© 1987 Heron Publishing—Victoria, Canada
Rapid field measurement of the assimilation rate versus internal CO2 concentration relationship in green ash (Fraxinus pennsylvanica Marsh.): the influence of light intensity
J. E. Davis (1), T. J. Arkebauer (2), J. M. Norman (2) and J. R. Brandle (1)
1. Department of Forestry, Fisheries and Wildlife, University of Nebraska-Lincoln, Lincoln, NE 68583-0814, USA / 2. Department of Agronomy, University of Nebraska-Lincoln, Lincoln, NE 68583-0817, USA / Received March 24, 1987
Summary
Assimilation rate (A) versus intercellular CO2 concentration (Ci) relationships for leaflets of five-year-old green ash (Fraxinus pennsylvanica Marsh.) trees were computed from gas exchange measurements obtained in the field with a closed-circuit, portable photosynthesis
measurement system comprising an LI-6200 gas analyzer and an LI-6000 computer, (Li-Cor, Inc., Lincoln, Nebraska, USA). Observations
were made over a range of light intensities achieved by attenuating direct sunlight with neutral density filters, and over
a range of ambient CO2 concentrations achieved by
breathing into the assimilation chamber and then lowering the CO2 concentration to the desired level with the LI-6200’s soda-lime scrubber. Boundary layer conductance was determined by use
of a leaf replica made of moist filter paper. Typically, A–Ci curves at four light intensities were obtained in three to four hours.
The initial slope (when A = 0) of the A–Ci curve obtained at a light intensity of 1750 μmol m–2 s–1 (full sunlight) was similar to that obtained at a light intensity of 840 μmol m–2 s–1. However, when light intensity was reduced further (to 370 and 160 μmol m–2 s–1), the initial slope of the A–Ci curve also decreased, indicating that at these light intensities, assimilation was limited by photochemical energy supply,
as well as CO2 concentration.
