Environmental regulation of carbon isotope discrimination and internal CO2 conductance in C3 leaves

Stable carbon isotopes are powerful tools for elucidating leaf- and ecosystem- level processes, and recent technological developments provide new opportunities to assess the isotopic flux during leaf gas exchange. In these studies I used a tunable diode laser spectroscope coupled to a infra-red gas analyzer to measure the isotopic composition of leaf gas exchange at high frequency in both field and greenhouse settings and assess environmental regulation of carbon isotope discrimination (Δ) and internal conductance of CO2 to sites of carboxylation (gi). I measured Δ and gi across diurnal and seasonal periods in field-grown Juniperus monosperma trees and used these data to 1) assess the diurnal variation in Δ in response to environmental drivers, 2) test predictions from existing models of Δ, 3) test the linearity of the relationship between Δ and the ratio of intercellular to ambient CO2 partial pressure (pi/pa), 4) test the hypothesis that gi varies at diurnal timescales and 5) test the influence of gi in Δ models. Results show photosynthetic photon flux density, soil water availability, and vapor pressure deficit were significant environmental drivers of diurnal Δ patterns, and that existing models generally produced model predictions of Δ within 1-3‰ of observed values. Linear models adequately described significant relationships between observed Δ and pi/pa, but second order models better described the relationship under some conditions. gi varied diurnally and ranged between 0.03-2.0 μmol m-2 s-1 Pa-1. Accounting for this variation improved model predictions of Δ compared with a model that omits gi, and parameterizing gi based on dynamic variables such as time of day produced the greatest improvement in predictions. These findings demonstrate the need for model improvements to better predict Δ under field conditions. Greenhouse studies were conducted to address the influence of soil water deficit (SWD) and leaf water potential (Ψw) on gi. Plants with isohydric tendencies were droughted and gi assessed using slope-based isotopic methods. Results showed no significant difference in Ψw or gi between droughted and control plants and suggest Ψw may buffer the gi response to SWD.Institute of Geophysics and Planetary Physics at Los Alamos National Laboratory (project 95566-001-05) National Science Foundation grant (IOS-0719118) UNM Biology Dept. Lynn A. Hertel Graduate Research Award and Grove ScholarshipDoctor of PhilosophyDoctoralUniversity of New Mexico. Biology Dept.Hanson, DavidMcDowell, NathanPockman, WilliamHelliker, Bren