Photosynthesis and Quantum Yield of Oil Palm Seedlings to Elevated Carbon Dioxide

Photosynthesis is a metabolic process through which green plants synthesize organic compounds from inorganic raw materials in the presence of sunlight. This process can be regarded as a procedure of converting radiant energy of the sun into chemical energy of plant tissues in the form of organic molecules. Photosynthesis increases the total free energy available to organism and provides energy to the world, directly or indirectly, necessary for sustaining all forms of life on earth. Farming is basically a system of exploiting solar energy to synthesize organic matter through photosynthesis. The yield of crop plants ultimately depends on the size and efficiency of their photosynthetic system (Anderson, 2000). The most important factors of biomass production of any crop are the amount of radiation intercepted by the crop and the effectiveness of using the radiation in dry matter production. All organisms on earth need energy for growth and maintenance. As a result, higher plants, algae and certain types of bacteria capture this energy directly from the sunlight and utilise it for the biosynthesis of essential food materials for dry matter increase. The plant photosynthetic apparatus contain the necessary pigments in leaf able to absorb light and channel the energy of the excited pigment molecules into a series of photochemical and enzymatic reactions. Light energy is absorbed by protein-bound chlorophylls that are located in light-harvesting complexes and the energy migration to photosynthetic reaction centres results in electron excitation and transfer to other components of the electron transfer chain (Hall and Rao, 1999). Carbon dioxide is a trace gas in the atmosphere, presently accounting for about 0.037%, or 370 parts per million (ppm), of air. The partial pressure of ambient CO2 (Ca) varies with atmospheric pressure and is approximately 36 pascals (Pa) at sea level. The current atmospheric concentration of CO2 is almost twice the concentration that has prevailed during most of the last 160,00 years, as measured from air bubbles trapped in glacial ice in Antarctica. For the last 200 years, CO2 concentrations during the recent geological past have been low, fluctuating between 180 and 260 ppm. These low concentrations were typical of times extending back to the Cretaceous, when Earth was much warmer and the CO2 concentration may have been as high as 1200 to 2800 ppm (Ehleringer et al., 1991). However, with the rapid increases in world population and economic activity, a doubling of the present atmospheric [CO2], assuming a mean annual increase rate of 1.5 ppm, which was observed over the past decade 1984–1993 (Stoskoptf, 1981), could be expected before the end


Introduction
Photosynthesis is a metabolic process through which green plants synthesize organic compounds from inorganic raw materials in the presence of sunlight.This process can be regarded as a procedure of converting radiant energy of the sun into chemical energy of plant tissues in the form of organic molecules.Photosynthesis increases the total free energy available to organism and provides energy to the world, directly or indirectly, necessary for sustaining all forms of life on earth.Farming is basically a system of exploiting solar energy to synthesize organic matter through photosynthesis.The yield of crop plants ultimately depends on the size and efficiency of their photosynthetic system (Anderson, 2000).The most important factors of biomass production of any crop are the amount of radiation intercepted by the crop and the effectiveness of using the radiation in dry matter production.All organisms on earth need energy for growth and maintenance.As a result, higher plants, algae and certain types of bacteria capture this energy directly from the sunlight and utilise it for the biosynthesis of essential food materials for dry matter increase.The plant photosynthetic apparatus contain the necessary pigments in leaf able to absorb light and channel the energy of the excited pigment molecules into a series of photochemical and enzymatic reactions.Light energy is absorbed by protein-bound chlorophylls that are located in light-harvesting complexes and the energy migration to photosynthetic reaction centres results in electron excitation and transfer to other components of the electron transfer chain (Hall and Rao, 1999).Carbon dioxide is a trace gas in the atmosphere, presently accounting for about 0.037%, or 370 parts per million (ppm), of air.The partial pressure of ambient CO 2 (Ca) varies with atmospheric pressure and is approximately 36 pascals (Pa) at sea level.The current atmospheric concentration of CO 2 is almost twice the concentration that has prevailed during most of the last 160,00 years, as measured from air bubbles trapped in glacial ice in Antarctica.For the last 200 years, CO 2 concentrations during the recent geological past have been low, fluctuating between 180 and 260 ppm.These low concentrations were typical of times extending back to the Cretaceous, when Earth was much warmer and the CO 2 concentration may have been as high as 1200 to 2800 ppm (Ehleringer et al., 1991).However, with the rapid increases in world population and economic activity, a doubling of the present atmospheric [CO 2 ], assuming a mean annual increase rate of 1.5 ppm, which was observed over the past decade 1984-1993(Stoskoptf, 1981)), could be expected before the end be low or high temperature, deficiency or over supply of water or nutrient, low CO 2 or high O 2 concentration, and low light intensity.In the meantime, many plant internal factors including developmental hormones and respiration, etc. may also have a significant effect on net photosynthetic rate.The main limitation site of net photosynthetic rate in C3 plants, however, is often in the reaction centre catalyzed by the enzyme ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco).Therefore, reducing or eliminating its oxygenase function or photorespiration or increasing the affinity of the enzyme for CO 2 is a long-term goal to increase productivity of the plant (Xu and Shen, 2000).

