tomato. Leaves of six C 3 dicots and four C 4 monocots from different families were compared. Their con-
clusion was substantiated by Gordon [2], who employed differential weight analysis and infrared gas
analysis to estimate immediate export. Using a steady-state^14 CO 2 labeling protocol to measure photo-
synthesis and immediate export rates, we have examined over 42 monocot and dicot species from differ-
ent families and genera including a number from the genera PanicumandFlaveria[33,34]. We examined
immediate export both as an absolute rate and as a rate relative to the fixation rate (i.e., as a percentage of
photosynthesis). Collectively, the data for all 24 species showed that the faster photosynthesis was, the
faster immediate export was (Figure 4). At ambient CO 2 , there was a high correlation coefficient (r
0.88) between the rate of photosynthesis and the absolute rate of immediate export (Figure 4A). Among
all species and within each of the Panicum(#21–34) and Flaveria(#9–15) genera, photosynthesis and ex-
port rates of the C 4 species were higher than those of C 3 species (Figure 4Aii) [34]. Previous studies also
showed that at ambient CO 2 , C 4 species have higher translocation rates than C 3 species [2,70,87–89].
However, the concept that leaves with a functional C 4 metabolic pathway inherently export newly fixed
C more readily than those with C 3 metabolism was challenged [33]. At ambient CO 2 , the percentage of C
exported immediately relative to photosynthesis was high in a number of C 3 dicot species (#3, 7, 8, 19,
38; Figure 4Ci) [33] that produce auxiliary transport sugars [39]. The notable exception was sunflower
(#17; Figure 4Ci), which not only translocated sucrose but also had a relatively high immediate export ca-
pacity [33,70]. Among the Flaveriaspecies,F. robusta(#14; Figure 4Cii), a C 3 that also translocated only
sucrose, seemed to have a relatively high export flux.
An interesting finding was that the C 3 -C 4 intermediate species can be very different in their ability
to export C immediately [34]. When immediate^14 C efflux was examined relative to the rate of^14 C as-
similation, “type I” C 3 -C 4 intermediatePanicumspecies (#25, 32) exported newly acquired^14 C as
quickly as the C 4 species (#21, 23, 24, 26, 28, 29, 30, 31, 34*) did (Figure 4C). In contrast to
this pattern, among the Flaveriaspecies, the “type II” C 3 -C 4 intermediates (#10, 11, 12**) had the
lowest export rates of the three photosynthetic types [34]. Collectively, the data in Figure 4A and C show
that the C 3 -C 4 intermediate type I and type II species of the two genera behave differently with respect to
immediate export. The reason for this difference in immediate export capacity remains unclear.
In both type I and type II C 3 -C 4 intermediate species, special anatomy and biochemistry lead to re-
duced rates of photorespiration compared with those of C 3 species [40,90–92]. In leaves of type I C 3 -C 4
intermediates, the mitochondrial enzyme glycine decarboxylase is localized in the bundle sheath cells
[90,91]. Photorespired CO 2 that is released in the bundle sheath may be refixed by Rubisco before es-
caping from the leaf and result in reduced rates of apparent photorespiration at ambient CO 2 [92].
Anatomical features such as partially developed Kranz anatomy and localization of a higher number of
organelles (e.g., mitochondria) in the bundle sheath cells would further facilitate the refixation of CO 2
[40,91,93–96]. In addition to compartmentation of glycine decarboxylase in the bundle sheath cells
[90,91], some elements of C 4 metabolism are found in the type II C 3 -C 4 intermediate species [40,97,98].
Although not as well developed as in C 4 Flaveriaspecies, aspects of Kranz-type anatomy are also evident
[40,93]. Clearly, both anatomical and biochemical characteristics need to be considered to explain why
the type I C 3 -C 4 intermediatePanicumspecies export newly fixed^14 C as quickly as their C 4 cousins
whereas the type II C 3 -C 4 intermediateFlaveriaspecies exported less^14 C (Figure 4C).
Consistent with the expected suppression of photorespiration and the increased availability of CO 2
for fixation [84,92,99,100], short-term CO 2 enrichment increased photosynthesis in all C 3 , and type I and
type II C 3 -C 4 intermediate species (Figure 4B). In most species except for a few C 4 species, the absolute
export rate increased at high CO 2 but not proportionally with photosynthesis (Figure 4B). In all species
the relative export rates decreased under CO 2 enrichment (Figure 4D). Collectively, these data indicate
that during CO 2 enrichment all species tended to accumulate excess C in their leaves in the light. Plant
productivity of C 3 and C 3 -C 4 intermediate and C 4 species depends on many factors including the ability
of the leaves to export C immediately [4,5,30,40]. More data are required to determine whether these ex-
tra reserves of C support export and new growth under sustained CO 2 enrichment [46,79].
V. SUMMARY
Over the last half of the 20th century the availability of radioisotopes of C (e.g.,^14 C) led to the elucida-
tion of major photosynthetic processes in algae and higher plants. For example, the discovery of the
Calvin cycle [35] helped to define the manner in which net CO 2 assimilation occurs in all plants [11,86]
QUANTIFYING IMMEDIATE C EXPORT FROM LEAVES 417