Leaf Conductance as Related to Xylem Water Potential and Carbon Dioxide Concentration in Sitka Spruce

Abstract

Current year shoots of Sitka spruce [Picea sitchensis Bong. (Carr.)] from the forest canopy were equilibrated in a leaf chamber. The shoots were excised in air, and removed at differing times in order to establish a relationship between stomatal conductance and xylem water potential. The experiment was repeated at five ambient CO2 concentrations. A second set of excised forest shoots, and shoots excised from 2‐year‐ old nursery seedlings were allowed to evaporate freely in a controlled environment wind tunnel until a constant rate of transpiration was measured, to establish a relationship between cuticular conductance and xylem water potential. Cuticular conductance was estimated to be 0.012 cm s‐1 at high water potential and declined linearly to 0.007 cm s‐1 at −3.5 MPa. The implication of this decline in the subsequent calculation of stomatal and mesophyll conductance is considered. Stomatal conductance remained constant at water potentials above −1.4 MPa and was not affected by ambient carbon dioxide concentrations between 20 and 600 cm‐3. At lower water potentials, stomatal conductance declined and approached zero at −2.5 to −2.6 MPa. The results suggest that stomatal aperture is not controlled by either ambient or intercellular space carbon dioxide concentration, and that stomatal closure at low water potential is unlikely to be mediated by carbon dioxide.

The growth and gas exchange response of soil‐planted Norway spruce [Picea abies (L.) Karst.] and red oak (Quercus rubra L.) exposed to elevated CO2 and to naturally occurring drought

“…3). Both Beadle, Jarvis & Neilson (1979) and Barton, Lee & Jarvis (1993) found increased stomatal conductance in the presence of CO, for Sitka spruce. Mansfield, Hetherington & Atkinson (1990) in a review of stomatal responses to CO, describe how a lack of response or opening in the presence of COj was considered exceptional, but that this view was established from experiments where plants were grown with very little variation in growth conditions.…”

Plant water relations and the effects of elevated CO2: a review and suggestions for future research

“…In some cases in which drought stress was the focus, excised branches of mature trees have been placed in, and then removed from, a water bath as a means of imposing short-term water stress (Beadle et al 1979) In studies in which environmental conditions other than Ca were controlled, light intensity, or photosynthetically active radiation (PAR), was commonly maintained at moderate levels of 450 to 750 #E m -2 s-1 (Sionit et al 1981;Conroy et al 1988a;Sasek & Strain 1989). Some workers, however, grew plants at low light levels of less than 150 #E m -2 s 1 (Khan & Madsen 1986) and some at high light levels between 1000 and 2000 /~E m -2 s-1 (Rogers 1983;Tolley & Strain 1984a;Reekie & Bazzaz 1989), depending on plant material and objectives.…”

“…In these studies, soil water potential (q~soi]) has been allowed to decrease to between circa -1.5 MPa (Sionit et al 1981;Rogers et al 1984;Sionit & Patterson 1985;Conroy et al 1988b) and circa -2.5 MPa (Beadle et al 1979;Tolley & Strain 1984b;Frederick et al 1990). In general, soil water status was gauged by observed predawn ud (Rogers et al 1984;Tolley & Strain 1984b;), expressed gravimetrically as soil water content (SWC) (Conroy et al 1988a;Barr et al 1990), or judged by 'time since last watered' (Sionit & Patterson 1985;Idso 1988).…”

“…Both methods can be reliable if used with knowledge and caution, but serious errors can result when inappropriately used. The pressure bomb method has been used by a number of researchers studying effects of CO2 on plant water relations (Beadle et al 1979;Tolley & Strain, 1984b;Higginbotham et al 1985;Teskey et al 1986;Sasek & Strain 1989;Frederick et al 1990). Readers are referred to Tyree and Jarvis (1982) for an account of how this analysis is done and examples of possible sources of error (Cheung et 49 al.…”

“…As plant water loss proceeds, the flow rate of dry air pumped into the chamber is adjusted automatically to maintain RH at the ambient level, and E can be calculated given flow rate, leaf area, RH and temperature. Open systems in which changes in RH ofingoing and outgoing air can be monitored have also been commonly used to measure E (Beadle et al 1979;Wong 1979;Morison & Gifford 1983), from which gl can be calculated (Barr et al 1990). Still others have estimated E gravimetrically, by changes in plant and pot weight (Conroy et al 1988a;Barr et al 1990).…”