CO2, assimilation rate

Farquhar, G.D., Hubick, K.T., Terashima, I., Condon, A.G. Richards, R.A. (1987). Genetic variation in the relationship between photosynthetic CO2 assimilation rate and stomatal conductance to water loss. In Progress in Photosynthesis Research, Vol. IV, ed. J. Biggins, pp. 5 209-12. Dordrecht Martinus Nijhoff. 

We will now develop an analytical framework to represent CO2 fixation in photosynthesis and its evolution in respiration and photorespiration (Fig. 8-16). The net flux of C02 into a leaf, Jcch> indicates the apparent (net) CO2 assimilation rate per unit leaf area by photosynthesis (see Fig. 8-1 for a measurement technique). The gross or true rate of photosynthesis, ygv minus the rate of CO2 evolution by respiration and photorespiration per unit leaf area, 7, equals Jcch-... 

Epron D., Godard D., Comic G., and Genty B. (1995) Limitation of net CO2 assimilation rate by internal resistances to CO2 transfer in the leaves of 2 tree species (Fagus Sylvaticah and Castanea-sativa Mill). Plant Cell Environ. 18(1), 43-51. 

Figure 2. Expected effects of improvements of RuBisCO properties on the photosynthetic CO2 assimilation rate. Kc, Km for CO2 ...

The CO2 assimilation rate (Ar) of salt-grown (no photoinhibition) plants showed a decrease of up to 22% in Ar. However, interaction of salt and photoinhibition resulted in an increased inhibition of Ar with 83-86% inhibition at 5 C and 35-55% at 20 C (Fig. 1). Stomatal conductance (gs) was more affected during interaction of salt and photoinhibition than the CO2... 

Fig.l The effect of salt stress and interaction with photoinhibition on CO2 assimilation rate... 

Changes in the capacity for carbon assimilation accompanied the loss of variable fluorescence. Following the transition to high irradiance carbon assimilation rapidly increased but as photoinhibition progressed the CO2 assimilation rate decreased slowly throughout the period of high... 

Catalase activity was completely inhibited 300 minutes after the addition of aminotriazole (Rgure 1a). This was accompanied by a decrease in the CO2 assimilation rate (Figure 1b). However, contrary to expectations the H2O2 concentration remained unaffected (Figure la). This decrease appears not to be due to increased H2O2 levels resulting in oxidation of the PCR cycle enzymes as previously suggested (9,10). 

Under drought Clare took a clear advantatge in photosynthesis rate with g values close to Seaton Park ones either bellow or above 1000 pE m s y so the superior A values must be atributed to nonstomatal effects that able Clare cvar to maintain higher CO2 assimilation rates (table 3). 

These data show that morphological and physiological modifications associated with water stress able cvar Clare to maintain higher rates of photosynthesis and plant production, but also that the water economy indexes were superior in this cultivar even under irrigation, showing the efficiency of this indexes and also the existence of intraespecific variability in photosynthesis rates under drought that able cultivar Clare to maintain better CO2 assimilation rates by water stressed leaves. 

CO2- assimilation rate The net CO assimilation rate (PN) was measured in a leaf chamber in which one of the leaves was... 

CO2 assimilation rate of leaves from Vicia faba plants fumigated for 38 days with different concentrations of SO ... 

Once the model was complete, it was adjusted to a steady state condition and tested using historic carbon isotope data from the atmosphere, oceans and polar ice. Several important parameters were calculated and chosen at this stage. Sensitivity analysis indicated that results dispersal of the missing carbon - were significantly influenced by the size of the vegetation carbon pool, its assimilation rate, the concentration of preindustrial atmospheric carbon used, and the CO2 fertilization factor. The model was also sensitive to several factors related to fluxes between ocean reservoirs. 

The possible effects of increased atmospheric CO2 on photosynthesis are reviewed by Goud-riaan and Ajtay (1979) and Rosenberg (1981). Increasing CO2 in a controlled environment (i.e., greenhouse) increases the assimilation rate of some plants, however, the anthropogenic fertilization of the atmosphere with CO2 is probably unable to induce much of this effect since most plants in natural ecosystems are growth limited by other environmental factors, notably light, temperature, water, and nutrients. 

In this chapter we review recent developments, concentrating mostly on the effects of water stress because of its great importance, but trying where possible to identify general problems. As noted above, accumulation of ABA in leaves is common under stress, and we review its effects on gas exchange. If the sole direct effect of ABA were to reduce stomatal aperture, then assimilation rate would also decrease because of a lower intercellular partial pressure of C02,pi. Some experiments have suggested that pi is little affected by ABA and that the capacity for CO2 fixation has actually decreased. The situation has become complicated with the observation (Terashima et al., 1988) that application of ABA can cause non-uniform... 

In the field, plants are normally not deprived of water rapidly. During slowly increasing water stress, the rate of CO2 assimilation generally decreases in virtually the same proportion as the rate of transpiration (or... 

Fig. 1. Rates of CO2 assimilation, A (/miol s ) leaf conductance, g (mol m s ) intercellular partial pressure of CO2, Pi (Pa) soil water potential and leaf water potential, xp (MPa) during gas-exchange measurements of a 30-day-old cotton plant, plotted against day after watering was withheld. Measurements were made with 2 mmol m sec" photon flux density, 30 °C leaf temperature, and 2.0 kPa vapour pressure difference between leaf and air (S.C. Wong, unpublished data).

