HR: 16:45h
AN: B14B-04    [Abstracts]
TI: Temporal Variability in Carbon Isotope Composition of Leaf-Respired Carbon Dioxide
AU: * Barbour, M M
EM: barbourm@landcareresearch.co.nz
AF: Landcare Research, PO Box Gerald St, Lincoln, 8152 New Zealand
AU: Hanson, D T
EM: dthanson@unm.edu
AF: University of New Mexico, 167 Castetter Hall, Albuquerque, NM 87131-1091 United States
AU: Bickford, C P
EM: bickford@unm.edu
AF: University of New Mexico, 167 Castetter Hall, Albuquerque, NM 87131-1091 United States
AU: McDowell, N G
EM: mcdowell@lanl.gov
AF: Los Alamos National Laboratory, MS-D462, Los Alamos, NM 85745 United States
AB: The stable carbon isotope composition of leaf-respired CO₂ (δ¹³C_Rl) has enormous potential to allow partitioning of ecosystem respiration into various components, to provide information on key physiological processes, and to trace carbon fluxes through plants and ecosystems. However, difficulties in measuring and understanding variation in δ¹³C_Rl have limited its application. We coupled an open gas exchange system (LI-6400, LiCor) to a tunable diode laser (TGA100A, Campbell Scientific) enabling measurement of leaf respiratory CO₂ fluxes and δ¹³C_Rl every three minutes, with a precision of at least ±0.3 per mil. We also measured oxygen consumption rates, allowing calculation of the respiratory quotient ( RQ) and indicating likely respiratory substrates. Castor bean ( Ricinus communis) plants grown at high and low light were placed in the dark after different lengths of time exposed to sunlight and variation in δ¹³C_Rl measured to test the patterns in variation in δ¹³C_Rl predicted by existing biochemical models. CO₂ respired by leaves previously exposed to high cumulative incident irradiance was up to 11 per mil more enriched than phloem sap sugars for the first 10 to 15 minutes after plants had been moved into the dark . This enrichment rapidly decreased, so that by 30 minutes in the dark δ¹³C_Rl was 5 per mil more enriched than phloem sap sugars. CO₂ production rates were also initially very high and rapidly decreased. RQ for plants grown in high light varied between 0.8 and 1.2, indicating that carbohydrates and/or organic acids were the respiratory substrates. δ¹³C_Rl measured 30 to 80 minutes after plants had been moved into the dark increased with increasing δ¹³C of phloem sap sugars. The RQ values of plants grown at low light suggested that the respiratory substrates were fatty acids or amino acids ( RQ of around 0.6), or lipids ( RQ less than 0.4). δ¹³C_Rl values were enriched by either 4 per mil ( RQ = 0.3) or 12 per mil ( RQ = 0.5) compared to phloem sap sugars. The highly enriched δ¹³C_Rl at ( RQ = 0.5) suggests gluconeogenesis of ¹³C enriched amino acids, like serine and glycine. These measurements at high temporal resolution over the first hour of a dark period suggest that existing models describing variation in δ¹³C_Rl need to be modified to include 1) the high rates of CO₂ evolution and highly enriched δ¹³C_Rl for the first minutes of a dark period, and 2) highly enriched δ¹³C_Rl at intermediate RQ.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0428 Carbon cycling (4806)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
SC: Biogeosciences [B]
MN: Fall Meeting 2005