Photosynthetic acclimation of Pinus taeda (loblolly pine) to long-term growth in elevated pCO₂ (FACE)

Growth in elevated pCO₂ generally leads to a stimulation of net CO₂ uptake rate. However, with long-term growth the magnitude of this stimulation is often reduced. This phenomenon, termed acclimation, has been largely attributed to a loss of Rubisco (ribulose 1,5 bisphosphate carboxylase). The mechanism by which Rubisco content declines with long-term growth is not certain. There is evidence for a sugar-mediated, selective down-regulation of Rubisco protein and also for a non-selective loss of total leaf nitrogen, which impacts Rubisco levels indirectly. Over a season, and including needles at different developmental stages, we investigated these two potential mechanisms in well-developed Pinus taeda grown for approximately 2.5 years in elevated (56 Pa) pCO₂ using free air CO₂ enrichment technology. Photosynthetic acclimation, as manifested by a decrease in the activity of Rubisco measured both in vivo (- 25%, via gas exchange) and in vitro (- 35%, via enzyme assays), was observed with growth in elevated pCO₂. This acclimation was observed in one-year-old needles but not in current-year needles. Needles exhibiting acclimation had reduced levels of Lsu Rubisco (- 25%) and an increased foliar carbohydrate content (+ 30%) but showed no evidence of a decrease in needle nitrogen or total protein content. These data support the concept that photosynthetic acclimation in elevated pCO₂ is caused by a selective down-regulation of Rubisco.