EFFECTS OF NITROGEN SUPPLY AND ELEVATED CARBON-DIOXIDE ON CONSTRUCTION COST IN LEAVES OF PINUS-TAEDA (L) SEEDLINGS

Seedlings of loblolly pine (Pinus taeda L.) were grown under varying c onditions of soil nitrogen and atmospheric carbon dioxide availability to investigate the interactive effects of these resources on the ener getic requirements for leaf growth. Increasing the ambient CO2 partial pressure from 35 to 65 Pa increased seedling growth only when soil ni trogen was high. Biomass increased by 55% and photosynthesis increased by 13% after 100 days of CO2 enrichment. Leaves from seedlings grown in high soil nitrogen were 7.0% more expensive on a g glucose g-1 dry mass basis to produce than those grown in low nitrogen, while elevated CO2 decreased leaf cost by 3.5%. Nitrogen and CO2 availability had an interactive effect on leaf construction cost expressed on an area bas is, reflecting source-sink interactions. When both resources were abun dant, leaf construction cost on an area basis was relatively high (81. 8 +/- 3.0 g glucose m-2) compared to leaves from high nitrogen, low CO 2 seedlings (56.3 +/- 3.0 g glucose m-2) and low nitrogen, low CO2 see dlings (67.1 +/- 2.7 g glucose m-2). Leaf construction cost appears to respond to alterations in the utilization of photoassimilates mediate d by resource availability.