Feature Article SUSHMA SARDANAnopr.niscair.res.in/bitstream/123456789/10194/1/SR 47(9) 23-25.pdf · SUSHMA SARDANA The rising carbon dioxide concentration in the atmosphere must be

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IN high school biology, we learned about a life-sustaining process called photosynthesis. Plants pullcarbon dioxide through tiny openings in their leaves,fix it as carbohydrates that, directly or indirectly,

supply almost all animal and human needs for food apartfrom providing oxygen. The principal factors affecting therate of photosynthesis are favourable temperature, the levelof light intensity, and the availability of carbon dioxide.

Most green plants respond quite favourably toconcentrations of CO2 well above current atmospheric levels.In fact, there is growing evidence that increases inatmospheric concentrations of CO2 may also have greatimpact on plant growth by affecting rates of photosynthesis.Trees absorb more carbon dioxide when the amount in theatmosphere is higher, but the increase is unlikely to offsetthe higher levels of CO2.

Actually it is one of the best-kept secrets in the debateon climate change that the vegetation on Earth would benefitgreatly from a higher level of carbon dioxide (CO2) in theatmosphere.

Positive EffectsThere are plenty of examples to show that if CO2 levelsincrease more than the present level of 360 parts per million,most plants would grow faster and larger because of moreefficient photosynthesis and a reduction in water loss. Therewould also be many other benefits for plants, among thembeing greater resistance to temperature extremes and otherforms of stress, better growth at low light intensities,improved root/shoot ratios, less injury from air pollutants,and more nutrients in the soil as a result of more extensivenitrogen fixation.

There are two important reasons for this productivityboost at higher CO2 levels. One is superior efficiency ofphotosynthesis. The other is a sharp reduction in water lossper unit of leaf area. This benefit comes from the partialclosing of pores in leaves, which is associated with higherCO2 levels. These pores, known as stomata, admit air intothe leaf for photosynthesis, but they are also a major sourceof transpiration or moisture loss. By partially closing thesepores, higher CO2 levels greatly reduce the plants’ waterloss—a significant benefit in arid climates.

There are marked variations in response to CO2 amongplant species. The biggest differences are among three broadcategories of plants—C3, C4, and Crassulacean AcidMetabolism or CAM—each with a different pathway forphotosynthetic fixation of carbon dioxide.

SUSHMA SARDANA

The rising carbon dioxide concentration in

the atmosphere must be viewed with

caution. It is inappropriate for public

discussion of the issue to focus only on the

hypothetical dangers of global

warming that might result from higher

carbon dioxide levels. It is important to

stress as well on the known benefits of

higher carbon dioxide concentration for

the productivity of food crops, trees, and

other plants.

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Most green plants, in forests that account for two thirdsof global photosynthesis, algae, and most major food cropsare C3 plants. C3 metabolism allows them to respond mostdramatically to higher levels of CO2. At current atmosphericlevels of CO2, up to half of the photosynthate in C3 plants istypically lost and returned to the air by a process calledphotorespiration, when photosynthesis rate is fast in theseplants. Elevated levels of atmospheric CO2 virtuallyeliminate photorespiration in C3 plants, makingphotosynthesis much more efficient.

Cereal grains with C3 metabolism, including rice,wheat, barley, oats, and rye, show yield increases rangingfrom 25% to 64%. Rice (the most eaten food in the world)was shown to increase mass and use less water with higherCO2 levels, meaning that the most important food in theworld highly benefits from CO2 increase. They alsoexperience a boost in photosynthetic efficiency in responseto higher carbon dioxide levels, but because there is littlephotorespiration in C4 plants, the improvement is smallerthan in C3 plants.

Instead, the largest benefit C4 plants receive from higherCO2 levels comes from reduced water loss. Loss of waterthrough leaf pores declines by about 33% in C4 plants witha doubling of the CO2 concentration from its currentatmospheric level as in case of corn, sugarcane, sorghum,millet, and some tropical grasses. As these plants arefrequently grown under drought conditions of hightemperatures and limited soil moisture, the yields improveeven when rainfall is lower than normal. They show yieldincreases ranging from 10 to 55%, resulting primarily fromsuperior efficiency in water.

The lowest response to higher CO2 levels is usuallyfrom the CAM plants that are already well adapted forefficient water use. However, some CAM plants likesucculents follow the C3 pathway when they are not underwater stress, that is, experience higher productivity atelevated levels of carbon dioxide.

