Photosynthesis as a Diffusion Process
Net photosynthesis, also called C02 assimilation (A), is the difference between biochemical C02
fixation (P) and respiration (Rl)> which takes place in the leaf simultaneously:
Respiration consists of a number of partial pro cesses:
P = Photosynthesis Net photosynthesis respiration
Fig. 2.4.1. Schematic presentation of C02 fluxes of a leaf. The flux of C02 into a leaf corresponds to the rate of photosynthesis minus respiration occurring at the same time. The net influx is known as net photosynthesis or C02 assimilation spaces of the mesophyll is smaller than in the external air. The following equation applies to the transport via stomata:
respiration in mitochondria of heterotrophic cells (Rhet)> which do not possess chloro-plasts, but are indispensable for the function of the leaf (conducting tissues, epidermis); respiration in mitochondria of photosynthe-tically active cells (Rmit) which depends on ATP concentration in the cytosol and thus on photosynthesis;
light-induced respiration (photorespiration,
Because it is difficult to quantify these partial processes in an illuminated leaf, net photosynthesis is used in ecology as the standard for assessments of the plant carbon balance (Fig. 2.4.1) and it is usually measured as the net C02 exchange. Only the net exchange can be measured in the field. Basically, it should also be possible to measure the 02 exchange. However, this is much more difficult because of the higher background concentration of 02 in the atmosphere. However, measurement of 02 becomes much more important globally (see Chap. 5).
C02 assimilation is a diffusion process, i.e. C02 from the atmosphere diffuses into the leaf as the C02 concentration in the intercellular
Ca = C02 concentration of atmosphere where Ca is the C02 concentration in the atmosphere, Q the C02 concentration in the mesophyll and gs the stomatal conductance. It is the same conductance which regulates the water vapour flux (see Chap. 2.2.3), but the conductance of stomata for C02 is lower by a factor of 1.6, corresponding to the higher molecular weight of C02 as compared with H20 (gco,/gH,o= 1-605; Jarvis 1971).
It would appear reasonable to also express the movement of C02 between Q and the chloroplast as a diffusion process. However, as already noted, respiratory processes occur in the leaf [Eq. (2.4.1)], and also the transport of C02 in the cell wall occurs not only in the dissolved form, but also partially as HCO3. Because of the respiratory processes taking place in different tissues, a C02 concentration, the compensation point (/), is reached with decreasing C02 concentration, where C02 assimilation and respiration are balanced. Therefore, the following analogue equation applies:
Fig. 2.4.1. Schematic presentation of C02 fluxes of a leaf. The flux of C02 into a leaf corresponds to the rate of photosynthesis minus respiration occurring at the same time. The net influx is known as net photosynthesis or C02 assimilation where gm is the conductance of the mesophyll and represents all leaf-internal transport processes. If the C02 concentration in the atmosphere is decreased experimentally below the compensation point, increased or decreased C02 release may occur, depending on the experimental conditions. Under such conditions the photosystems may be irreversibly damaged (Schäfer 1994; see also Chap. 1.2.1).
The response of C02 assimilation to the C02 concentration in the atmosphere describes the physiological state of photosynthesis in the leaf and is called a response curve (Fig. 2.4.2) with three important ranges:
• a linear range above the compensation point: In this range C02 assimilation is limited by the activation of RuBP-carboxylase by the substrate C02 (Farquhar and von Caemmerer 1982; Lange et al. 1987). The increase (AA/ Ac) describes the efficiency of RuBP-carboxy-lase. According to Eq. (2.44), gm = A/(C!-r);
• a saturation range at high C02 concentration: With increasing supply of C02, RuBP-carb-
Fig. 2.4.2. Diagram of the dependence of C02 gas exchange on the C02 concentration in the air. The y-axis shows positive values as C02 assimilation and the negative values as respiration. The linear part of the curve shows enzyme limitation, the flat part shows saturation because of substrate limitation. The inserts show that change in light only affects the saturated region, whereas drought operates via enzyme limitation. For explanation, see text
Fig. 2.4.2. Diagram of the dependence of C02 gas exchange on the C02 concentration in the air. The y-axis shows positive values as C02 assimilation and the negative values as respiration. The linear part of the curve shows enzyme limitation, the flat part shows saturation because of substrate limitation. The inserts show that change in light only affects the saturated region, whereas drought operates via enzyme limitation. For explanation, see text oxylase is fully activated, but C02 assimilation is limited by the supply of the substrate ribu-lose-l,5-bis-P (RuBP), to which the carbon of C02 is bound. In this range the turnover rate of RuBP-carboxylase is saturated and it cannot use the supplied C02. Because of the limited capacity of the Calvin cycle, C02 assimilation finally reaches a maximum rate at which it is C02 saturated; • the range below the compensation point: In C02-free air with 20% 02 the oxygenase function of RuBP-carboxylase/oxygenase is fully activated, i.e. light-induced respiration (Rphot) reaches its highest value. At the same time, dependent on ATP consumption in the cyto-sol, mitochondrial dark respiration (Rmit) i® activated. Rmit decreases with increasing rates of photosynthesis as ATP is also formed in photosynthesis and is probably reduced under conditions of assimilation above the compensation point (Loreto et al. 2001). With de creasing C02, the Rmit approaches asymptotically the rate of respiration of chlorophyll-free heterotrophic leaf cells (Rhet)- Light-induced respiration (Rphot) decreases dependent on the C02/02 ratio and is very low at C02 saturation.
