Evaluation of the importance of acclimation of needle structure, photosynthesis, and respiration to available photosynthetically active radiation in a Scots pine canopy

We analyzed the combined effect of differences in the photosynthetic light response curve and in the distributions of photosynthetically active radiation (PAR) irradiance within the canopy on the CO2 exchange rates of Scots pine (Pinus sylvestris L.) shoots. Nitrogen concentration did not vary with depth within the canopy, but leaf mass per area (LMA) ranged from 58.2 to 95.2 g.m(2) (all needle age-classes pooled) and increased with increasing available PAR. The photosynthetic light response curves of 75 randomly sampled, 1-year-old shoots (with a fixed structure) were measured in the laboratory. No statistically significant differences in photosynthetic parameters or stomatal conductance either on an area or mass basis were detected between the top, middle, and bottom zones of the canopy. However, a significant decrease occurred in the area-based dark respiration rate (R-d) with increasing depth in the canopy. The area-based maximum CO2 exchange rate was weakly correlated with needle nitrogen content (N) and LMA, whereas R showed a higher correlation with both N and LMA. Estimates of the CO2 exchange rate over a day (24 h) in July suggest that the apparently small differences in mean light response curves of the canopy zones are reflected in the enhanced performance of shade needles in low light conditions because of reduced respiration costs. Based on our results, structural acclimation of needles along the light gradient, rather than changes in biochemical machinery, appears to be the more important acclimation process in Scots pine.