问题描述：
英语翻译
Crassulacean acid metabolism (CAM),a water-conserving
mode of photosynthesis whereby plants take up CO2 at
night,has been under investigation since its discovery over
200 years ago (Winter and Smith 1996).It has evolved
multiple times in nearly 30 plant families,both terrestrial
and aquatic (Crayn et al.2004;Lu¨ttge 2004),and is found
in at least 6% of all vascular plant species (Ehleringer and
Monson 1993).There are four phases of CAM:(1) night-
time CO2 uptake and fixation by phosphoenolpyruvate
carboxylase (PEPC),(2) early morning CO2 uptake and
fixation by ribulose 1,5-bisphosphate carboxylase/oxygen-
ase (Rubisco),(3) daytime decarboxylation and refixation
of CO2 under closed stomates,and (4) late afternoon CO2
uptake and fixation by Rubisco (Griffiths et al.2002;
Lu¨ttge 2004).Although CAM is typically defined as
nighttimeCO2 uptake,inmanyCAMspecies,daytimeCO2
fixation by Rubisco raises questions as to whether the
amount of CO2 fixed through the C3 pathway varies with
environmental conditions (e.g.,Nobel et al.1992; Winter
and Holtum 2002; Winter et al.2008).The significance ofvariation in activity of Rubisco in strong CAM plants has
not yet been fully incorporated into an ecological
framework.
13
The C stable isotopic composition (d C) of plant
material can be used as an indicator of the relative pro-
portion of CO2 assimilated by CAM and C3 photosynthetic
pathways because of differential discrimination by the
two carboxylating enzymes,PEPC and Rubisco.If only
Rubisco is used,as occurs during light CO2 fixation,the
13
d C of bulk plant material will be close to -27%.In
contrast,if only PEPC is used,as occurs during dark CO2
13
fixation,the d C of plant material will be near -13%.
13
Foliar d C values therefore reflect the weighting of these
two processes (Farquhar et al.1989).A mixing model
13
using plant d C values in combination with CAM and C3
can provide information on the proportional use of the two
photosynthetic pathways.Moreover,taking into consider-
ation variation in C3 and CAM activity can allow a more
accurate description of the contribution of each photosyn-
thetic pathway (Winter and Holtum 2002).
Crassulacean acid metabolism (CAM),a water-conserving
mode of photosynthesis whereby plants take up CO2 at
night,has been under investigation since its discovery over
200 years ago (Winter and Smith 1996).It has evolved
multiple times in nearly 30 plant families,both terrestrial
and aquatic (Crayn et al.2004;Lu¨ttge 2004),and is found
in at least 6% of all vascular plant species (Ehleringer and
Monson 1993).There are four phases of CAM:(1) night-
time CO2 uptake and fixation by phosphoenolpyruvate
carboxylase (PEPC),(2) early morning CO2 uptake and
fixation by ribulose 1,5-bisphosphate carboxylase/oxygen-
ase (Rubisco),(3) daytime decarboxylation and refixation
of CO2 under closed stomates,and (4) late afternoon CO2
uptake and fixation by Rubisco (Griffiths et al.2002;
Lu¨ttge 2004).Although CAM is typically defined as
nighttimeCO2 uptake,inmanyCAMspecies,daytimeCO2
fixation by Rubisco raises questions as to whether the
amount of CO2 fixed through the C3 pathway varies with
environmental conditions (e.g.,Nobel et al.1992; Winter
and Holtum 2002; Winter et al.2008).The significance ofvariation in activity of Rubisco in strong CAM plants has
not yet been fully incorporated into an ecological
framework.
13
The C stable isotopic composition (d C) of plant
material can be used as an indicator of the relative pro-
portion of CO2 assimilated by CAM and C3 photosynthetic
pathways because of differential discrimination by the
two carboxylating enzymes,PEPC and Rubisco.If only
Rubisco is used,as occurs during light CO2 fixation,the
13
d C of bulk plant material will be close to -27%.In
contrast,if only PEPC is used,as occurs during dark CO2
13
fixation,the d C of plant material will be near -13%.
13
Foliar d C values therefore reflect the weighting of these
two processes (Farquhar et al.1989).A mixing model
13
using plant d C values in combination with CAM and C3
can provide information on the proportional use of the two
photosynthetic pathways.Moreover,taking into consider-
ation variation in C3 and CAM activity can allow a more
accurate description of the contribution of each photosyn-
thetic pathway (Winter and Holtum 2002).
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