Fig. 2. Rates of CO2 assimilation,. 4, and leaf conductances, g, as functions of intercellular partial pressure of CO2, p in Zea mays on various days after withholding watering. Measurements made with 9.5,19.0,30.5, and 38.0 Pa ambient partial pressure of CO2, 2 mmol m" s" photon flux density, 30 °C leaf temperature, and 2.0 kPa vapour pressure differences between leaf and air. Closed symbols represent measurements with 30.5 Pa ambient partial pressure of COj. Leaf water potentials were 0.05, - 0.2, - 0.5 and - 0.8 MPa on day 0, 4, 11 and 14, respectively (after Wong et al., 1985).

Fig. 3. Rate of CO2 assimilation. A, and leaf conductance, g, in Zea mays on various days after withholding watering. Data are those shown in Fig. 2 with 30.5 Pa ambient partial pressure of CO2 (after Wong et al., 1985).

At low irradiances, photosynthesis uses virtually 100% of the quanta, but in full sunlight, about 2000 imol quanta s , more quanta are available than can be used in photochemistry. Maximum rates of photosynthesis by Populus or Spinacia leaves of 15 and 70 jumol O2 m s , respectively, would require only 15 x 9 = 135 to 630 jumol quanta m s , or 10-40%. Leaves, therefore, need to be able to dissipate 60-90% of the quanta at high irradiance in an orderly manner such as non-radiative decay if they are to avoid the potentially damaging formation of oxygen radicals from reduced ferredoxin (Asada Takahashi, 1987). When plants are under a stress that restricts CO2 assimilation, excessive light will be reached at even lower irradiances. 

Wong, S.C., Cowan, I.R. Farquhar, G.D. (1985). Leaf conductance in relation to rate of CO2 assimilation. III. Influence of water stress and photoinhibition. Plant... 

Vegetation formation (see Table 3.9) Rate of CO2 assimilation (106tC/yr) ... 

Example 4.8. pH Change Resulting from Photosynthetic CO2 Assimilation As a result of photosynthesis with NOs assimilation, a surface water with an alkalinity of 8.5 X 10 eq liter" showed within a 3-h period a pH variation from 9.0 to 9.5. What is the rate of net CO2 fixation [Assume a closed aqueous system, that is, no exchange of CO2 with the atmosphere and no deposition of CaC03 (25°C), pATi = 6.3, pK2 = 10.2.]... 

Lakes often become greatly undersaturated in CO2 during photosynthesis because CO2 assimilation is faster than the resupply by gas exchange from the atmosphere. Under such circumstances—high pH, higher film thickness—the invasion rates of CO2 into the lake are more likely to be enhanced by reactions of CO2 to form HCOf. 

Table 2. CO2 fixation rates and optimum photosynthesis temperature (OPT) of two species- Acaena cylindrostachya and Senecio formosus of neotropical superparamo along an elevation gradient. Assimilation rates...

Under conditions of low photorespiration, about 90% of photochemical product is allocated to COj fixation. The progression from light limitation to light saturation in photosynthesis is illustrated in Pig. 3 A which shows a typical light saturation curve for photosynthesis illustrated in the shmb Juanulloa aurantiaca. The quantum requirement per mole COj fixed (on an incident irradiance basis) increases as the irradiance and rate of CO2 assimilation increase. The higher the flux the lower the efficiency of COj assimilation (Fig. 3B). As the ATP and NADPH requirements for COj fixation are independent of... 

CO2 assimilation was inhibited by 30 minutes of high light in the abscence of both O2 and CO2. On returning the plant to normal CO2, O2 and low light, assimilation rate recovered and was saturated by 60 minutes. Recovery was, however, incomplete (Fig. 1). Changes in stomatal conductance paralled the recovery of assimilation rate following inhibition. 

Effect of varying irradiance on the rate of CO2 assimilation and calculated internal CO2, Ci (A) and the xanthophyll contents of the leaf (B). Conditions of irradiance are given at the top of the figure. Other details are as foi figure 2. 

Aminotriazole (2mM) was applied through the transpiration stream of excised pea seedlings held in a water-jacketed glass cuvette. Rates of net CO2 assimilation (A) and transpiration (E) were measured by conventional gas exchange techniques under different CO2 and O2 concentrations, with or without PGA added to the transpiration stream. Photosynthetic photon flux density (PPFD) was maintained at 500 /nmol m s temperature at 24 2°C and atmospheric vapour deficit at 1 kPa. Results were normalized to steady state assimilation rates prior to commencement of treatment. 

The rate of inhibition of CO2 assimilation was proportional to the rate of carbon flow through the photorespiratory cycle. The inhibition was greatest at low CO2 concentrations and high O2 concentrations (Figure 2a, b). Enhanced rates of photorespiration wiil resuit in increased rates of H2O2 production both in the presence and absence of aminotriazoie. In the presence of aminotriazole the H2O2 must be metabolised by an aiternative method. This appears to interfere with the mechanism of CO2 assimiiation. 

---