the light photosynthesis and photorespiration occur simultaneously while C is being exported from the
tissue. Our analyses showed that in healthy source leaves when photorespiration was suppressed by low
O 2 and high CO 2 levels, both photosynthesis and immediate export increased [18,63]. Significantly, us-
ing our steady-state^14 CO 2 labeling procedure, we were able to show that leaf warming resulted in a re-
duction in immediate export prior to any inhibition of the photosystems that would have altered the C-fix-
ation processes per se [18,63]. These data challenge conventional literature which focuses on the concept
of disruption of thylakoid membranes in chloroplasts and loss of the photosystem activity being the pri-
mary limiting event during heat stress of leaf tissue. One applied outcome of the studies of immediate ex-
port at elevated leaf temperatures [18] has been a reexamination of the effect of high temperature on the
maintenance of valuable greenhouse crops, such as alstroemeria [63] and roses [79]. In the case of roses,
for example, our data provide an explanation for reduced production of flowers at high temperatures. Pho-
tosynthesis is maintained but export during photosynthesis is inhibited, leading to poorer flower devel-
opment. Carbohydrate limitation does not appear to be compensated for by nighttime export of stored re-
serves because most of the C in roses is exported in the light even when the plants have acclimated to high
CO 2 and more C is stored in the light [79].
In another study of bean leaves infected with bacterial blight (i.e., Xanthomonas), the immediate ex-
port rate was reduced before a drop in photosynthesis was observed [78]. This conclusion was not read-
ily evident from other analyses of export. In a related study with geranium infected with bacterial blight,
photosynthesis and immediate and subsequent nighttime export rates were inhibited more at high CO 2
than at ambient levels even though bacterial numbers in the leaves were lower in plants grown at high
CO 2 [80]. The data show that classical definitions of disease severity (virulence) based on bacteria num-
bers [e.g., colony-forming units (CFU)] are not accurate. In plants that were acclimated to high CO 2 but
not infected, the immediate and daily export rates were greater than those in control plants grown at am-
bient CO 2. Collectively, these results are interesting because they provide some insight into the acclima-
tion processes that might operate in plants growing in a “CO 2 -enriched world” as well as the manner in
which plants compete with pathogens or symbiotes for assimilates. Using^14 C pulse-chase analysis, it ap-
pears in geranium that the healthy uninfected control plants [80], like the healthy rose plants [79], export
the bulk of the^14 C fed during a light period (i.e., during photosynthesis). Long-term CO 2 enrichment in-
creased the storage of sugars and starch in the leaves but proportionally both ambient-grown plants and
those that were acclimated to CO 2 enrichment had similar patterns of immediate (daytime) and nighttime
export. Nighttime respiration was generally higher in the CO 2 -enriched plants. Using destructive weight
analyses to study diurnal export patterns in castor bean that had been acclimated to either ambient or el-
evated (700 l L1) CO 2 , Grimmer and Komor [46] also showed that in both ambient- and high
CO 2 –grown plants more export occurred during the light period than during the dark period. The rates of
export at night and respiration at night were also greater in the high CO 2 –acclimated leaves, but during
CO 2 enrichment the C content (as a percentage) was not altered by CO 2 enrichment.
Growth rate and plant development might be markedly altered as more or less of the total transport
proceeds during the day versus the night [5,46,80,83]. A number of studies with both C 3 and C 4 species
have shown the magnitude of export during daytime rather than during the night and therefore the im-
portance of export that occurs concurrently with photosynthesis [1–3,46,79,80].
B. Immediate Export in Natural Photosynthetic Variants
Differences in photosynthetic capacity are recognized to occur naturally among genetically similar
species and genera. Anatomical, biochemical, and physiological differences among C 3 , C 3 -C 4 intermedi-
ate, and C 4 photosynthetic pathways in genera such as PanicumandFlaveriaare well documented
[40,84–86]. One advantage of examining the leaf photosynthesis and export patterns in Panicumand
Flaveriamore closely is that these genera represent a range of naturally occurring variants of photosyn-
thetic types. Both genera contain C 3 -C 4 intermediate types in addition to species with well-defined C 3 and
C 4 traits. Although the impact of these different photosynthetic pathways on photosynthetic and pho-
torespiration rates has been examined, studies that provide measurements of export capacity during pho-
tosynthesis in leaves with different photosynthetic types are limited and have often employed different
techniques to quantify translocation [2,33,34,70].
On the basis of the rate of disappearance of^14 C during a 6-hr chase period, Hofstra and Nelson [70]
showed that the C 4 species corn and sorghum had higher export rates than C 3 species such as soybean and
QUANTIFYING IMMEDIATE C EXPORT FROM LEAVES 415