Quantum yield
The quantum yield of a process in which molecules give up their excitation energy (known as "decay") is the fraction of excited molecules that decay via that pathway (Wells et al., 1982;Taiz and Zieger, 1991).The quantum yield of a process, such as photochemistry, has been defined (Clayton 1971;1980) mathematically as the yield of photochemical products divided by total number of quanta absorbed.For a particular process, the value of quantum yield can range between 0 (when the process does not involve any decay of the excited state) and 1.0 (when the process involves deactivation of the excited state).Taiz and Zieger (1991) explained that all possible processes would ultimately contribute to a sum 1.0 of the quantum yields.Basically, in functional chloroplast that is kept in dim light, the quantum yield of photochemistry is approximately 0.95, the quantum yield of fluorescence is 0.05 or lower, and the quantum yields of other processes are negligible.The vast majority of excited chlorophyll molecules, therefore, lead to photochemistry.The reciprocal of the quantum yield is called the quantum requirement.For a high yielding crop, it is not only a high photosynthetic rate in strong light is important but a high quantum yield in weak light is also crucial.Therefore, Ort and Baker (1988) believed that the improvement of future crop production should aim at increasing their quantum yield.It is well known that in photosynthesis of the Calvin-Benson cycle, the assimilation of one molecule of CO 2 into carbohydrate requires 2NADPH and 3ATP.The production of 2NADPH is the result of transporting four electrons from 2H 2 O to 2NADP along an electron transport chain.Because the chain includes two photosystems in series, two photons are needed for one electron transport.Thus, at least eight photons are required for the production of 2NADPH.Therefore, the maximal or theoretical quantum yield for photosynthetic carbon assimilation is 0.125 mole CO 2 /mole photons.In field studies Xu (1988) found that the apparent quantum yield of photosynthetic carbon assimilation often displayed a significant midday decline in many C3 plants such as soybean and wheat but not in C4 plants such as maize and sorghum.It was deduced that photoinhibition may be a cause of the midday decline of the photosynthetic efficiency (Xu et al. 1990).The molecular mechanism of photoinhibition, however, is still not fully understood.For more than a decade photoinhibition has been considered almost synonymous with photodamage to the photosynthetic apparatus (Nigoyi, 1999).In addition to photoinhibition, enhanced photorespiration is another cause of the midday decline in the photosynthetic efficiency of C3 plants (Guo et al., 1994).For a long time photorespiration has been considered a wasteful process.Many efforts have been made to eliminate or reduce photorespiration but no success has yet been reported.Extensive screening programs involving several species (wheat, barley, oats, soybean, potato, tall fescue) failed to identify genotypes with low CO2 compensation point (Hay and Walker, 1989).Attempts to select C3 plants with low rates of photorespiration and high rates of net photosynthesis also have had little success (Xu and Shen, 2000).
