CO2 compensation point

Figure 17.15 Carbon dioxide exchange rate (net photosynthetic rate) of a 20 potato stand at different CO2 concentrations. The photosynthetic rates saturated above 1200 ppm while the CO2 compensation point occurred at 97 ppm (source Wheeler et al., 2008a).

The atmospheric CO2 concentration at which the CO2 evolved by respiration and photorespiration is exactly compensated by CO2 consumption in photosynthesis, leading to no net CO2 uptake, is known as the C02 compensation point. We can use the electrical circuit in Figure 8-17 and Equation 8.30 derived from it to demonstrate the CO2 compensation point for photosynthesis in terms of forces and fluxes. In particular, if we steadily decrease for a leaf initially having a net uptake of CO2, Jcoz will decrease and eventually will become zero when is... 

The CO2 compensation point is much higher for a C3 plant than for a C4 plant. At the compensation point, c1 - is equal to and... 

The electrical circuit in Figure 8-17 and Equations 8.30 and 8.31c developed from it are able to portray the CO2 compensation point, the light compensation point, as well as the general interrelations of the fluxes of CO2 for photosynthesis, photorespiration, and respiration in the light and in the dark. Our discussion and Figure 8-17 are for C3 plants—to apply an electrical circuit analog to C4 and Crassulacean acid metabolism (CAM)... 

C4 also metabolism reduces transpiration by lowering the CO2 compensation point (Fig. 13.14). Photosynthesis is slower when there is less CO2 about, and the compensation point is definerl ac fVio TPOd needed to give net photosynthesis. 

The ratio of the specificity in the carboxylase reaction to that of the oxygenase reaction is called "relative specificity (Sr)" [7]. This is a constant determined by the enzymatic properties of RuBisCO. RuBisCO from a photosynthetic bacterium, Rhodospirillum rubrum, has the Sr value of 10 the oxygenase reaction proceeds two-times faster than the carboxylase reaction in the ordinary atmosphere. The upper most in the Sr value has been thought to be possessed by RuBisCO from higher plants. The value reaches 90 to 95 [8] the carboxylase reaction is three to four times faster than the oxygenase reaction. The Sr value is a determinant of the CO2 compensation point, where CO2 fixation by RuBisCO and CO2 release from photorespiration are balanced and no net CO2 fixation is observed. 

In another project in our laboratories, transformation of the tabacco plastid genome has been achieved [20]. Introduction of the gene for improved RuBisCO into the plant will be our next interest. If a Cs plant has Galdieria RuBisCO for its photosynthetic CO2 fixation, the CO2 compensation point of the transformant will decrease from 50 ppm to 20 ppm. The transformant can fix CO2 at a considerable rate photosynthetically at 50 ppm where the ordinary Ca plants cannot adsorb CO2. Thereby, the water-use efficiency of the transformant will be substantially improved. 

These benefits, thought to be the result of direct influences on physiological processes of the treated plants, are referred as to physiological effects [48a], and have been most extensively studied with kresoxim-methyl and pyradostrobin. These include effects like delayed senescence, altered CO2 compensation point, reduced stomatal aperture and water consumption, and better tolerance of oxidative stress. Significantly altered levels of enzyme activities (ACC synthase, nitrate reductase, peroxidases, alternative oxidase AOX) could be observed or inferred indirectly in vivo, but in no attempted cases directly with isolated enzymes in vitro. The simplest and therefore most convindng hypothesis to explain all these many... 

We recognize that the area participating in gas exchange does not appear in the equation. The computed Cj is correct for that part of the leaf in which stomata are open, but not for the other part in which they are closed. In the latter, Cj should be at the CO2 compensation point. If ABA caused a binary stomatal response, it would produce a reduction of the photosynthetically active area which would appear as a depression at constant Cj and a reduced photosynthetic capacity of the leaf over the whole range of partial pressures of CO2 (dashed line in Fig. 2, right). 

Osmond and Bjorkman (1975) found that light-dependent CO2 fixation in K. daigremontiana was inhibited by O2 in the same manner as C3 plants, whereas dark CO2 fixation was independent of 4%, 26%, or 36% O2. Further, O2 inhibition of photosynthetic CO2 fixation is accompanied by a high CO2 compensation point (Allaway et al., 1974a). Unlike C3 plant photorespiration, however, the O2 inhibition of photosynthetic CO2 fixation is not eliminated by high CO2 tension, and the postillumination CO2 burst is present at low O2 (Osmond and Bjorkman, 1975 see Fig. 5.13). 

In Kalanchoe daigremontiana, Allaway et al. (1974 a) estimated the CO2 compensation point at the end of a 16-h day, when some constancy of light CO2 fixation was approached, at 51)0,1/1 ( = 92ngcm ) (saturating light intensity). This value is comparable to that of C3 plants, i.e., higher than that of C4 plants (cf. Downton, 1971). 

