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.) needles for plants exposed to elevated atmospheric CO2 may provide insight into the potential influences of global CO2 increases on plant productivity. Longleaf pine seedlings were grown in open-top field chambers supplied with either ambient (∼365 μmol mol−1) or elevated (∼720 μmol mol−1) atmospheric CO2 for 20 mo. Two levels of soil nitrogen (40 and 400 g ha−1 yr−1) and two soil moisture regimes (−0.5 or −1.5 MPa predawn xylem pressure potential) were used in combination with CO2 treatments. Needle tissue was collected 12 and 20 mo after treatment initiation and subjected to light and scanning electron microscopy. There was no effect of elevated CO2 on stomatal distribution or the proportion of internal leaf area allocated to a given tissue type at either sampling date. Although the relationships between vascular, transfusion, mesophyll, and epidermal tissue cross-sectional areas to total leaf cross-sectional areas appear nonplastic, leaves grown in elevated CO2 with low N availability exhibit anatomical characteristics suggestive of reduced capacity to assimilate carbon, including decreased mesophyll cell surface area per unit needle volume (in low-N soil). Significantly greater (8%) needle fascicle volume as a result of growth in elevated CO2 was observed after 12 mo because of thicker needles. After 20 mo of exposure, there was a trend indicating smaller fascicle volume (8%) in plants grown with elevated CO2 compared with those grown in ambient conditions, resulting from shorter needles and smaller mesophyll, vascular tissue, and epidermal cell cross-sectional areas. These results indicate short-term stimulation and long-term 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 dicotyledons, including other tree species.