Significance was stated when P=0.05 and extreme significance when P=0.01.

Key findings

This paper highlights five important key findings, the first being elevated CO2 improves growth, yield and WUE of wheat (figure 6). The weight to height ratio of the shoots is also altered and lateral growth rather than vertical growth occurs.

Increases in grain yield are also found at elevated CO2. At high FWC the grain weight per plant increased by 166% and by 78% in low FWC, this is mostly due to increases in the number of grains per plant.

The quality of the grain was found to decrease under elevated CO2. Crude protein content was found to have decreased by 15.2%, Zn by 32.6%, K by 23.2% and P by 36.6% from ambient CO2 levels at high soil water content. Only starch content was found to have increased by 9.7%. Due to the increased number of grain per plant the overall nutritive values per hectare increased by 126.2% for nitrogen.

The beneficial effects of CO2 enrichment on plants is increased when water is not a restricting factor although elevated CO­2 is still important in low water conditions as it reduces transpiration rates and improves WUE making it an important aspect of future crop growth in drought prone areas.

Conclusions

This paper is highly significant as it builds on many previous studies in the area and tries to gain a more whole plant view on the effect of elevated CO2 by adding the effects of water deficit and grain quality as well as yield and growth measurements.

The experiments took place in growth chambers and the wheat plants were therefore exposed to an artificial environment devoid of many aspects of the natural ecosystem that could possibly shape a plant response to elevated CO2. The effect of elevated CO2 has been show to decrease when plants were grown in pots possibly due to nutrient exhaustion or interaction of roots and solid barriers (Arp 1991).

References

  • Arp WJ. 1991. Effects of source-sink relations on photosynthetic acclimation to elevated CO2. Plant Cell Environ. 14:869-75

Responses of rice cultivars to the elevated CO2

Uprety, D.C et al. (2003) Biologia Plantarum 46:35-39.

Why the paper was chosen

This paper was chosen as it provides a look into the extent of the differences between cultivars of the agriculturally important crop of rice. This study is one of few that focus on the biochemical and physiological aspects of rice under elevated CO2. The paper helps answer the question posed in this review as it shows the effects of elevated CO2 on many rice qualities rice protein, sugar levels, yield and growth.

Methods

Two cultivars of Indian rice (Oryza sativa L.) were used in this experiment, Pusa Basmati-1 and Pusa-677. The cultivars were transplanted into 3 m open top chambers lined with transparent polythene where the conditions had been set up to mimic normal agriculture practice for this crop. The elevated CO2 condition was between 575-620 µmol mol-1 and the plants were thinned to 200 per m2.

The youngest fully developed top most leaf of main shoot was measured for stomatal conductance and net photosynthetic rate. The fully expanded uppermost leaf of main shoot was used to record sugar and starch content and the 3rd and 5th leaves of the main shoot were used to analyse protein.

The rice plants were harvested and the grain yield, grain mass and spike number recorded. Number of leaves, height and tiller production were recorded and the constant mass found via drying at 80oC. A leaf area meter was used to measured leaf area.

Key findings

Variation in the response between Pusa-677 and Basmati-1 under elevated CO2 was observed. Pusa Basmati-1 experienced a greater photosynthetic rate (46%) and greater decrease in stomatal conductance (44%) than Pusa-667 (25% & 34%).

Elevated CO2 was also found to alter the sugar compositions of both rice cultivars. Basmati-1 was again found to have a higher response compared to Pusa-677 with reducing sugars, non-reducing sugars and soluble sugars increasing by 29%, 25% and 26% compared with increases of 16%,17% and 22.7% seen in Pusa-677, although starch content was slightly higher in Pusa-677.

Protein profiling (figure 8) showed that both cultivars experienced a reduction of less molecular mass proteins and Rubisco protein under elevated CO2 and that Pusa-677 had a greater loss of 69% & 27% in the 3rd and 5th leaf compared to a loss of 53% & 17% in Basmati-1 rice (figure 8). There was an incease seen in proteins between 68.0 and 97.4 kDa. Despite the reduction of the small and large subunits of Rubisco elevated CO2 still had a posertive effect on the photosynthetic rate.

