Thursday, August 5, 2010

PS 82-145: Elevated CO2 and different forms of N alters stem hydraulic conductivity in conifer seedlings

Anisul Islam, Tobias Gebauer, and Hormoz BassiriRad. University of Illinois at Chicago

Background/Question/Methods

Rising atmospheric CO2 has been shown to affect plant water status but the underlying mechanism(s) are not understood. Therefore, factors that control water status of plants are critical in models designed to predict hydrologic exchanges between biosphere and atmosphere. Plant water status is governed by hydraulic properties along the soil-plant-and-atmosphere-continuum which includes the axial conductivity in the stem.  In this study we examined stem conductivity of four conifer seedlings in response to different CO2 levels. Plant hydraulics has also been shown to be affected by the availability and/or form of nitrogen (N). Therefore, we also examined the interacting effects of N-form (ammonium vs. nitrate) and CO2. One year-old seedlings of Douglas-fir (Pseudotsuga menziesii), Norway spruce (Picea abies), Colorado spruce (Picea pungens), and Fraser Fir (Abies fraseri) were treated at either near ambient (400 ppm), or elevated (600 or 800 ppm) CO2 for six months. Thermal and photo periods were identical for all treatments but seedlings received a modified Hoagland solution which contained either ammonium, nitrate, or ammonium nitrate. We measured stem specific conductivity (ks) and leaf specific conductivity (LSC). ks and LSC was calculated by dividing the stem hydraulic conductivity by stem cross sectional area and total needle area, respectively.
Results/Conclusions

Elevated CO2 significantly affected kS but this effect was highly species dependent. For example, in Colorado spruce, increasing CO2 levels from 400 to 800 ppm, increased ks by an average of 23 % across all N treatments but decreased ks in Douglas fir by 24 %.  Overall, there was no significant effect of N form on ks however, the CO2 induced changes in ks was highly depended on N form. For example, in Colorado spruce ks increased by 102% in nitrate but decreased by 20% in ammonium treated seedlings with the doubling of CO2 concentration. Statistically LSC responses to CO2 were similar to ks i.e., the effect of elevated CO2 on LSC was highly dependent on species and within species LSC depended on N form.  Despite this statistical similarity, ks and LSC responses within species were quite dissimilar suggesting that the CO2-induced changes in ks are not mediated by an adjustment in canopy size and/or canopy water conductance. The results suggest that as CO2 levels increases, there is a significant adjustment in ks which is apparently not coordinated with the canopy conductance. Preliminary data indicates that ks responses to rising CO2 levels are more closely coupled to root hydraulic conductance.