The primary reactions of photosynthesis can be roughly divided into four parts:
formation of NADPH and ATP, incorporation of CO2
into ribulose 1,5-bisphosphate
(RuBP) by RuBisCO to produce 3-phosphoglycerate (PGA), regeneration of
RuBP in the PCR cycle (Fig. 4.1), and sucrose synthesis using triose phosphate
exported into the cytosol and counterchanged with phosphate released by this
synthesis. The accepted photosynthesis model (Farquhar et al.
, 1981) is based on
the prediction that the rate of synthesis of NADPH and ATP is calculated from the
flux of electrons in the photosynthetic electron transport chain, with three protons
transported for every ATP formed. In situ
RuBisCO activity is calculated using the
concentration of the activated catalytic site and kinetic parameters of RuBisCO
(Farquhar, 1979). The steady-state concentration of RuBP is balanced both by the
rate of regeneration and the utilization by RuBisCO for CO2
information has been provided by simultaneous measurements of rates of gas
exchange and steady-state concentrations of metabolites in the PCR cycle using
part of a single attached leaf under a range of conditions. The photosynthetic rate
of an attached leaf has been found to match the rate calculated with RuBisCO kinetics at CO2
concentrations in the intercellular space below 40 Pa and at
saturating light intensities, while the photosynthetic rate calculated by taking
electron flux into consideration significantly exceeds the photosynthetic rate
(Badger et al.
, 1984). The intraplastidic concentration of RuBP reaches levels that
several fold higher than the concentration of the RuBisCO active site
under these conditions (Badger et al.
, 1984; Geiger and Servaites, 1994). This
indicates that photosynthesis is limited by either RuBisCO or the CO2
pathway. As the intercellular CO2
concentration increases, photosynthesis enters
an RuBP-limited phase and transport of inorganic phosphate back into chloroplasts
becomes rate limiting (Sage, 1990; Sage et al.
, 1989). In contrast, the capacity
for NADPH and ATP formation limits photosynthesis at nonsaturating light
intensities (Farquhar et al.
, 1981). Moreover, photosynthesis in source organs
may occasionally become limited by the capacities of sink organs to accumulate
photosynthates (Paul and Foyer, 2001).
|FIGURE 4.1 Photosynthetic carbon reduction cycle. Vmax of each enzyme is given in micromoles per milligram chlorophyll per hour (Robinson and
Walker, 1981). (See Page 3 in Color Section.)