If plants respond so well to additional carbon dioxide,then we would expect to see positive responses to thesubstantial increase in atmospheric CO2 over recent decades.Several pieces of evidence suggest exactly such a response.

Some EvidencesAccording to a report published by scientists with theFinnish Forest Research Institute in the 3 April 1992 issue ofScience magazine, a 25 to 30% increase was reported in thegrowing stock of forests in Austria, Finland, France, Sweden,

Switzerland, and West Germany between 1971 and 1990,and this growth was attributed in part to a 9% increase inatmospheric carbon dioxide during the same period.

In most green plants, productivity continues to rise upto CO2 concentrations of 1,000 ppm and above. For rice, theoptimal CO2 level is between 1,500 and 2,000 ppm. Forunicellular algae, the optimal level is 10,000 to 50,000 ppm.

An indoor garden with the carbon dioxide amountincreased from an ambient level of 300 ppm to a high levelof 2,000 ppm can nearly double plant growth. Experimentshave shown that plants can handle up to 10,000 ppmof CO2 with no ill effects on maintaining all plantresources at maximum and at a temperature not exceeding30°C (86°F).

Richard Norby of the Department of Energy and hiscolleagues have examined the responses to elevated carbondioxide levels in sweet gum trees. The experiment consistedof pumping tonnes of carbon dioxide into the plots, raisingthe concentration of carbon dioxide in the tree. The totalamount of carbon dioxide fixed into organic matter such asleaves, stems and roots, was found to be higher in plotsgiven extra carbon dioxide. The average increase was 24%.Fine root and wood production increased significantlyduring only the first year of treatment in response toelevated carbon dioxide.

Fine roots are important for water and nutrient uptake,but they have a short life and their carbon returns to the soilwithin a year. Initial results suggest that the increase incarbon supply to fine roots has increased the carbon contentof the soil (carbon dioxide fertilization). Norby cautions,however, that the positive effect of carbon dioxidefertilization is insufficient to halt the rising level ofatmospheric carbon dioxide.

A Russian study from 1961-1998 found that as carbondioxide increased the forest increased at the same rate. Pinetrees grown for 2 years at 600 ppm grow more than 200%faster compared to normal rates. If some types of foresttrees grow more rapidly then higher atmospheric CO2 holdsthe prospect of lowering timber costs and hence loweringhousing and furniture costs!

Trees and their seedlings grown under controlledenvironments or in open top chambers simulating theoutdoors have shown remarkable growth responses toelevated levels of CO2. Addition of carbon dioxide to blackwalnut, sugar maple, oak, ash, sweet gum, pine, andeucalyptus shows good results. The forestry department atMichigan State University has produced plantable trees inmonths, rather than years, by subjecting seedlings to 1000-ppm CO2 concentrations under optimal conditions of light,temperature, day length, and nutrients.

The Water Conservation Laboratory of the U.S.Department of Agriculture has shown that orange treesaccumulated 2.8 times more biomass in five years, and intheir first two years of production produced 10 times moreoranges.

It is also standard practice for laboratory scientistsworking with algae cultures to conduct their research inCO2-enriched environments. They cut costs by shorteningtheir season and better crops.

For over 100 years, nurserymen have been addingcarbon dioxide to their greenhouses to raise the yields ofvegetables, flowers, and ornamental plants. These

Trees absorb morecarbon dioxidewhen the amountin the atmosphereis higher, but theincrease is unlikelyto offset thehigher levels ofCO

2.

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greenhouse-grown vegetables, including tomatoes,cucumbers, and lettuce, show earlier maturity, larger fruitsize, greater numbers of fruit, a reduction in cropping time,and yield increases ranging from 10 to 70%, averaging 20 to50%. Tuber and root crops, including potatoes and sweetpotatoes, show dramatic increase in tuberization (potatoes)and growth of roots (sweet potatoes). Yield increases rangefrom 18 to 75%.

Greenhouse-grown flower crops, including roses,carnations, and chrysanthemums, grow to earlier maturity,and have longer stems and larger, longer-lived, morecolorful flowers. Yield increases range from 9 to 15%, witha mean of 12%.