3.1 Factors affecting quantum yield 3.1.1External factors Emerson and Lewis (1943) showed that the values of quantum yield were related to the quality of light.A high quantum yield was measured at red light around 680 nm.The quantum yields of sun and shade leaves grown under different light intensities were similar, although there was a significant difference in light saturated photosynthetic rate between them (Oquist and Hallgren, 1982).At 21% O 2 and a temperature range of 15-35°C the quantum yield decreased gradually with temperature increase in C3 plants but not in C4 plants (Ku and Edwards, 1978;Xu and Shen, 2000).Water deficiency and excessive water or flooding could lead to a decline in quantum yield (Mohanty and Boyer, 1976).After several rainy days, the photosynthetic quantum efficiency became lower in spinach leaves (Li et al. 1991).The reason may be that the reduction of NADP is severely hindered in swollen chloroplasts under hypotonic conditions (Ye et al., 1995).Decreasing O 2 concentration or increasing CO 2 concentration in air could increase quantum yield in C3 plants but not in C4 plants (Monson et al., 1982).This may be due to decreased excitation energy transport from antenna pigments to PSII reaction centers and enhanced excitation energy dissipation as heat under phosphate deficiency conditions (Jacob, 1995;Xu and Shen, 2000).

Internal factors
Among all internal factors, photorespiration has the most significant effect on quantum yield.The effects of air temperature and CO 2 or O 2 concentration on quantum yield mentioned earlier, in fact, are related to the changes in photorespiratory rate caused by these factors.In normal air and at 20-25°C, the quantum yields of C3 and C4 plants were similar.However, when the air temperature was over 30°C, the quantum yield in C4 plants was slightly higher than that in C3 plants (Ehleringer and Pearcy, 1983).When photorespiration was inhibited by high CO 2 and/or low O 2 , C4 plants had about 30% lower quantum yields than C3 plants because they used two additional ATP molecules in the C4 pathway for fixation of one molecule of CO 2 to form carbohydrate (Osmond et al. 1980;Xu and Shen, 2000).

C 3 species response to elevated CO 2
The present atmospheric [CO 2 ] limits the photosynthetic capability, growth, and yield of many agricultural crop plants, among which the C3 species show the greatest potential for response to rising [CO 2 ] (Xu et al., 1984).Current atmospheric CO 2 and O 2 levels and C3 Rubisco specificity factors translate into photorespiratory losses of 25% or more for C3 species (Farquhar and Sharkey, 1982).The projection that a rise in atmospheric [CO 2 ] will reduce the deleterious effect of O 2 on C3 photosynthesis but that it has a negligible effect on C4 photosynthesis is indeed supported by experimental growth data.Exposure of C3 plants to elevated [CO 2 ] generally results in stimulated photosynthesis and enhanced growth and yield (Sharkawy et al., 1990).A compilation of the existing data available from the literature for C3 agricultural crops, including agronomic, horticultural, and forest tree species, shows an average enhancement in net CO 2 exchange rates up to 63% and growth up to 58% with a doubling of the present atmospheric [CO 2 ] ( Brinkman and Frey, 1978).Long-term exposure to elevated [CO 2 ] leads to a variety of acclimation effects, which include changes in the photosynthetic biochemistry and stomatal physiology and alterations in the morphology, anatomy, branching, tillering, biomass, and timing of developmental events as well as life cycle completion (Evan and Dunstone, 1970).A greater number of mesophyll cells and chloroplasts have been reported for plants grown under elevated [CO 2 ] (Poskuta and Nelson, 1986).In terms of dark respiration, the exposure of plants to elevated [CO 2 ] usually results in lowering the dark respiration rate, which can be explained by both direct and indirect effects (Pettigrew and Meredith, 1994).The mechanism for the direct effect appears to be an inhibition of the enzymes in the mitochondrial electron transport system, and for the indirect (acclimation) effect of elevated [CO 2 ] on dark respiration may be related to changes in tissue composition (Yin et al., 1956;Xu and Shen, 2000).Many C3 species grown for long periods at elevated [CO 2 ] show a down-regulation of leaf photosynthesis (Zelith, 1982); and carbohydrate source-sink balance is believed to have a major role in the regulation of photosynthesis through the feedback inhibition (Wells et al., 1986).Source-sink imbalances may occur during exposure to elevated [CO 2 ] when photosynthetic rate exceeds the export capacity or the capacity of sinks to use photosynthates for growth resulting in an accumulation of carbohydrates in photosynthetically active source leaves (Dong, 1991).Although growth is enhanced under elevated [CO 2 ] the extent to which starch and soluble sugars accumulate depends largely on the species differences.In many plants, the increase in starch also seems to be greater than that of soluble sugars.More frequently observed is the correlation between starch accumulation and inhibition of leaf photosynthesis (Wells et al., 1986) implying that high starch content may be responsible for down regulation of photosynthesis under elevated [CO 2 ].For many plant species, the long exposure to elevated [CO 2 ] has also resulted in a down-regulation of Rubisco (Mohanty and Boyer, 1976).Zhang et al. (1992) observed the down-regulation in cotton, cucumber, parsley, pea, radish, soybean, spinach, tobacco and wheat exposed to elevated [CO 2 ] due to increased leaf acid invertase activities, an indication of starch accumulation in the leaf; conversely, an up-regulation of photosynthesis in bean, plantain and sunflower was also detected suggesting variations in responses by species differences to elevated [CO 2 ].Levels of soluble sugars in plant cells have been shown to influence the regulation of expression of several genes coding for key photosynthetic enzymes (Osmond et al., 1980;Xu and Shen, 2000).The buildup in carbohydrates may signal the repression but does not directly inhibit the expression of Rubisco and other proteins that are required for photosynthesis (Oquist et al., 1982).Although the signal transduction pathway for regulation of the sugar-sensing genes may involve phosphorylation of hexoses, derived from sucrose hydrolysis by acid invertase via hexokinase (Guo et al., 1996), unknown gaps still exist between hexose metabolism and repression of gene expression at elevated growth [CO 2 ] (Hong and Xu, 1998;Xu and Shen, 2000).

Photosynthetic and quantum yield up-regulation under elevated CO 2
Carbon dioxide is the substrate that through the light and dark reactions of photosynthesis, are combined into dry mass (Pinkard et al., 2010).Thus [CO 2 ] can be a major factor limiting photosynthesis (Hall and Rao, 1992).Stomata regulate the diffusion of CO 2 into leaves; stomata can respond sensitively to [CO 2 ] as part of a proportionate response to the CO 2 requirement for photosynthesis; increasing concentrations are, therefore, associated with a closing response and vice-versa (Pinkard et al., 2010).Hence, elevated [CO 2 ] is anticipated to increase or up-regulate photosynthesis, decrease stomatal conductance and increase intrinsic water-use efficiency i.e. the ratio of leaf photosynthesis to stomatal conductance (Long et al., 2004).Many factors other than [CO 2 ] determine photosynthetic rate, and the law of limiting factors (von Liebig, 1840) will ultimately determine photosynthetic responses to [CO 2 ] where more than one limiting factors may be involved.Sala and Hoch (2009) suggested that elevated CO 2 would improve carbon balance in light-limited as well as highlight environments through the CO 2 enhancement of quantum yield.The present CO 2 :O 2 ratio of the air constrains photosynthesis by 30-40% because of O 2 inhibition of carboxylation and associated photorespiration (Booth and Jayanovic, 2005).As CO 2 concentration increases, quantum yield is increased because the ratio of carboxylation to oxygenation by Rubisco increases and photorespiration decreases (Pinkard et al., 2010).Several studies have shown that CO 2 enrichment enhances photosynthesis and growth under limiting irradiance condition, and in some cases the relative enhancement was greater at low rather than at high irradiances (Gifford et al., 1981).In C3 plants, elevated [CO 2 ] increases the quantum yield of photosynthesis by reducing photorespiration caused by the oxygenase activity of Rubisco.Maximum, single-leaf quantum yield was increased from 0•065 to 0•080 (Long and Drake, 1992).Up-regulation of photosynthesis refers to a significant increase in