© 2001 Heron Publishing—Victoria, Canada
Canopy position and needle age affect photosynthetic response in field-grown Pinus radiata after five years of exposure to elevated carbon dioxide partial pressure
David T. Tissue (1), Kevin L. Griffin (2), Matthew H. Turnbull (3) and David Whitehead (4)
1. Department of Biology, Texas Tech University, Lubbock, TX 79409-3131, USA / 2. Department of Earth and Environmental Sciences, Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964-8000,
USA / 3. Department of Plant and Microbial Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand / 4. Landcare Research, PO Box 69, Lincoln 8152, New Zealand / Received August 18, 2000
Summary
Photosynthesis of tree seedlings is generally enhanced during short-term exposure to elevated atmospheric CO2 partial pressure, but longer-term studies often indicate some degree of photosynthetic adjustment. We present physiological
and biochemical evidence to explain observed long-term photosynthetic responses to elevated CO2 partial pressure as influenced by needle age and canopy position. We grew Pinus radiata D. Don. trees in open-top chambers for 5 years in sandy soil at ambient (36 Pa) and elevated (65 Pa) CO2 partial pressures. The trees were well watered and exposed to natural light and ambient temperature. In the fourth year of
CO2 exposure (fall 1997), when foliage growth had ceased for the year, photosynthetic down-regulation was observed in 1-year-old
needles, but not in current-year needles, suggesting a reduction in carbohydrate sink strength as a result of increasing needle
age (Turnbull et al. 1998). In 5-year-old trees (spring 1997), when foliage expansion was occurring, photosynthetic down-regulation
was not observed, reflecting significantly large sinks for carbohydrates throughout the tree. Net photosynthesis was stimulated
by 79% in trees growing in elevated CO2 partial pressure, but there was no significant effect on photosynthetic capacity or Rubisco activity and concentration. Current-year
needles were more responsive to elevated CO2 partial pressure than 1-year-old needles, exhibiting larger relative increases in net photosynthesis to elevated CO2 partial pressure (98 versus 64%). Lower canopy and upper canopy leaves exhibited similar relative responses to growth in
elevated CO2 partial pressure. However, needles in the upper canopy exhibited higher net photosynthesis, photosynthetic capacity, and
Rubisco activity and concentration than needles in the lower canopy. Given that the ratio of mature to juvenile foliage mass
in the canopy will increase as trees mature, we suggest that trees may become less responsive to elevated CO2 partial pressure with increasing age. We conclude that tree response to elevated CO2 partial pressure is based primarily on sink strength and not on the duration of exposure.
Keywords:
carbohydrates, carboxylation capacity, electron transport capacity, forest trees, photosynthetic adjustment, Rubisco.
