title: High-frequency field measurements of diurnal carbon isotope discrimination and internal conductance in a semi-arid species, Juniperus monosperma.
abstract: We present field observations of carbon isotope discrimination (Delta) and internal conductance of CO(2) (g(i)) collected using tunable diode laser spectroscopy (TDL). Delta ranged from 12.0 to 27.4 per thousand over diurnal periods with daily means from 16.3 +/- 0.2 per thousand during drought to 19.0 +/- 0.5 per thousand during monsoon conditions. We observed a large range in g(i), with most estimates between 0.04 and 4.0 micromol m(-2) s(-1) Pa(-1). We tested the comprehensive Farquhar, O'Leary and Berry model of Delta (Delta(comp)), a simplified form of Delta(comp) (Delta(simple)) and a recently suggested amendment (Delta(revised)). Sensitivity analyses demonstrated that varying g(i) had a substantial effect on Delta(comp), resulting in mean differences between observed Delta (Delta(obs)) and Delta(comp) ranging from 0.04 to 9.6 per thousand. First-order regressions adequately described the relationship between Delta and the ratio of substomatal to atmospheric CO(2) partial pressure (p(i)/p(a)) on all 3 d, but second-order models better described the relationship in July and August. The three tested models each best predicted Delta(obs) on different days. In June, Delta(simple) outperformed Delta(comp) and Delta(revised), but incorporating g(i) and all non-photosynthetic fractionations improved model predictions in July and August.
authors.affiliation: Department of Biology, University of New Mexico, Albuquerque, NM 87131, USA. bickford@unm.edu
authors.forename: Christopher P
authors.initials: CP
authors.lastname: Bickford
authors.forename: Nate G
authors.initials: NG
authors.lastname: McDowell
authors.forename: Erik B
authors.initials: EB
authors.lastname: Erhardt
authors.forename: David T
authors.initials: DT
authors.lastname: Hanson
mesh_terms: D002244:Carbon / Q000032:analysis*; D002245:Carbon Dioxide / Q000032:analysis; D002247:Carbon Isotopes / Q000032:analysis; D018653:Juniperus / Q000737:chemistry*; D008954:Models, Biological; D010313:Partial Pressure; D018515:Plant Leaves / Q000737:chemistry*; D013057:Spectrum Analysis; D013997:Time Factors
chemical_list: D002247:Carbon Isotopes; D002245:Carbon Dioxide; D002244:Carbon