the light-saturated rate of photosynthesis (A sat ), the rate of photosynthesis under ambient light (A), and/or diurnal photosynthesis (A).Elevated [CO 2 ] up-regulates photosynthesis by increasing the carboxylation rate (Vc) of ribulose bisphosphate carboxylase (Rubisco) and competitively inhibiting the oxygenation of ribulose bisphosphate (RuBP), thereby reducing photorespiration (Luo and Reynold, 1999).Elevated [CO 2 ] is also associated with the expression of several other changes that affect photosynthesis.The common observation of reduced stomatal conductance, gs, will tend to dampen the extent to which any upregulation is expressed at a leaf-scale, but may conserve water such that stand-scale responses are positive (Ainsworth and Rogers, 2007).Photosynthetic acclimation refers to longer-term adaptive changes in the photosynthetic responses to external stimuli that reduce the net level of the initial response; acclimation is also referred to as down-regulation (Pinkard et al., 2010).Acclimation is commonly observed and arises from the plant's need to balance all resources that are allocated for photosynthetic processes, including the external [CO 2 ] ( Gunderson and Wullschleger, 1994).For elevated [CO 2 ], acclimation is mechanistically linked to decreased maximum apparent carboxylation velocity (Vc max ) and reduced investment in Rubisco (Rogers and Humphries, 2000).It is also associated with reduction in N content.These changes are linked to a decrease in control of A max by Vc max but an increase in the regeneration of RuBP, J max (Long and Drake, 1992).There is also an increase in starch and sugar content.In the short term, rising [CO 2 ] increases photosynthesis in many of the woody species as have been studied by Ainsworth and Long (2005).These species have the potential to yield significantly with increases in the rates of biomass accumulation.The allocation of dry mass to the above-ground parts in forest free air carbon dioxide enrichment (FACE) experiment was also found to increase by 28% on the average; this includes a greater allocation to woody components.In general, larger responses in growth, biomass production and leaf area index to elevated [CO 2 ] have been observed in trees than other functional types (Ainsworth and Long, 2005).Nevertheless, there is often a poor correlation between photosynthetic capacity measured as A max and total biomass production under elevated [CO 2 ] enrichment (Oren et al., 2001).

Environmental factors determining the response of photosynthesis to elevated [CO 2 ]
6.1 Nitrogen supply At elevated [CO 2 ] condition, when plant photosynthesis becomes RubP limited, Rubisco will be in excess of requirements (Ainsworth and Rogers, 2007).The excess capacity for carboxylation could be reduced through a reduction in the activation state of Rubisco (Cen and Sage, 2005).Alternatively, because less Rubisco is required by these plants at elevated [CO 2 ], redistribution of the excess N invested in Rubisco could further increase N use efficiency at elevated [CO 2 ] without negatively impacting potential C acquisition (Parry et al., 2003;Ainsworth and Rogers, 2007).However, there is only benefit in reducing the amount of N invested in Rubisco at elevated [CO 2 ] when the resources invested in it can be usefully deployed elsewhere (Parry et al. 2003).Ainsworth and Long (2005) reported that the stimulation in A sat at elevated [CO 2 ] was 23% lower in plants grown with a low N supply.Meanwhile, under elevated [CO 2 ] the Vc max was decreased at both high and low N, with greater reduction of 85% in low N condition.This result is in agreement with summaries of earlier studies conducted in controlled environments and field enclosures by Ainsworth and Rogers (2007), Drake et al. (1997), Moore et al. (1999) and Stitt and Krapp (1999), and is consistent with current understanding of the mechanism underlying acclimation under elevated [CO 2 ].When plants are N limited, sink development is restricted, C supply is in excess of demand, and the sugar feedback mechanism as outlined earlier can operate to reduce Rubisco content and increase N use efficiency.As N supply increases, the limitation imposed by sink capacity decreases and the sugar linked signal for down-regulating Rubisco content is reduced (Ainsworth and Rogers, 2007;Drake et al. 1997;Rogers et al. 1998;Long et al., 2004).