Pusa Basmati-1 showed a higher grain yield (40%) under elevated CO2 than Pusa-677 (24%) the tillers and leaves also appeared earlier under high CO2 for both cultivars. Number of tillers, and leaf area increased by 23.5% and 21% in Basmati-1 compared to an increase of 14% and 9.4 % in Pusa-677. The increased grain yield under elevated CO2 was due to an increase in grains per plant in Pusa-677 and grain number and partially grain mass in Basmati-1.

Conclusion

The most important information to emerge from this study is the differing extent of elevated CO2 impact on rice of different cultivars. It suggests that in order to form predictions of future crop development it is too reductionist to look at one cultivar of an important agricultural crop such as rice and make a blanket prediction of the extent at which future levels of CO2 will impact upon it.

The fact that the study was conducted in open top chambers may have influenced the results as the temperature and irradiance differed compared to the outside environment. To come to a more reliable prediction of the future impact of CO2 on rice a FACE study could be conducted.

CO2 enrichment increases water-use efficiency in sorghum

Conley, M.M. et al. (2001) New Phytologist 151:407-412.

Why the paper was chosen

Sorghum bicolor is a crop of particular importance in arid regions found in parts of Africa and Asia. Water limitation is responsible for large decreases in yield of sorghum; this may be reduced under future CO2 levels in the atmosphere. This study is therefore important for future predictions of sorghum yield in water limited environments. This paper investigates the evapotranspiration of sorghum under ample and deficient water conditions and using the FACE technique to expose the crop to elevated CO2 and an ambient control condition.

Methods

A total of eight FACE plots were used in this experiment with four under ambient levels of CO2 exposure (370 µmol mol-1) and four under elevated levels (570 µmol mol-1). The plots were maintained at elevated CO2 24 hours a day over two growing seasons.

Level-basin flood irrigation was used to create the wet (W) and dry (D) treatments. In the wet treatments 100% of the evapotranspiration was replaced and in dry water was supplied twice during the growing season.

The sorghum crop was planted on June 15th 1998 and July 16th 1999 and harvested on December 21st for the first year and October 26th for the second year. By adding the grain and stover together after harvest the total above grown biomass was found.

Key findings

The Sorghum bicolor grown in 1998 experienced two periods of drought stress (figure a) defined as a drop of soil water moisture below the field capacity. In 1999 the plant was under water stress three times the final of which continued to maturity (figure b).

In 1998 and 1999 seasons the cumulative evapotranspiration was significantly different in wet and dry plots. The FACE dry and ambient dry plots did not show significant difference over the season in 1998 but in 1999 a 6% reduction in cumulative evapotranspiration was observed. In both 1998 and 1999 the cumulative evapotranspiration for wet plots under FACE was reduced compared to ambient by 11% and 9% respectively.

There was a significant difference in wet and dry conditions for WUE-B and WUE-G in 1998 and 1999. In 1999 the FACE dry plot experienced a 45% greater WUE-G than ambient dry plots although no significant change was observed between the two wet conditions. The interaction between soil moisture and CO2 enrichment was not significant for WUE-B in 1999 although the CO2 effect was larger (26%) in the dry conditions that the wet (8%).

When an average was taken over both years and soil moisture conditions a decrease in evapotranspiration of 7% was found under elevated CO2. WUE-B and WUE-G were also increased by 16% and 14% under higher atmospheric elevated CO2.

Conclusion

The evidence found in this paper suggests that under elevated CO2 the WUE of Sorghum biolor will improve as drought stress increases. The productivity will be greater if future CO2 levels continue to rises as predicted and irrigation requirements will be decrease.

References

  • Assmann, S.M. (1999). The cellular basis of guard cell sensing of rising CO2. Plant cell and environment. 22:629-637.
  • Hetherington, A.M., Woodward, F.I. (2003). The role of stomata in sensing and driving environmental change. Journal of Nature. 424:901-908.
  • Raven, J. (2002). Selection pressures on stomatal evolution. New phytol. 153:371-386.
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