THE EFFECT OF SOURCE OR SINK TEMPERATURE ON PHOTOSYNTHESIS AND C-14 PARTITIONING IN AND EXPORT FROM A SOURCE LEAF OF ALSTROEMERIA

The influence of source and sink temperature on leaf net C exchange ra te (NCER), export, and partitioning in the C-3 monocotyledon Alstroeme ria sp. cv. Jacqueline were examined. Leaf (i.e. source) temperature w as varied between 12 and 35 degrees C while source leaves were exposed to photorespiratory and nonphotorespiratory conditions during a 2-h s teady-state (CO2)-C-14 labelling period. Between 12 and 20 degrees C, at ambient CO2 and O-2. leaf NCER and export were similar with maximum rates of 9.71 +/- 0.51 and 3.06 +/- 0.36 mu mol C m(-2) s(-1), respec tively. Both NCER and export decreased above 20 degrees C. At 35 degre es C NCER was 30% of the rate at 20 degrees C, but export was totally inhibited. Between 12 and 35 degrees C, at the end of the 2-h feeding period, C-14 was partitioned in the leaf as ethanol insolubles (3-10%) , H2O solubles (88-92%), and chloroform solubles (2-8%). However, abov e 25 degrees C, less C-14 was recovered in the starch fraction and mor e in the sugar fractions. At all temperatures, 86 to 94% of the labell ed sugars was C-14-sucrose. In nonphotorespiratory conditions (i.e. 18 00 mu l(-1) CO2 and 2% O-2), NCER and export were higher than the rate s obtained at ambient CO2 and O-2 at each temperature. Carbon dioxide enrichment sustained high NCER and export rates even at 35 degrees C. Although CO2 enrichment increased partitioning of C-14 into starch, st arch synthesis at 35 degrees C was markedly reduced. Cooling the root- zone mass (i.e. a dominant sink) to 10 degrees C, which simulated the commercial practice used to induce flowering, had no significant effec t on source leaf NCER and export rates either during a 2-h steady-stat e labelling period or subsequently during a 21-h light-dark chase peri od. Furthermore, partitioning of C-14 among leaf products at the end o f the feed-chase period was not affected. Additional pulse and chase e xperiments using (CO2)-C-11 fed to source leaves of control and root-c ooled plants showed that there was no difference in the direction of m ovement of C-11-assimilates towards the flower or the root zone as a c onsequence 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 exp ort than manipulating sink activity.