Sink strength
Defined here as the capacity to utilize photosynthate, sink strength can be a major constraint on carbon acquisition (Ainsworth and Rogers, 2007).A reduced or insufficient sink capacity may be the result of many potentially limiting processes e.g.N supply (Rogers et al., 1998), genetic constraints (Ainsworth et al., 2004), temperature (Ainsworth et al., 2003b) or developmental changes (Bernacchi et al., 2005;Rogers and Ainsworth, 2006).However, the net result is the same, i.e. the appearance of a carbohydrate-derived signal that can lead to the subsequent down-regulation (acclimation) of photosynthetic machinery, principally Rubisco (Stitt and Krapp, 1999;Long et al., 2004).Davey et al. (2006) showed that poplar grown at elevated [CO 2 ] had a large sink capacity.Poplar was able to export >90% of its photosynthate during the day; it also had a large capacity for the temporary storage of overflow photosynthate as starch (Ainsworth and Rogers, 2007;Stitt and Quick 1989;Davey et al., 2006).These two traits enabled poplar to maintain high photosynthetic rates at elevated [CO 2 ] and avoid a major source-sink imbalance that could lead to a reduction in the potential for C acquisition (Ainsworth and Rogers, 2007).In contrast, L. perenne can become extremely sink limited at elevated [CO 2 ] ( Rogers and Ainsworth, 2006).As reported, large accumulations of carbohydrate which build up in grasses over several days and weeks are common (Fischer et al., 1997;Isopp et al., 2000;Rogers & Ainsworth 2006).The most likely explanation for the sink limitation observed in L. perenne is an insufficient N supply (Fischer et al., 1997;Rogers et al., 1998).The excess of C and shortage of N may explain why grasses reduced their Rubisco content at elevated [CO 2 ] despite the negative impact on potential carbon gain (Rogers et al., 1998).Therefore, a reduction in carboxylation capacity would be expected.However, legumes can trade photosynthate for reduced forms of N with their bacterial symbionts (Rogers et al., 2006b).The benefit of an increase in N use efficiency resulting from the reduction of Rubisco content and the sugar-derived signal required for a reduction in carboxylation capacity would not be expected (Ainsworth & Rogers, 2007).It follows that acclimation in legumes is likely to occur through reduction in Rubisco activity rather than through a loss of Rubisco protein content.This occurs in order to maintain the balance between the supply and demand for the products of the light reactions (Ainsworth & Rogers, 2007).Alternatively, other nutrient limitations may also impact N-fixation and sink capacity at elevated [CO 2 ] (Almeida et al., 2000;Hungate et al., 2004).

Oil palm responses to elevated CO 2
An experiment was carried out using three levels of [CO 2 ] (400, control; 800 and 1200 µmol mol -1 CO 2 ) to demonstrate the responses of oil palm seedlings on photosynthesis and quantum yield to elevated [CO 2 ].Photosynthetic light response curves of oil palm were measured at growth CO 2 concentrations on with an open flow infrared gas analyzer with an attached red LED light source (LI-6400, Li-Cor, Inc., Lincoln, NE).Measurements began with approximately 5 minutes of saturating light (1500 µmol m −2 s −1 ) followed by nine incremental reductions until the irradiance was 0 µmol m −2 s −1 .Decreasing light was used rather than increasing light to reduce the equilibrium time required for stomatal opening and photosynthetic induction (Kubiske & Pregitzer, 1996).Preliminary trials indicated that photosynthetic rates reached steady state within 2 minutes following each incremental decrease in light.Measurements were made on fully expanded leaves.Gas exchange measurements were restricted to the hours between 0800 and 1200 hours on sunny days to minimize diurnal effects on photosynthesis.Leaf temperatures averaged 28.89 ± 0.89°C within each measurement period.The differences in light response curves due to CO 2 concentration were examined by calculating and statistically comparing light-saturated photosynthesis (A sat ), dark respiration (Rd), light compensation point (Γ) (where A = Rd), and apparent quantum yield (φ). Apparent quantum yield (φ) and Rd were estimated from the measured data.Values of φ were calculated as the slope of photosynthesis (A) versus the incident irradiance.Light compensation points (Γ) were estimated by extrapolating between measured data.The shape of the average light response curve in each CO 2 concentration and canopy position was modeled by fitting data to a non-rectangular hyperbola (Leverenz, 1987;Leverenz, 1995) by means of a nonlinear least squares curve-fitting