© 1991 Heron Publishing—Victoria, Canada
Gas exchange characteristics of Populus trichocarpa, Populus deltoides and Populus trichocarpa × P. deltoides clones
John H. Bassman and John C. Zwier
Department of Natural Resource Sciences, Washington State University, Pullman, WA 99164-6410, USA / Received July 13, 1990
Summary
Responses of net photosynthesis, dark respiration, photorespiration, transpiration, and stomatal conductance to irradiance,
temperature, leaf-to-air vapor density difference (VDD), and plant water stress were examined in two Populus trichocarpa clones (one from a moist, coastal climate in western Washington
and one from a dry, continental climate in eastern Washington), one P. deltoides clone, and two P. trichocarpa × P. deltoides clones.
Light saturation of photosynthesis in greenhouse-grown trees occurred at about 800 μmol m–2 s–1 for
P. deltoides, P. trichocarpa × P. deltoides, and the eastern Washington ecotype of P. trichocarpa, but at about 600 μmol m–2 s–1 for the western Washington ecotype of P. trichocarpa. Average net photosynthesis (at saturating irradiance and the optimum temperature of 25 °C) was 20.7, 18.8, 18.2 and 13.4
μmol CO2 m–2 s–1 for P. deltoides, P. trichocarpa × P. deltoides, and the eastern and western Washington clones of P. trichocarpa, respectively. In all clones, net photosynthesis decreased about 14% as VDD increased from 3 to 18 g H2O m–3. Stomatal conductance decreased sharply with decreasing xylem pressure potential (XPP) in all clones except the western Washington
clone of P. trichocarpa. Stomata in this clone were insensitive to changes in XPP and did not control water loss. Complete stomatal closure (stomatal
conductance < 0.05 cm s–1) occurred at about –2.0 MPa in the eastern Washington clone of P. trichocarpa and around –1.25 MPa in the P. deltoides and P. trichocarpa × P. deltoides clones. Transpiration rates were highest in the P. trichocarpa × P. deltoides clone and lowest in the western Washington clone of P. trichocarpa. The P. deltoides clone and eastern Washington clone of P. trichocarpa had the highest water use efficiency (WUE) and the western Washington clone of P. trichocarpa had the lowest WUE. The hybrids were intermediate.
It was concluded that: (1) gas exchange characteristics of eastern and western Washington clones of P. trichocarpa reflected adaptation to their native environment; (2) crossing the western Washington clone of P. trichocarpa with the more drought resistant P. deltoides clone produced plants better adapted to the interior Pacific Northwest climate, although the stomatal response to soil water
deficits in the hybrid was conservative compared with that of the eastern Washington clone of P. trichocarpa; and (3) introducing eastern Washington clones of black cottonwood into breeding programs is likely to yield lines with favorable
growth characteristics combined with enhanced WUE and adaptation to soil water deficits.
