HR: 1340h
AN: B23A-0920    [Abstracts]
TI: Water-Use Efficiency and Stable Carbon Isotopes: Accounting for Photosynthetic Refixation
AU: * Ubierna Lopez, N
EM: nerea.ubiernalopez@vandals.uidaho.edu
AF: University of Idaho, College of Natural Resources, Moscow, ID 83844-1133, United States
AU: Marshall, J D
AF: University of Idaho, College of Natural Resources, Moscow, ID 83844-1133, United States
AB: Three processes are performed by every green plant tissue: photosynthesis, respiration and refixation. Each of these affects the ratio of stable isotopes, ¹²C and ¹³C. Refixation allows plants to fix a portion of the CO₂ produced via respiration prior to releasing the remaining CO₂ back into the atmosphere. The process begins with a pool of CO₂ already depleted in ¹³C and subsequently depletes it further, resulting in two simultaneous effects: enrichment of CO₂ released into the atmosphere and depletion of biomass that is formed. Recently, considerable research has concentrated on identifying processes that determine the isotopic composition of a given plant tissue. A convincing explanation for the observed enrichment of stems versus leaves has still not been derived. We advocate that refixation can explain currently inexplicable patterns.
We hypothesized that leaves re-fix carbon during their entire lifespan when light intensity is below the light compensation point and above total darkness. We grew Idaho hybrid poplars under controlled conditions in a growth chamber. Light intensity was regulated to create three different treatments: (1) Light (PAR=270 μmol/m²s), (2) Shade (PAR=89 μmol/m²s) and (3) Dark (PAR=0 μmol/m²s). For each treatment we modified respiration values by regulating the light environment between total darkness and the light compensation point. For the light treatment group, leaf respired CO₂ was collected at 5% (PAR=14) and 22% (PAR=59) of the light growing environment. For the shade treatment group, leaf respired CO₂ was collected at 22% (PAR=20) of the light growing environment. We estimated the amount of refixation as (Ddark- Dlight)/Ddark, where Ddark represents dark respiration (μmol/gs) and Dlight respiration during light periods (μmol/gs).
Light treatments plants exhibited a maximum refixation level of 53% at PAR=59, with an associated enrichment of leaf respired C isotopic composition (δ¹³C_LR) of 3.3‰. At PAR=14, refixation rate for Light plants decreased to 10% and the observed enrichment on δ¹³C_LR was 1‰. Correspondingly, Shade plants showed a 37% level of refixation and 3.6‰ enrichment at PAR=20.
Our findings support the hypothesis that leaves re-fix carbon under low-light intensity environments. The degree of refixation is proportional to light intensity, with higher refixation rates associated with higher light intensities and more depleted leaf biomass. The continuous refixation of the internal leaf carbon pool during leaf expansion together with diurnal refixation periods in mature leaves adds depleted biomass into the leaf that is likely to account for the patterns described in the literature (i.e. 2‰ depletion of leaf versus stem biomass). This can influence the interpretation of δ¹³C leaf biomass data of previous studies and can compromise the utility of δ¹³C from leaf tissue as a precise meter of water-use efficiency.
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0428 Carbon cycling (4806)
DE: 0465 Microbiology: ecology, physiology and genomics (4840)
DE: 0476 Plant ecology (1851)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting