CO₂ enrichment and plant response, change in quality and quantity of crop produce

                             The beneficial effects of atmospheric CO₂ enrichment may be divided into three distinct growth response phases. First is a well-watered optimum-growth-rate phase where a 300 parts per million increase in the CO₂ content of the air generally increases plant productivity by approximately 30%.

           Next comes a nonlethal water-stressed phase where the same increase in atmospheric CO₂ is more than half again as effective in increasing plant productivity. Finally, there is a water-stressed phase normally indicative of impending death, where atmospheric CO₂ enrichment may actually prevent plants from succumbing to the rigors of the environment and enable them to maintain essential life processes, as life ebbs from corresponding ambient-treatment plants.

Empirical records provide incontestable evidence of global changes: foremost among these changes is the rising concentration of CO₂ in the earth's atmosphere. Plant growth is nearly always stimulated by elevation of CO₂. Photosynthesis increases, more plant biomass accumulates per unit of water consumed, and economic yield is enhanced.

The profitable use of supplemental CO₂ over years of greenhouse practice points to the value of CO₂ for plant production. Plant responses to CO₂ are known to interact with other environmental factors, e.g. light, temperature, soil water, and humidity. Important stresses including drought, temperature, salinity, and air pollution have been shown to be ameliorated when CO₂ levels are elevated. In the agricultural context, the growing season has been shortened for some crops with the application of more CO₂; less water use has generally, but not always, been observed and is under further study; experimental studies have shown that economic yield for most crops increases by about 33% for a doubling of ambient CO₂ concentration. However, there are some reports of negligible or negative effects.

Plant species respond differently to CO₂ enrichment, therefore, clearly competitive shifts within natural communities could occur. Thought of less importance in managed agro-ecosystems, competition between crops and weeds could also be altered. Tissue composition can vary as CO₂ increases (e.g. higher C: N ratios) leading to changes in herbivory, but tests of crop products (consumed by man) from elevated CO₂ experiments have generally not revealed significant differences in their quality. However, any CO₂-induced change in plant chemical or structural make-up could lead to alterations in the plant's interaction with any number of environmental factors—physicochemical or biological. Host-pathogen relationships, defense against physical stressors, and the capacity to overcome resource shortages could be impacted by rises in CO₂.

Root biomass is known to increase but, with few exceptions, detailed studies of root growth and function are lacking. Potential enhancement of root growth could translate into greater rhizodeposition, which, in turn, could lead to shifts in the rhizosphere itself. Some of the direct effects of CO₂ on vegetation have been reasonably well-studied, but for others work has been inadequate. Among these neglected areas are plant roots and the rhizosphere. Therefore, experiments on root and rhizosphere response in plants grown in CO₂-enriched atmospheres will be reviewed and, where possible, collectively integrated.

In general, C₃ plant species are more responsive to atmospheric carbon dioxide (CO₂) enrichment than C₄-plants. Increased relative growth rate at elevated CO₂ primarily relates to increased Net Assimilation Rate (NAR), and enhancement of net photosynthesis and reduced photorespiration.

Transpiration and stomatal conductance decrease with elevated CO₂, water use efficiency and shoot water potential increase, particularly in plants grown at high soil salinity. Leaf area per plant and leaf area per leaf may increase in an early growth stage with increased CO₂, after a period of time

Leaf Area Ratio (LAR) and Specific Leaf Area (SLA) generally decrease. Starch may accumulate with time in leaves grown at elevated CO₂. Plants grown under salt stress with increased (dark) respiration as a sink for photosynthates, may not show such acclimation to increased atmospheric CO₂ levels.

Plant growth may be stimulated by atmospheric carbon dioxide enrichment and reduced by enhanced UV-B radiation but the limited data available on the effect of combined elevated CO₂ and ultraviolet B (280–320 nm) (UV-B) radiation allow no general conclusion. CO₂-induced increase of growth rate can be markedly modified at elevated UV-B radiation.

Plant responses to elevated atmospheric CO₂ and other environmental factors such as soil salinity and UV-B tend to be species-specific, because plant species differ in sensitivity to salinity and UV-B radiation, as well as to other environmental stress factors (drought, nutrient deficiency). Therefore, the effects of joint elevated atmospheric CO₂ and increased soil salinity or elevated CO₂ and enhanced UV-B to plants are physiologically complex.

High CO₂ Makes Crops Less Nutritious

Crops grown in the high-CO₂ atmosphere of the future could be significantly less nutritious. a new challenge as society reckons with both rising carbon emissions and malnutrition in the future.

Scientists generally predict that crop yields could fall in a warmer world—though higher atmospheric CO₂ by itself should raise yields, as plants find it easier to extract CO₂ from the air to make carbohydrates.

The effect climate change might have on the nutritional value of crops, as opposed to their yield, has been even murkier. Previous studies have given conflicting results.

The CO₂ levels expected in the second half of this century will likely reduce the levels of zinc, iron, and protein in wheat, rice, peas, and soybeans. Some two billion people, the researchers note, live in countries where citizens receive more than 60 percent of their zinc or iron from these types of crops. Deficiencies of these nutrients already cause an estimated loss of 63 million life-years annually.

