title: Photosynthetic responses to slowly decreasing leaf water potentials in Encelia frutescens.
abstract: The importance of reduced leaf conductance (stomatal and boundary layer) in limiting photosynthetic rates during water stress was studied in Encelia frutescens, a drought-deciduous leaved subshrub of the Mohave and Sonoran Deserts. Light-saturated CO2assimilation rates of greenhouse grown plants decreased from 42.6±1.6μmol CO2m-2s-1(x±s.e.) to 1.7±1.7 μmol CO2m-2s-1as leaf water potential decreased from-1.5 MPa to-4.0 MPa. The dependence of light saturated, CO2assimilation rate on leaf intercellular CO2concentrations between 60 and 335 μl l-1was also determined as leaf water potential decline. This enabled us to compare the effects of leaf water potentials on limitations to carbon assimilation imposed by leaf conductance and by intrinsic photosynthetic capacity. Both leaf conductance and intrinsic photosynthetic capacity decreased with decreasing leaf water potential, but the decrease in leaf conductance was proportionately greater. The relative stomatal limitation, defined as the percent limitation in photosynthetic rate due to the presence of gas-phase diffusional barriers, increased from (x±s.e.) to 41±3% as water potentials became more negative. Since both leaf conductance and intrinsic photosynthetic capacity were severely reduced in an absolute sense, however, high photosynthetic rates could not have been restored at low leaf water potentials without simultaneous increases in both components.
authors.affiliation: Department of Biology, University of Utah, 84112, Salt Lake City, UT, USA.
authors.forename: Joanthan
authors.initials: J
authors.lastname: Comstock
authors.affiliation: Department of Biology, University of Utah, 84112, Salt Lake City, UT, USA.
authors.forename: James
authors.initials: J
authors.lastname: Ehleringer