program (JMP, SAS Institute, Inc., Cary, NC) Photosynthetic light response curves of oil palm are shown in Figure 1.Light-saturated net photosynthesis (A sat ) was greater in elevated treatment than in ambient CO 2 .The elevated [CO 2 ] exposure of oil palm seedling resulted in higher rates of A sat (P < 0.001; Figure 2a).The enhancement of A sat by [CO 2 ] enrichment was significantly greater for 1200 µmol mol -1 CO 2 followed by 800 µmol mol -1 and 400 µmol mol -1 CO 2. During the experiment, elevated CO 2 had caused the A sat of oil palm to increase by 52 to 78% compared to the ambient.Apparent quantum yield, calculated from the initial slope of the light response curves, was slightly lower in ambient treatment compared to the elevated CO 2 treatments (800 and 1200 µmol mol -1 CO 2 ; Figure 2b).During measurements, the elevated treatment exhibited a higher quantum yield than the ambient leaves (P < 0.05).Quantum yield was enhanced by 2 fold and 3 fold respectively in the 800 and 1200 µmol mol -1 CO 2 treatments compared to 400 µmol mol -1 CO 2 one.than the ambient leaves (Figure 2c; 2d).Estimated from the photosynthetic light response curves, it was also demonstrated a significant effect of elevated CO 2 on Γ or Rd (P≤0.01).Using the light response curves, the light compensation point for ambient CO 2 was recorded at 78.21 followed by 66.21 for 400 and 800 µmol mol -1 CO 2 respectively and 30.24 for 1200 µmol mol -1 CO 2 treatment.It was also observed that the dark respiration was reduced by 1.72 to 3.21 µmol m -2 s -1 compared to ambient levels that recorded 5.71 µmol m -2 s -1 .Many studies have suggested that the enriched CO 2 leaves respond to atmospheric CO 2 enrichment to a greater extent than the ambient-CO 2 leaves as a result of increased quantum yields (Hanstein & Felle, 2002).A small increase in quantum yield may increase daily carbon gain under low light conditions (Kiirats et al., 2002).In our study, elevated CO 2 increased apparent quantum yields in the 800 and 1200 µmol mol -1 CO 2 treatments.Light response curve analysis of oil palm seedling had showed that CO 2 enriched seedlings had reduced their dark respiration rate by 43 to 70% through enhancement of their A sat and apparent quantum yield (φ) by 52 to 78% and 15 to 62%, respectively.The enhancement of Г and φ signify direct inhibition of the activity of key respiratory enzymes under elevated CO 2 (Drake et al., 1997).This result has been supported by Henson and Haniff (2005) who reported productivity or dry matter production of plants would increase if respiration could be minimized without affecting gross assimilation, or if gross assimilation could be increased without increasing respiration.This shows that increase in CO 2 would enhance gross assimilation and reduce respiration by compensating respiration rate with high carbon gain.Usually, compensation irradiance is reduced while quantum efficiency is increased in plant under elevated CO 2 (Vavin et al., 1995).The same result was also observed by Kubiske and Preigitzer (1996) with red oak seedlings grown at elevated CO 2 in shaded open top chamber.In oil palm, it was found that enrichment with high levels of [CO 2 ] have enhanced the leaf gas exchange of oil palm seedlings.As CO 2 levels increased from 400 to 800 and 1200 µmol mol -1 CO 2 the net photosynthesis (Figure 3) and water use efficiency (Figure 4) were also improved.Net photosynthesis (A) and water use efficiency (WUE) were been enhanced by respective 211 to 278% and 158 to 224% when enriched with [CO 2 ] (800 and 1200 µmol mol -1 CO 2 ).As CO 2 levels increased, it was observed that the intercellular CO 2 (Ci) increased higher in oil palm seedling treated with high levels of [CO 2 ] (Figure 5).The Ci for 1200 µmol mol -1 CO 2 recorded the highest (361.11µmol mol -1 CO 2 ) value followed by that of 800 µmol mol -1 CO 2 (311.11µmol mol -1 CO 2 ) with the lowest at 400 µmol mol -1 CO 2 that recorded 289.12 µmol mol -1 .Up-regulation of A may as represented by increases in leaf intercellular CO 2 concentration (C i ) that could also be related to increase in the thickness of the leaves (high SLA) achieved under elevated [CO 2 ] that contains high photosynthetic protein a especially Rubisco (Ramachandra & Das, 1986).The latter might also up-regulate several enzymes related to carbon metabolism which simultaneously increase the C i (Anderson et al., 2001).This data imply that high A under elevated CO 2 could be due to more efficient net assimilation resulting from extra carbon fixation as exhibited by high C i per unit area which is related to increased thickness of mesophyll layer, mainly due to increased