Plant Physiol, April 2003, Vol. 131, pp. 1727-1736
Natural Abundance Carbon Isotope Composition of Isoprene Reflects
Incomplete Coupling between Isoprene Synthesis and Photosynthetic
Carbon Flow
Hagit P.
Affek and
Dan
Yakir*
Department of Environmental Sciences and Energy Research, Weizmann
Institute of Science, Rehovot 76100, Israel
Isoprene emission from leaves is dynamically coupled to
photosynthesis through the use of primary and recent photosynthate in
the chloroplast. However, natural abundance carbon isotope composition
(
13C) measurements in myrtle (Myrtus
communis), buckthorn (Rhamnus alaternus), and
velvet bean (Mucuna pruriens) showed that only 72% to
91% of the variations in the 13C values of fixed carbon
were reflected in the 13C values of concurrently emitted
isoprene. The results indicated that 9% to 28% carbon was contributed
from alternative, slow turnover, carbon source(s). This contribution
increased when photosynthesis was inhibited by CO2-free
air. The observed variations in the 13C of isoprene
under ambient and CO2-free air were consistent with contributions to isoprene synthesis in the chloroplast from pyruvate associated with cytosolic Glc metabolism. Irrespective of alternative carbon source(s), isoprene was depleted in 13C relative to
mean photosynthetically fixed carbon by 4
to 11. Variable
13C discrimination, its increase by partially inhibiting
isoprene synthesis with fosmidomicin, and the associated accumulation
of pyruvate suggested that the main isotopic discrimination step was
the deoxyxylulose-5-phosphate synthase reaction.
*
Corresponding author; e-mail
dan.yakir{at}weizmann.ac.il; fax 972-8-9344124.
© 2003 American Society of Plant Biologists
