© 1997 Heron Publishing—Victoria, Canada
Light availability and photosynthesis of Pseudotsuga menziesii seedlings grown in the open and in the forest understory
Han Y. H. Chen and Karel Klinka
Department of Forest Sciences, University of British Columbia, 270-2357 Main Mall, Vancouver, B.C. V6T 1Z4, Canada / Received January 30, 1996
Summary
The light environment, photosynthetic dynamics and steady-state net photosynthetic rates of lateral branch shoots of Pseudotsuga menziesii var. glauca (Beissn.) Franco seedlings growing in the open and in the forest understory were investigated in situ. Mean incident photosynthetic
photon flux density (PPFD) was 702.5 µmol m–2 s–1 on open-grown branches and 52.0 µmol m–2 s–1 on understory-grown branches. Mean daily durations of PPFD greater than 500, 200, and 50 µmol m–2 s–1 were 8.5, 31.5, and 270.3 min, respectively, on understory-grown branches, and 559.1, 700.7, and 803.3 min, respectively,
on open-grown branches. Sunflecks accounted for 32.4% of total daily photosynthetically active radiation incident on understory
branches. Following 10 min at a PPFD of 50 µmol m–2 s–1, the induction time required for net photosysnthesis to reach 50 and 90% of steady-state rates was shorter at a PPFD of 200
than at a PPFD of 500 µmol m–2 s–1 and shorter in understory-grown branches than in open-grown branches. On a leaf area basis, dark respiration rates of understory-grown
branches were lower and net photosynthetic rates were higher than those of open-grown branches exposed to low PPFD. However,
at high PPFDs, understory-grown branches had lower photosynthetic rates than open-grown branches. When measurements were expressed
on a leaf dry mass basis, there was no difference in dark respiration rates between understory branches and open-grown branches,
but net photosynthetic rates of understory branches were equal to or higher than those of open-grown branches at all PPFDs.
Keywords:
dark respiration, light acclimation, net photosynthesis, photosynthetic induction, sunflecks.