Whilst these processes of photosynthesis and respiration occur in an intact leaf, C02 supply to the mesophyll is limited by the diffusion resistance of stomata. Analogous to a current/voltage diagram, C02 concentration decreases down the diffusion path from ambient air to the intercellular spaces of the mesophyll (see Fig. 2.4.2). Starting from the ambient C02 concentration, the C02 influx into the mesophyll increases with decreasing concentration in the intercellular spaces (Q). Plotting Ca against Q results in a straight line corresponding to the increase in stomatal conductance. According to Eq, (2.4.3), gs = A/(Ca-Q). Thus stomata regulate the C02
flux, so that the concentration of C02 in the mesophyll is lower than in the ambient air (Ca), and lies between r and Ca. On average, Ci/ Ca= 0.7-0.8.
The response of photosynthesis to C02 concentration may also be expressed as a function of consumption (Raschke 1979). Stomatal conductance (function of supply) and efficiency of carboxylase (function of consumption) decrease the C02 concentration in the mesophyll.
The C02 gradient across the stomata (from the atmosphere to the intercellular spaces), Ac, is proportional to the ratio of A to gs:
Stomatal closure means reduced conductance of gs, leading to a decrease in the supply function of Fig. 2.4.2, and at constant photosynthetic capacity (consumption function) to a decrease in Ci. Removing the epidermis experimentally would bypass the influence of stomata, and enable C02 assimilation to be measured at Ca and thus determine the RuBP-carboxylase activity. In this case, the measured rate (ACa) would reach the RuBP limiting range (Farquhar and von Caemmerer 1982).
A decrease in light intensity has its primary effect on the chloroplast which results in decreased C02 assimilation and to a decrease in the saturated rate of photosynthesis at the same C02 concentration, despite constant carboxylase activity. There need not necessarily be an effect on Ci- In contrast, water stress or N deficiency causes reduced efficiency of carboxylase, i.e. at constant stomatal conductance Ci increases. C02 assimilation (consumption function) increases more slowly and reaches saturation earlier than in plants with sufficient water supply or N nutrition.
Efficiency of C02 fixation may be evaluated by the number of C02 molecules diffusing through the stomata and exchanging oxygen with the cell water and diffusing back into the atmosphere, as they are not assimilated by photosynthesis. Of 1000 C02 molecules entering the leaf only one is reduced, the others diffuse out to the atmosphere (Lloyd et al. 1996). This process can be shown by the 180 content in C02. At the evaporating surfaces in the mesophyll cells 180 is enriched in the cell wall water (Roden and Ehleringer 1999). This H2180 exchanges oxygen with the oxygen of C02, as a consequence of gaseous C02 dissolving in the cell wall water and then reacting with the dissociated 18OH" ions to give HClsOO". In this process O atoms are "exchanged" so that the C02 diffusing from the leaf back to the atmosphere contains a new 180 atom as a label showing the significance of the physical process of diffusion in C02 uptake: It is possible to increase C02 assimilation by increased C02 concentration, but large amounts of C02 apparently diffuse back into the atmosphere without being used for photosynthesis by the plant because Rubisco is limiting. In this context, turnover of C02 by an-hydrase plays an additional important role (Gil-Ion and Yakir 2001).
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