The effect of source or sink temperature on photosynthesis and 14C‐partitioning in and export from a source leaf of Alstroemeria

Abstract

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.