Longleaf pine photosynthetic response to soil resource availability and elevated atmospheric carbon dioxide

Gas exchange responses during a drought cycle were studied in longleaf pine (Pinus palustris Mill.) seedlings after prolonged exposure to varying leve ls of atmospheric CO2 (approximate to 365 or approximate to 730 mu mol CO2 mol(-1)), soil N (40 or 100 kg N ha(-1) yr(-1)), and water ("adequate" and "stressed"). Elevated atmospheric CO2 concentration increased photosynthesi s, tended to decrease stomatal conductance, and increased water-use efficie ncy (WUE). Although soil resource availability influenced gas exchange meas urements, it generally did not affect the magnitude or direction of the res ponse to CO2 concentration. However, significant interactions among treatme nt variables Here observed for plant xylem pressure potential. In seedlings grown with high N, a positive growth response to elevated atmospheric CO2 increased whole-plant water use resulting in more severe plant water stress , despite increased leaf-level WUE; however, under low N conditions the lar k of a growth response to elevated CO2 reduced wholeplant water use, decrea sed water stress severity, and increased WUE. Photosynthetic response to CO 2 was greatest in the high N treatment at the beginning of the drought cycl e, but diminished as water stress increased: however, plants grown with low N showed greater photosynthetic responses to CO2 later in the drought cycl e. Therefore, plant gas exchange rates interact with growth response in det ermining the severity of water stress under drought and, thus, the ability of elevated atmospheric CO2 to ameliorate the effects of drought and allow plants to maintain increased rates of photosynthesis mag be influenced bq t he availability of other resources, such as N and water.