C₃ Crops Hit Hardest

Conducted over six growth years on field sites in Japan, Australia, and the United States, the study compared crops grown in normal conditions with ones grown in nearby experimental plots where the air is enriched with CO₂ via open-air sprayers. The current atmospheric CO₂ level is 400 parts per million; in the enriched plots, it was between 546 and 586 parts per million, a level scientists expect the atmosphere to reach in four to six decades.

In addition to wheat, rice, peas, and soybeans, which all use a form of photosynthesis known as C₃, corn and sorghum, which use C₄ photosynthesis, a faster kind. There was relatively little effect of CO₂ enrichment on the nutritional value of the C₄ crops.

In the C₃ crops, however, there was significant declines in zinc and iron. The largest was a 9.3 percent drop in the zinc level in wheat. They also found reduced levels of protein in wheat, rice, and peas, but not in soybeans. In general, crops are losing nutrients as CO₂is going up.

Unfortunately, the new study sheds little light on why more CO₂ in the atmosphere should mean less nutritious plants. One hypothesis has been that plants in an enriched atmosphere produce so much carbohydrate that it dilutes the other nutrients.

The new study seems to rule out that hypothesis: Instead of a uniform dilution of all other nutrients in the crops, it found that nutrients changed unevenly when CO₂ was higher.

Quality and Quantity

The need to balance changes in yield against changes in the nutritional value of crops makes predicting the future of agriculture an even more complicated task.

"Rising global CO₂ increases yield and decreases water use by crops, and this is often presented as one positive of atmospheric change, environments will mean less nutritional crops, so that "increased quantity is at the expense of quality."

There should be a global effort to develop new breeds of wheat, rice, peas, and soybeans that show resistance to higher CO₂ levels. While the various cultivars of wheat, peas, and soybeans in the study all suffered similar nutrient losses in response to higher CO₂, rice offered a ray of hope: Its cultivars varied wildly in their response.

CO₂ concentration

CO₂ is vital for plant growth, as it is the substrate for photosynthesis. Plants take in CO₂ through stomatal pores on their leaves. At the same time as CO₂ enters the stomata, moisture escapes. This trade-off between CO₂ gain and water loss is central to plant productivity. The trade-off is all the more critical as Rubisco, the enzyme used to capture CO₂, is efficient only when there is a high concentration of CO₂ in the leaf. Some plants overcome this difficulty by concentrating CO₂ within their leaves using C₄ carbon fixation or Crassulacean acid metabolism. However, most species used C₃ carbon fixation and must open their stomata to take in CO₂ whenever photosynthesis is taking place.

The concentration of CO₂ in the atmosphere is rising due to deforestation and the combustion of fossil fuels. This would be expected to increase the efficiency of photosynthesis and possibly increase the overall rate of plant growth. This possibility has attracted considerable interest in recent years, as an increased rate of plant growth could absorb some of the excess CO₂ and reduce the rate of global warming. Extensive experiments growing plants under elevated CO₂ using Free Air Concentration Enrichment (FACE) have shown that photosynthetic efficiency does indeed increase. Plant growth rates also increase, by an average of 17% for above-ground tissue and 30% for below-ground tissue. However, detrimental impacts of global warming, such as increased instances of heat and drought stress, mean that the overall effect is likely to be a reduction in plant productivity. 

Reduced plant productivity would be expected to accelerate the rate of global warming. Overall, these observations point to the importance of avoiding further increases in atmospheric CO₂ rather than risking runaway climate change.

A regional climate change model (PRECIS) for China, developed by the UK's Hadley Centre, was used to simulate China's climate and to develop climate change scenarios for the country. Results from this project suggest that, depending on the level of future emissions, the average annual temperature increase in China by the end of the twenty-first century may be between 3 and 4 °C. Regional crop models were driven by PRECIS output to predict changes in yields of key Chinese food crops: rice, maize and wheat. Modelling suggests that climate change without carbon dioxide (CO₂) fertilization could reduce the rice, maize and wheat yields by up to 37% in the next 20–80 years. Interactions of CO₂ with limiting factors, especially water and nitrogen, are increasingly well understood and capable of strongly modulating observed growth responses in crops.

More complete reporting of free-air carbon enrichment experiments than was possible in the Intergovernmental Panel on Climate Change's Third Assessment Report confirms that CO₂ enrichment under field conditions consistently increases biomass and yields in the range of 5–15%, with CO₂ concentration elevated to 550 ppm Levels of CO₂ that are elevated to more than 450 ppm will probably cause some deleterious effects in grain quality. It seems likely that the extent of the CO₂ fertilization effect will depend upon other factors such as optimum breeding, irrigation and nutrient applications.