A = CO2 concentration difference between air source and tissue sink. Usually, it is assumed that the ambient air is constant at about 300 ppm CO2 or 58.9 mg CO2 cm 10 at standard pressure, and the tissue sink is 0. Alternatively, A can be calculated as internal CO2 minus the CO2 compensation point, r = as above (for typical r values see Table 6.1). 

Figure 8-18. Dependence of net CO2 uptake on external C02 level for leaves of representative C3 and C4 plants. C3 plants require a higher Noo2at tlie 02 compensation point (Jco2= 0) and for C02 saturation than C4 plants. We note that because photosynthesis for C4 plants is already nearly saturated at current atmospheric CO2 levels, higher C02 levels generally will not substantially enhance their photosynthetic rates, whereas the increasing atmospheric C02 levels will progressively increase net C02 uptake for C3 plants.

If we reduce the amount of light incident on a leaf from the value for direct sunlight, we eventually reach a PPF for which there is no net CO2 uptake (Fig. 8-19). This PPF for which Jcch is zero is known as the light compensation point for photosynthesis. Because photorespiration depends on photosynthetic products, both photorespiration and gross photosynthesis decrease as the PPF is lowered. Hence, the light compensation point for leaves is approximately the same for C3 and C4 plants—at 20° C and 380 pmol CO2 mol-1 in the turbulent air near a leaf, light... 

We next consider the light compensation point for CO2 fixation by leaves. As we mentioned in Chapter 8 (Section 8.4D), light compensation generally occurs at a PPF of about 10 pmol m-2 s-1 for a leaf temperature near 20°C... 

A. If the light compensation point for CO2 fixation is at a PPF of 8 pmol m-2 s-1, what are the cumulative leaf areas per unit ground area for light compensation when 2000, 200, 20, and 0 pmol m-2 s-1 are incident on the canopy ... 

D. The maximum upward flux of CO2 will occur for the level at the light compensation point (leaves below this level have a net evolution of C02)- At 200 pmol m-2 s-1 incident on the canopy, light compensation occurs at F= 4.6 (calculated in A). Because the leaf area index is 8.0, Fincreases by 1.0 every 2.0 m (16 m/8.0). Therefore, an F of 4.6 occurs at 9.2 m from the top of the canopy, or 6.8 m above the ground. 

Stomata close under dry conditions, and when this happens, the levels of CO2 in the mesophyll air spaces fall towards the compensation point. Since this is lower in C4 plants, they can photosynthesise with narrower stomatal apertures, so only lose half as much water per CO2 fixed as C3 plants. So although we may regard the C4 syndrome as an adaptation that reduces photorespiration, as far as plants are concerned, it is an adaptation to reducing water loss in hot climates. 

Hence, a net C02 uptake, and therefore a net C02-incorporation into carbohydrates by photosynthesis versus CC>2-release by respiration, is only possible above a critical external CO2 concentration cgo2 referred to as the compensation point (c.p.). Evidently, according to Eq. (9) the compensation point in photorespiring1 plants is considerably higher than in other plants. 

Typical values at 23 °C for the compensation point are 5—10 ppm C02 for non-photorespiring plants (e.g. sugarcane, maize, sorghum) and 40—100 ppm CO2 for photorespiring plants (e.g. wheat, tomato, tobacco)21 as compared with normal C02 concentration in air of 320 ppm. The numerical values of the types of resistances of Eq. (9) vary considerably for different plants and environmental conditions (s. e.g. Ref.134. The C02 input rates under good conditions are of the order of 1 -A nmoles... 

Fig. 4.2. Dependence of photosynthesis intensity on light intensity for sun-adapted and shade-adapted plants. At the beginning, CO2 consumption is less than the amount released by respiration. At certain intensity of illumination the compensation point is achieved when the C02 assimilation is equal to C02 production. Photosynthesis is accelerated only up to optimal intensity of Ulumination over-intensive light can inhibit... 

The observed quenching of Chi a fluorescence when 25 pM DIC was added to Illuminated cells at the compensation point (Fig. 2) was due primarily to HC03 transport as little quenching occurred in the absence of 25 mM NaCl (not shown) and CO2 transport was greatly limited by the CO2 supply rate. Uptake of HCO3" was also indicated by the observation that photosynthesis exceeded the CO2 supply rate by 11.7 fold. (Fig. 2). That CO2 transport occurred, however, was shown by the fact the extracellular [CO2] remained near zero and well below the anticipated equilibrium concentration. 

Light compensation point the light intensity at which the rate of photosynthesis (COj incorporation) and the rate of respiration (CO2 production) are balanced. See C02-compensation point. 

Under both laboratory and field conditions, a burst of CO2 uptake occurs immediately after the onset of illumination (see Chap.5.1.3.1 and Fig. 5.1). Normally, this initial burst reaches its maximum within 30-60 min after illumination. Subsequently, the CO2 exchange curve declines steeply towards the compensation point. 

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