Anatomical and morphological alterations in longleaf pine needles resulting from growth in elevated CO2: Interactions with soil resource availability

Studies of anatomical changes in longleaf pine (Pinus palustris Mill.) need les for plants exposed to elevated atmospheric CO2 may provide insight into the potential influences of global CO2 increases on plant productivity. Lo ngleaf pine seedlings were grown in open-top field chambers supplied with e ither ambient (similar to 365 mu mol mol(-1)) or elevated (similar to 720 m u mol mol(-1)) atmospheric CO2 for 20 mo. Two levels of soil nitrogen (40 a nd 400 g ha(-1) yr(-1)) and two soil moisture regimes (-0.5 or -1.5 MPa pre dawn xylem pressure potential) were used in combination with CO2 treatments . Needle tissue was collected 12 and 20 mo after treatment initiation and s ubjected to light and scanning electron microscopy. There was no effect of elevated CO2 on stomatal distribution or the proportion of internal leaf ar ea allocated to a given tissue type at either sampling date. Although the r elationships between vascular, transfusion, mesophyll, and epidermal tissue cross-sectional areas to total leaf cross-sectional areas appear nonplasti c, leaves grown in elevated CO2 with low N availability exhibit anatomical characteristics suggestive of reduced capacity to assimilate carbon, includ ing decreased mesophyll cell surface area per unit needle volume (in low-N soil). Significantly greater (8%) needle fascicle volume as a result of gro wth in elevated CO2 was observed after 12 mo because of thicker needles. Af ter 20 mo of exposure, there was a trend indicating smaller fascicle volume (8%) in plants grown with elevated CO2 compared with those grown in ambien t conditions, resulting from shorter needles and smaller mesophyll, vascula r tissue, and epidermal cell cross-sectional areas. These results indicate short-term stimulation and longterm inhibition of needle growth in longleaf pine as a result of exposure to elevated CO2 and suggest at the leaf level that pine species are less responsive to elevated CO2 than are dicotyledon s, including other tree species.