palisade layer (Lawson et al., 2002).Up-regulation of A may as represented by increases in leaf intercellular CO 2 concentration (C i ) may also be related to increase in the thickness of the leaves under elevated CO 2 that contain high photosynthetic protein especially Rubisco (Ramachandra and Das, 1986).The latter might up-regulate several enzyme related to carbon metabolism that simultaneously increase the C i (Anderson et al., 2001).This data implied that high A under elevated CO 2 could be due to more efficient net assimilation due to extra carbon fixation exhibited by high C i per unit area which is related to increased thickness of mesophyll layer, mainly due to increased palisade layer (Lawson et al., 2002).) Fig. 6.Stomatal conductance, gs as affected by CO 2 levels in oil palm seedlings Statistically, higher A was observed over the controlled plants as the levels of enrichment increased.Similar result was obtained by Downtown (1990) and Van and Megonigal (2001).Theoretically, exposure to higher [CO 2 ] would increase A by increasing the availability of the substrate (CO 2 ; Downtown, 1990).In the present study, the increase in A might be justified by reduced light compensation point (┌) and dark respiration rate (R d ), with the plant enriched with high CO 2 having enhanced apparent quantum yield and net assimilation rates (Kubiske & Preigitzer, 1996).Despite increases in A and WUE, stomatal conductance of oil palm seedlings enriched with high levels of CO 2 decreased as levels of CO 2 increases (Figure 6).In 400 µmol mol -1 CO 2 , stomatal conductance recorded a value at 42.3 mmol m -2 s -1 ; with increasing [CO 2 ] to 800 and 1200 µmol mol -1 the stomatal conductance documented lower values (27.1 to 32.3 mmol m -2 s -1 ).Further enhancing the plants to 800 and 1200 µmol mol -1 had shown to reduce stomatal conductance (g s ) of the CO 2 -enriched seedlings versus the ambient CO 2treated plants with lowest g s .The decreased g s simultaneously reduced the transpiration rate (E) of plant under elevated CO 2 .This phenomenon is usually reported in plant treated with high than ambient CO 2 (Rashke, 1986;Lodge et al., 2001;Lawson et al., 2002).It was believed that reduced g s might contribute to plant acclimation to high intercellular CO 2 (C i ) (Morrison & Jarvis, 1980).It was also found that nitrogen levels were influenced by CO 2 levels applied to the oil palm seedlings.From Figure 7 it is observed that the nitrogen levels were highest in leaves followed by stems and lowest in the roots.As [CO 2 ] levels increased from 400 to 1200 µmol mol -1 CO 2 the nitrogen content decreased highly in 1200 followed by 800 and lowest in 400 µmol mol -1 CO 2 .This implies that plant enriched with high levels of [CO 2 ] have high dilution of nitrogen content in the plant tissues.Nitrogen content was influenced by the application of CO 2 levels to the seedlings.As the levels of CO 2 increased from 400 to 1200 µmol mol -1 CO 2 , nitrogen content were found to be reduced.The decrease in nitrogen content with increasing CO 2 levels has been reported by Porteus et al. (2009).Several researchers attributed this phenomenon to decreasing uptake of nitrogen as transpiration rate (E) was decreased due to reduction in stomata conductance (g s ) under elevated CO 2 level (Conroy and Hawking, 1993).

Conclusion
The results demonstrated by the oil palm seedlings indicated a positive response of oil palm seedling to elevated CO 2 enrichment in term of enhanced photosynthesis rate and quantum yield as compared with ambient CO2 condition.The positive impact of oil palm seedlings to CO2 enrichment was shown by enhancement of the leaf gas exchange characteristics of oil palm.The positive responses have been shown to cause increases in net photosynthesis, Asat, apparent quantum yield and reduction of dark respiration rate and light compensation point.The findings suggest that in the next 22 nd century, it would be expected that oil palm to benefit from changes in the climate as long as temperature does not increase beyond the palm optimum level.Further research in the future needs to be conducted to confirm these effects especially involving many environmental conditions under elevated [CO 2 ].
Producing crops under climate change conditions, then, would be a growing challenge in the new agriculture of the world.

Fig. 2 .
Fig. 2. The maximum net assimilation rate Asat (a), apparent quantum yield (b), Light compensation point (c) and dark respiration rate (d) as affected by different CO2 levels in oil palm seedlings

Fig. 7 .
Fig. 7. Nitrogen levels as affected by CO 2 levels in different part of oil palm seedlings