© 1997 Heron Publishing—Victoria, Canada
Seasonal photosynthetic responses to light and temperature in white spruce (Picea glauca) seedlings planted under an aspen (Populus tremuloides) canopy and in the open
Rongzhou Man and Victor J. Lieffers
Department of Renewable Resources, University of Alberta, Edmonton, AB T6G 2E3, Canada / Received August 2, 1996
Summary
Photosynthetic light and temperature response curves were measured seasonally in seedlings of white spruce (Picea glauca (Moench.) Voss) grown for two years in the understory of aspen (Populus tremuloides Michx.) or in the open in central Alberta. Light-saturated rate of net photosynthesis, the optimum temperature for net photosynthesis,
transpiration rate, photochemical efficiency, and stomatal and mesophyll conductances increased from spring to summer and
declined thereafter, whereas dark respiration rate and compensation and saturation points were highest in spring. Depression
of photosynthetic parameters was greater in open-grown seedlings than in understory seedlings during the periods in spring
and autumn when night frosts were common. Net photosynthetic rates were similar in understory and open-grown seedlings in
summer, but they were significantly lower in open-grown seedlings in spring and autumn. Significantly lower transpiration
rates and stomatal conductances in open-grown seedlings than in understory seedlings were also observed at 15 and 25 °C in
the autumn. Shoot and needle growth were less in open-grown seedlings than in understory seedlings. In summer, when irradiances
were low in the aspen understory, understory white spruce seedlings maintained a positive carbon balance by decreasing their
compensation and saturation points and increasing their photochemical efficiency compared to spring and autumn.
Keywords:
compensation point, photochemical efficiency, saturation point, seasonal photosynthesis, stomatal conductance, transpiration,
understory.