New Mutations and AdaptationsSome researchers have reported the discovery of the firstknown plant with a genetic mutation that makes it stronglyinsensitive to increased levels of carbon dioxide, which willprovide additional information about the mechanism ofplants’ response to carbon dioxide levels. However, theresearchers caution that a number of factors in addition tofuture atmospheric carbon dioxide concentrations, such astemperature, precipitation and available nutrient levels, willneed to be considered before it will be possible tothoughtfully predict plant behavior based on molecularmechanisms.

Using Arabidopsis thaliana, a fast-growing, floweringplant used for genetic and developmental studies,Dominique Bergmann, an assistant professor of biology,Gregory Lampard, a postdoctoral fellow, and CoraMacAlister, a PhD student, found a unique structural regionon a protein with 10 sites that can be modified by a well-known, environmentally-controlled signaling pathway todictate the number of stomata a plant makes.

Japanese researchers have found a way to make plantleaves absorb more carbon dioxide in an innovation thatmay one day help ease global warming and boost foodproduction. According to chief researcher Ikuko Hara-Nishimura of Kyoto University, soaking germinated seedsin a protein solution raised the number of pores, or stomas,on the leaves that inhale CO2 and release oxygen. A largernumber means there are more intake windows for carbondioxide, contributing to lowering the density of the gas.Another effect is higher starch production in photosynthesis,the process in which green plants use CO2 and water toproduce sugar and other organic compounds as food andbio fuel.

In the experiments, the team used budding leaves ofthale cress, or Arabidopsis, which has a short life span oftwo months and is widely used as a model plant in biology.They found that the number of pores multiplied relative to

the concentration of the solution of the protein, which theresearchers named Stomagen, an expensive product,achieving a maximum of four times the number of pores ofan untreated plant. An alternative may be to geneticallymodify plants to have more pores.

Good News or Bad NewsSo, there’s an argument to be made that carbon dioxideconcentrations increase plant growth and abundance.Actually it’s misleading to say that, if CO2 is good for plants,it’s good for the environment. Research shows that withhigher carbon dioxide concentrations in the atmosphere wewould see more wood growth, but that there may also bemore pests due to higher temperatures.

While scientists disagree about the likely effects ofadditional carbon dioxide on global temperature, theygenerally agree a doubling of the carbon dioxideconcentration in the atmosphere, as is projected, wouldincrease plant productivity by almost one-third. Most plantswould grow faster and bigger, with increases in leaf sizeand thickness, stem height, branching, and seed production.The number and size of fruits and flowers would also rise.Root/top ratios would increase, giving many plants betterroot systems for access to water and nutrients.

It does not necessarily mean that such a doubling isgood for the planet. We do not know what the optimal levelof atmospheric carbon dioxide should be. So many variablescould be affected by a major increase in CO2 includingtemperature and a redistribution of water resources, thatthe honest observer has to conclude he does not really knowwhat will happen. Even so, the good news about plantgrowth makes it possible to project a number of features ofthe global ecosystem in the next century.

First, we can expect a rapid expansion of foodproduction that may offset some of the presumed adverseclimate effects. As crop yields rise with higher CO2 levels,the amount of land devoted to agriculture could decline. Itwill be much easier to protect environmentally sensitiveland areas from over-cultivation for crops.

Since C3 plants will benefit somewhat more than C4plants from higher CO2 levels, there will be some shift inthe mix of plants. Trees are C3 plants, so we can expectmore rapid reforestation and an enormous expansion inforest biomass. Of the 21 most important food crops, 17have C3 pathways. They include rice, wheat, barley, oats,rye, soybeans, field beans, mung beans, cowpeas, chickpeas,pigeonpeas, potatoes, sweet potatoes, cassava-yams, sugarbeets, bananas, and coconuts. The exceptions are corn,sorghum, millet, and sugarcane, which have C4 pathways,and which will probably decline in relative production. Onthe other hand, since 14 of the 18 most noxious weeds areC4 plants, rising levels of atmospheric CO2 will generallyfavour crop production over weeds.

Plants, directly or indirectly, provide 95% of the totalfood of the earth. Since plants are at the bottom of the foodchain, a boost in plant production should lead to majorincreases in bird, fish, and mammal populations as well.

Ms Sushma Sardana has been teaching biology in the higher classes at theDelhi Public School, R.K. Puram since the last 18 years. Address: K1/107,SF, Chittaranjan Park, New Delhi-110019

Most plants would growfaster and larger becauseof more efficientphotosynthesis and areduction in water loss.

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