The modelling work took into account climatic variables, irrigation, soil variables and the influence of higher atmospheric concentrations of carbon dioxide (CO₂) on plant metabolism. In general, climate change itself tends to reduce crop yield but the fertilization effect of CO₂ tends to increase yield. The balance between these two effects is likely to depend, in reality, on factors such as the availability of water and nutrients and the prevalence of pests and diseases, all of which are also likely to be affected by climate change.

CO₂ fertilization effectively offsets yield decreases caused by shorter growth duration due to higher temperatures in rice

If the direct effect of CO₂ is included, average yields are projected to increase for rainfed maize and decrease for irrigated maize in the 2080s. The increase is likely to be highest for rainfed maize because the higher CO₂ concentration would boost the yield of rainfed maize under the current water-limited conditions prevalent in North China (the biggest maize cultivation area). Without the CO₂ fertilization effect, the average yield of both rainfed and irrigated maize is likely to fall because the higher temperature may shorten the growth period by between 4 and 8 days. While irrigation might counteract the trend towards a decrease in yield (assuming sufficient water is available), it is not expected to stop it completely.

Yield decreases would be greatest if higher temperatures occur during the period when the maize ears are swelling. These results show a large relative benefit to maize yields from elevated CO₂. This is in contrast to most C₄ crop experiments which show minor absolute changes in yield due to CO₂ enrichment. As with rice, there is large regional variability in the yield change. This could be due to the use of calibrated irrigation and nutrition parameters in the model which were validated under present CO₂ concentration rather than in a higher CO₂ environment.

In wheat, if the effect of CO₂ fertilization is included, average wheat yields are shown to increase in China in the 2020s, 2050s and 2080s for both rainfed and irrigated wheat. Spatial variability is again large. The response of wheat to future atmospheric CO₂ increases is likely to significantly constrain potential increases in yield. But for irrigated wheat to benefit from the effects of CO₂ fertilization, nutrients need to be non-limiting. Without CO₂ fertilization, wheat yields are expected to be 10-20% lower by 2080 compared with current yields.

Effects of elevated CO₂ on grain quality

Wheat of two genotypes was grown in the field under CO₂ gradient enrichment (CGE—half of open) with a controlled chamber in the Chinese Academy of Agricultural Science experiment station in Beijing in 2001–2002. The gradient CO₂ enrichment was from 451 to 565 mg kg⁻¹. Measurement for effects of elevated CO₂ on grain quality showed that: (i) protein content for flour was found to significantly decrease with CO₂concentration gradient enrichment (at range 57 μmol  mol⁻¹; (ii)the sedimentation value of ZhongYu five was found to decrease a little. Even though there were some errors and uncertainties due to limited samples, significant differences between different varieties still exist after strict measurements. These results indicate that elevated CO₂ levels may cause a decrease in the quality of bread wheat due to generally lowered protein content.

The above results from CGE experiments have confirmed the results from previous studies: elevated CO₂ can cause more or less deleterious effects on grain quality. In addition, there would be distinct differences between varieties. Most of the experiments showed reductions in grain nitrogen content or grain protein at elevated levels of CO₂, although some found no significant effect of elevated CO₂ on grain quality.

Rises in the concentration of CO₂ in the atmosphere are likely to be accompanied by temperature increases. Small increases in temperature (2–4 °C) had a larger effect than elevated CO₂ on grain quality. Moreover, the effects of elevated CO₂ on grain quality may be partially balanced because temperature increases can enhance grain protein content. However, it is unlikely that any high temperature effects will totally compensate for CO₂ enrichment. Data from Kimball's experiments (2001) suggest that adequate fertilizer is necessary to attain good quality grain and that with ample fertilizer the deleterious effects of elevated CO₂ will be minor. Furthermore, crops grown with limiting levels of nitrogen probably have poorer quality grain than they could have. CO₂ enrichment in the atmosphere during coming decades is likely to make the quality poorer still.

Potential adaptation technologies of acclimation to CO₂ fertilization

Measurements have shown that with prolonged exposure to elevated atmospheric CO₂, the photosynthetic rate gradually declined, approaching or even less than that in ambient. These results indicate an acclimation or down regulation to the higher CO₂ levels. But CO₂ fertilization still can be favoured for adaptation in a future climate. A detailed understanding of CO₂ fertilization should be taken into account in developing adaptation technology.

New varieties by seed selection are one of the key methods of increasing crop yield and improving crop quality as well as adapting to environment change. With increasing ambient CO₂ concentration and a warmer climate, especially in winter, new crop varieties with high yield, warm-winter resistance under higher CO₂ should be favoured for adaptation in a future climate. For rice, cross breeding of Indica and Japanica varieties is considered ideal for enhancing morphological characteristics.

Improving crop cultivation would be another helpful technique for acclimation of crops to the CO₂ fertilization effect. For example, adjusting crop planting time could avoid light energy loss while adjusting the planting area and region of C₃ and C₄ crops and increasing plant density could increase the accumulation and efficient use of CO₂.

It is very difficult to understand the interactive impact of elevated atmosphere CO₂ and raising temperature on crop growth and yield formation. More CO₂ fertilization can be practiced through adjusting planted crop distributions and sowing times.