The ability of 21 C 3 and C 4 monocot and dicot species to rapidly export newly fixed C in the light at both ambient and enriched CO 2 levels was compared. Photosynthesis and concurrent export rates were estimated during isotopic equilibrium of the transport sugars using a steady-state 14 CO 2 -labeling procedure. At ambient CO 2 photosynthesis and export rates for C 3 species were 5 to 15 and 1 to 10 mol C m ؊2 s ؊1 , respectively, and 20 to 30 and 15 to 22 mol C m ؊2 s ؊1 , respectively, for C 4 species. A linear regression plot of export on photosynthesis rate of all species had a correlation coefficient of 0.87. When concurrent export was expressed as a percentage of photosynthesis, several C 3 dicots that produced transport sugars other than Suc had high efflux rates relative to photosynthesis, comparable to those of C 4 species. At high CO 2 photosynthetic and export rates were only slightly altered in C 4 species, and photosynthesis increased but export rates did not in all C 3 species. The C 3 species that had high efflux rates relative to photosynthesis at ambient CO 2 exported at rates comparable to those of C 4 species on both an absolute basis and as a percentage of photosynthesis. At ambient CO 2 there were strong linear relationships between photosynthesis, sugar synthesis, and concurrent export. However, at high CO 2 the relationships between photosynthesis and export rate and between sugar synthesis and export rate were not as strong because sugars and starch were accumulated.

A computer controlled semiclosed net CO2 exchange measurement system, employing an infrared gas analyzer and mass flow controllers to inject pure CO2 at preset rates, has been developed for measuring whole plant net CO2 exchange and net C gain in a controlled environment (i.e. CO2, light, and temperature). Data Many gas-exchange systems based on infrared gas analysis of CO2 have been designed to measure photosynthesis, photorespiration, and dark respiration of single plant leaves (2, 9, 15). However, measurement of leaf photosynthesis does not necessarily predict plant growth and crop productivity (3), since individual leaves are not representative of the photosynthetic behavior of the entire canopy. Furthermore, analysis of the metabolism of a single leaf does not take into account dark respiration of the entire plant and ignores both the problem of partitioning of photoassimilates and the evaluation of crop quality (5,14). A positive correlation between leaf photosynthetic rate and crop productivity requires considerable sampling (3,12) and is most easily obtained when studying a crop for which the vegetative portion of the plant (e.g. leaves or roots) is harvested for market (8,14,17). In spite of the problems in correlating photosynthesis with yield it is well known that over 95% of the dry matter of a plant is derived from photosynthesis and further that carbon (C) obtained from photosynthesis comprises approximately 40% of the plant dry weight under most growth conditions (1, 7). Even so, CO2 analysis itself is rarely used as a means of measuring growth rate.Growth rate is frequently defined as an increase in the physical size of the plants expressed simply as an absolute increase in dry weight with time (e.g. g gained * week-1) or as a relative increase I

Export and photosynthesis i n leaves of Salvia splendens were measured concurrently during steady-state 14C0, labeling conditions. Under ambient CO, and O, conditions, photosynthesis and export rates were similar at 15 and 25"C, but both declined as leaf temperature was raised from 25 to 40°C. Suppressing photorespiration between 15 and 40°C by manipulating CO, and O, levels resulted in higher rates of leaf photosynthesis, total sugar synthesis, and export. There was a linear relationship between the rate of photosynthesis and the rate of export between 15 and 35°C. At these temperatures, 60 to 80% of the carbon fixed was readily exported with sucrose, raffinose, and stachyose, which together constituted over 90% of phloem mobile assimilates. Above 35"C, however, export during photosynthesis was inhibited both in photorespiratory conditions, which inhibited photosynthesis, and in nonphotorespiratory conditions, which did not inhibit photosynthesis. Sucrose and raffinose but not stachyose accumulated i n the leaf at 40°C. When leaves were preincubated at 40°C and then cooled to 3 5 T , export recovered more slowly than photosynthesis. These data are consistent with the view that impairment of export processes, rather than photosynthetic processes associated with light trapping, carbon reduction, and sucrose synthesis, accounted for the marked reduction i n export between 35 and 40°C. Taken together, the data indicated that temperature changes between 15 and 40°C had two effects on photosynthesis and concurrent export. At all temperatures, suppressing photorespiration increased both photosynthesis and export, but above 35"C, export processes were more directly inhibited independent of changes i n photorespiration and photosynthesis.

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Whole-plant diurnal C exchange analysis provided a noninvasive estimation of daily net C gain in transgenic tobacco (Nicotiana tabacum L.) plants deficient in leaf cytosolic pyruvate kinase (PK c ؊). PK c ؊ plants cultivated under a low light intensity (100 mol m ؊2 s ؊1 ) were previously shown to exhibit markedly reduced root growth, as well as delayed shoot and flower development when compared with plants having wild-type levels of PK c (PK c ؉). PK c ؊ and PK c ؉ source leaves showed a similar net C gain, photosynthesis over a range of light intensities, and a capacity to export newly fixed 14 CO 2 during photosynthesis. However, during growth under low light the nighttime, export of previously fixed 14 CO 2 by fully expanded PK c ؊ leaves was 40% lower, whereas concurrent respiratory 14 CO 2 evolution was 40% higher than that of PK c ؉ leaves. This provides a rationale for the reduced root growth of the PK c ؊ plants grown at low irradiance. Leaf photosynthetic and export characteristics in PK c ؊ and PK c ؉ plants raised in a greenhouse during winter months resembled those of plants grown in chambers at low irradiance. The data suggest that PK c in source leaves has a critical role in regulating nighttime respiration particularly when the available pool of photoassimilates for export and leaf respiratory processes are low.PK catalyzes the synthesis of pyruvate and ATP from PEP and ADP and is believed to be a major control point of plant and nonplant glycolysis (Plaxton, 1996). The enzyme has been demonstrated to be significantly displaced from equilibrium in vivo and has pronounced regulatory properties in vitro. Plant PK exists as cytosolic and plastidic isozymes (PK c and PK p , respectively), which differ substantially in their molecular and kinetic/regulatory properties. PK c plays an important role in generating the precursor pyruvate for various biosynthetic pathways and mitochondrial respiration. The biosynthetic role of cytosolic glycolysis is central in actively growing autotrophic tissue (Plaxton, 1996), in which a significant proportion of the C that enters the glycolytic pathway is incorporated into numerous compounds such as amino acids, nucleic acids, fatty acids, and secondary metabolites. The exact contribution that these enzymatic steps provide in source leaves and in developing sink tissues remains unclear.PK c deficiency in nonplant species causes serious detrimental effects. However, our earlier studies revealed that transgenic tobacco plants (Nicotiana tabacum L.) deficient in leaf PK c (PK c Ϫ) grew from seed to seed, demonstrating the remarkable flexibility of plant PEP metabolism (GottlobMcHugh et al., 1992;Knowles et al., 1998). Plant cells can use a variety of alternative metabolic routes to directly or indirectly circumvent the reaction catalyzed by PK c . These could include the action of PEP phosphatase or the combined action of PEP carboxylase, malate dehydrogenase, and NAD-malic enzyme (Plaxton, 1996). It is also possible that the elevated levels of PEP observed in the PK c Ϫ ...