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 radiat ion (PAR) irradiance within the canopy on the CO2 exchange rates of Scots p ine (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 9 5.2 g.m(2) (all needle age-classes pooled) and increased with increasing av ailable PAR. The photosynthetic light response curves of 75 randomly sample d, 1-year-old shoots (with a fixed structure) were measured in the laborato ry. No statistically significant differences in photosynthetic parameters o r stomatal conductance either on an area or mass basis were detected betwee n the top, middle, and bottom zones of the canopy. However, a significant d ecrease occurred in the area-based dark respiration rate (R-d) with increas ing depth in the canopy. The area-based maximum CO2 exchange rate was weakl y correlated with needle nitrogen content (N) and LMA, whereas R showed a h igher correlation with both N and LMA. Estimates of the CO2 exchange rate o ver a day (24 h) in July suggest that the apparently small differences in m ean light response curves of the canopy zones are reflected in the enhanced performance of shade needles in low light conditions because of reduced re spiration costs. Based on our results, structural acclimation of needles al ong the light gradient, rather than changes in biochemical machinery, appea rs to be the more important acclimation process in Scots pine.