Seasonal change in the balance between capacities of RuBP carboxylation and RuBP regeneration affects CO2 response of photosynthesis in Polygonum cuspidatum
Yusuke Onoda
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Kouki Hikosaka
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Tadaki Hirose
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Graduate School of Life Sciences, Tohoku University
, Aoba, Sendai 980-8578,
Japan
The balance between the capacities of RuBP (ribulose1,5-bisphosphate) carboxylation (Vcmax) and RuBP regeneration (expressed as the maximum electron transport rate, Jmax) determines the CO2 dependence of the photosynthetic rate. As it has been suggested that this balance changes depending on the growth temperature, the hypothesis that the seasonal change in air temperature affects the balance and modulates the CO2 response of photosynthesis was tested. Vcmax and Jmax were determined in summer and autumn for young and old leaves of Polygonum cuspidatum grown at two CO2 concentrations (370 and 700 lmol mol21). Elevated CO2 concentration tended to reduce both Vcmax and Jmax without changing the Jmax:Vcmax ratio. The seasonal environment, on the other hand, altered the ratio such that the Jmax:Vcmax ratio was higher in autumn leaves than summer leaves. This alternation made the photosynthetic rate more dependent on CO2 concentration in autumn. Therefore, when photosynthetic rates were compared at growth CO2 concentration, the stimulation in photosynthetic rate was higher in young-autumn than in young-summer leaves. In old-autumn leaves, the stimulation of photosynthesis brought by a change in the Jmax:Vcmax ratio was partly offset by accelerated leaf senescence under elevated CO2. Across the two seasons and the two CO2 concentrations, Vcmax was strongly correlated with Rubisco and Jmax with cytochrome f content. These results suggest that seasonal change in climate affects the relative amounts of photosynthetic proteins, which in turn affect the CO2 response of photosynthesis.
Introduction
Because CO2 is a substrate for photosynthesis, elevation of
atmospheric CO2 concentration is expected to increase the
carbon assimilation of plants (Mott, 1990). Many studies
have focused on the photosynthetic response to elevated
CO2 concentration with short- and long-term experiments
(for a review see Cure and Acock, 1986; Poorter, 1993;
Gunderson and Wullschleger, 1994; Curtis, 1996; Curtis
and Wang, 1998; Wand et al., 1999). CO2 responses of
photosynthesis are determined by stomatal conductance,
and the capacity of RuBP (ribulose-1,5-bisphosphate)
carboxylation and RuBP regeneration (Farquhar et al.,
1980; Mott, 1990). Stomatal conductance regulates the CO2
concentration in the leaf, while RuBP carboxylation and
RuBP regeneration limit the photosynthetic rate at low and
high CO2 concentrations, respectively. Transition of the
limitation from RuBP carboxylation to RuBP regeneration
usually occurs between an ambient and a twice-ambient
CO2 concentration (Stitt, 1991).
Long-term treatment of elevated CO2 modulates the leaf
traits and stimulates photosynthesis to various extents
(Poorter, 1993; Gunderson and Wullschleger, 1994; Curtis,
1996). Plants grown in elevated CO2 tend to have a lower
stomatal conductance (reviewed by Drake et al., 1997;
Medlyn et al., 2001) and a lower photosynthetic rate than
those grown in ambient CO2 when compared at a given
CO2 concentration (for a review see Stitt, 1991; Gunderson
and Wullschleger, 1994; Sage, 1994). The lower rate of
photosynthesis has been ascribed to a reduction in the
capacity of RuBP carboxylation (Vcmax) and/or RuBP
regeneration (expressed as the maximum electron transport
rate, Jmax) (Sage, 1994). Reduction in Vcmax is usually
attributed to decreased amounts of Rubisco (RuBP
carboxylase/oxygenase) (Jacob et al., 1995; Nakano et al., 1997;
Tissue et al., 1999), and sometimes to a low activation state
(Sage et al., 1989; Cook et al., 1998). The mechanism of
lowering in Jmax, on the other hand, is less clear, because
the capacity of RuBP regeneration is determined through
many biochemical steps in electron transport (von
Caemmerer and Farquhar, 1981; Evans and Terashima,
1987; Sudo et al., 2003) and the Calvin cycle (Strand et al.,
2000; Sudo et al., 2003).
Changes in the balance between Vcmax and Jmax also
affect the CO2 stimulation of photosynthesis, because the
CO2 dependence of the photosynthetic rate is greater when
the rate is limited by RuBP carboxylation than by RuBP
regeneration (Fig. 2). As Vcmax and Jmax limit the
photosynthetic rate at low and high concentrations, respectively,
an increase in the Jmax:Vcmax ratio increases the ratio of the
photosynthetic rate at high CO2 to that at low CO2
concentrations (Webber et al., 1994; Sage, 1994; Medlyn,
1996; Hikosaka and Hirose, 1998). This indicates that the
increase in the Jmax:Vcmax ratio enhances the CO2
stimulation of photosynthesis. Although higher Jmax:Vcmax ratios
were reported in several studies in plants grown with
elevated CO2 (Sage et al., 1989; Lewis et al., 1996), many
studies found no change in the ratio (reviewed by Medlyn
et al., 1999). Hence, the effects of changes in the Jmax:Vcmax
ratio have been largely discounted in studies of plant
response to elevated CO2.
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