Canopy position and needle age affect photosynthetic response in field-grown Pinus radiata after five years of exposure to elevated carbon dioxide partial pressure

Photosynthesis of tree seedlings is generally enhanced during short-term ex posure to elevated atmospheric CO2 partial pressure, but longer-term studie s often indicate some degree of photosynthetic adjustment. We present physi ological and biochemical evidence to explain observed longterm photosynthet ic responses to elevated CO2 partial pressure as influenced by needle age a nd canopy position. We grew Pinus radiata D. Don. trees in open-top chamber s for 5 years in sandy soil at ambient (36 Pa) and elevated (65 Pa) CO2 Par tial pressures. The trees were well watered and exposed to natural light an d 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, suggestin g a reduction in carbohydrate sink strength as a result of increasing needl e age (Turnbull et al. 1998). In 5-year-old trees (spring 1997), when folia ge expansion was occurring, photosynthetic downregulation 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 ca pacity or Rubisco activity and concentration. Current-year needles were mor e responsive to elevated CO2 Partial pressure than I -year-old needles, exh ibiting larger relative increases in net photosynthesis to elevated CO2 par tial pressure (98 versus 64%). Lower canopy and upper canopy leaves exhibit ed similar relative responses to growth in elevated CO2 partial pressure. H owever, needles in the upper canopy exhibited higher net photosynthesis, ph otosynthetic capacity, and Rubisco activity and concentration than needles in the lower canopy. Given that the ratio of mature to juvenile foliage mas s in the canopy will increase as trees mature, we suggest that trees may be come less responsive to elevated CO2 partial pressure with increasing age. We conclude that tree response to elevated CO2 partial pressure is basedpri marily on sink strength and not on the duration of exposure.