The influence of source and sink temperature on leaf net C exchange rate (NCER), export, and partitioning in the C3 monocotyledon Alstroemeria sp. cv. Jacqueline were examined. Leaf (i.e. source) temperature was varied between 12 and 35°C while source leaves were exposed to photorespiratory and nonphotorespiratory conditions during a 2-h steady-state 14CO2 labelling period. Between 12 and 20°C, at ambient CO2 and O2, leaf NCER and export were similar with maximum rates of 9.71 ± 0.51 and 3.06 ± 0.36 μmol C m-2 s-1, respectively. Both NCER and export decreased above 20°C. At 35°C NCER was 30% of the rate at 20°C, but export was totally inhibited. Between 12 and 35°C, at the end of the 2-h feeding period, 14C was partitioned in the leaf as ethanol insolubles (3–10%), H2O solubles (88–92%), and chloroform solubles (2–8%). However, above 25°C, less 14C was recovered in the starch fraction and more in the sugar fractions. At all temperatures, 86 to 94% of the labelled sugars was 14C-sucrose. In nonphotorespiratory conditions (i.e. 1 800 μI I-1 CO2 and 2% O2). NCER and export were higher than the rates obtained at ambient CO2 and O2 at each temperature. Carbon dioxide enrichment sustained high NCER and export rates even at 35°C, Although CO2 enrichment increased partitioning of 14C into starch, starch synthesis at 35°C was markedly reduced.
Cooling the root-zone mass (i.e. a dominant sink) to 10°C, which simulated the commercial practice used to induce flowering, had no significant effect on source leaf NCER and export rates either during a 2-h steady-state labelling period or subsequently during a 21-h light-dark chase period. Furthermore, partitioning of 14C among leaf products at the end of the feed-chase period was not affected. Additional pulse and chase experiments using 11CO2 fed to source leaves of control and root-cooled plants showed that there was no difference in the direction of movement of 11C-assimilates towards the flower or the root zone as a consequence of root cooling. Together, the data indicate that changing source strength, by manipulating photosynthesis and photorespiration, by varying the leaf temperature had a more profound effect on leaf export than manipulating sink activity.