© 1990 Heron Publishing—Victoria, Canada
Gas exchange in Quercus rubra (northern red oak) during a drought: analysis of relations among photosynthesis, transpiration, and leaf conductance
J. A. Weber (1, 2) and D. M. Gates (1)
1. Biological Station and Department of Biology, University of Michigan, Ann Arbor, MI 48109-1048, USA / 2. US Environmental Protection Agency, Environmental Research Laboratory, 200 SW 35th Street, Corvallis, OR 97333, USA /
Summary
Development of water stress in leaves of mature Quercus rubra L. caused a marked midday depression in photosynthesis (A) and transpiration (E). At external CO2 partial pressures of 100–110 Pa, a constant temperature of 30 °C and a constant photosynthetic photon flux density of about
1000 μmol m–2 s–1, A was 8 μmol m–2 at low leaf water potentials (–1.5 to –2.0 MPa), whereas it was 20 μmol m–2 s–1 in non-stressed leaves (–1.0 MPa). At lower external CO2 partial pressures, the effect of low leaf water potential on A was less. The midday depression in gas exchange was relieved by an overnight rain of 2.5 cm. No difference in carboxylation
efficiency or CO2 compensation point was found between leaves before and after rain, The relationship between A and E was linear for a given external CO2 partial pressure, but the slope varied with CO2 concentration. Modification of the model of stomatal response proposed by Ball et al. (1987) produced a linear relationship
between leaf conductance and a factor incorporating A, relative humidity, and CO2. The data indicate that gas exchange in leaves of mature northern red oak respond rapidly to relief of drought with no indication
of long-term photoinhibition.
