The influence of elevated atmospheric carbon dioxide on longleaf pine needles

The influence of rising atmospheric CO{dollar}sb2{dollar} on leaf structure and tissue quality of trees must be understood before it will be possible to predict the fate of trees, ecosystems, and entire biomes to atmospheric CO{dollar}sb2{dollar} levels predicted for the next century. Therefore, longleaf pine (Pinus palustris Mill.) seedlings were grown for 20 months at two levels of CO{dollar}sb2{dollar} (365 and 720 {dollar}mu{dollar}mol mol{dollar}sp{lcub}-1{rcub}{dollar}), in two levels of soil N (4 and 40 g m{dollar}sp{lcub}-2{rcub}{dollar}), and with two levels of soil moisture ({dollar}-{dollar}0.5 and {dollar}-{dollar}1.5 MPa xylem pressure potential). Leaf tissue was collected 4, 8, 12 and 20 months after initiation of the experiment and prepared for light microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Needle phenolic content was determined using the Folin-Denis method, and condensed tannins were estimated with a protein precipitation Assay at the final (20 month) harvest. Although significant interactions of soil N with CO{dollar}sb2{dollar} were observed for leaf anatomy and morphology at the first harvest (4 months), few significant main effects or interactions of CO{dollar}sb2{dollar}, soil N or water levels were detected at later harvests. At the 12 month harvest, a CO{dollar}sb2{dollar} by N by water interaction was observed for the size of starch grains within chloroplasts. Disruption of chloroplast integrity by large starch inclusions was pronounced in needles from trees grown in elevated CO{dollar}sb2{dollar} when water and N were both limiting. At 20 months, chloroplasts grown in high CO{dollar}sb2{dollar} exhibited stress symptoms including increased numbers of plastoglobuli and shorter grana. Needle surface wax density was decreased and epicuticular wax morphology was altered by growth in elevated CO{dollar}sb2{dollar} only when soil N was limiting. Total leaf polyphenol and condensed tannin contents were increased by main effects of elevated CO{dollar}sb2{dollar}, low soil N and adequately watered conditions. Elevated CO{dollar}sb2{dollar} and low N decreased deposition of calcium oxalate crystals within needle phloem compared to ambient CO{dollar}sb2{dollar} and high N. Needle tissue quality, and thus interactions between pathogens/herbivores and longleaf pine, may be altered under elevated CO{dollar}sb2{dollar}. Furthermore, decreasing effects of elevated CO{dollar}sb2{dollar} on needle morphology and anatomy with increasing length of the study, coupled with negative effects of elevated CO{dollar}sb2{dollar} on ultrastructural characteristics of the photosynthetic apparatus suggest that some degree of photosynthetic acclimation may have occurred. Results from this study provide data useful in understanding how the longleaf pine ecosystem will respond to future CO{dollar}sb2{dollar} levels. Furthermore, they suggest that pine species may be inherently less able to exploit extra carbon in a high CO{dollar}sb2{dollar} world than broadleaf species.