Morphological and physiological features of roots and shoots are often changed when seedlings are exposed to water stress. In this study, we analyzed the influence of elevated CO₂ and nitrogen (N) fertilization on water relations and gas exchange in one-year-old loblolly pine (Pinus taeda L.) seedlings experiencing high and low water availability. For one growing season, loblolly pine seedlings were exposed to either ambient or elevated CO₂ (700 µl L^-1), and watered with an automated irrigation system to maintain 20% or 90% soil water holding capacity. Seedlings were fertilized twice a week to obtain high (0.2 mg N g^-1 soil yr^-1) and low N availability (0.02 mg N g^-1 soil yr^-1). At the end of the growing season, we withheld irrigation and examined shoot physiological responses of seedlings to water stress. Specifically, we examined stomatal conductance, transpiration rate, main stem and root hydraulic conductance, and root morphology. 

Results/Conclusions

The ratio of root hydraulic conductance to stem hydraulic conductance (k_r/k_s) was lower under high N availability, indicating that N promoted an increase in stem hydraulic conductance. The high water availability increased root hydraulic conductance, especially in fine roots, whereas elevated CO₂ increased in hydraulic conductance in coarse roots. Higher stem hydraulic conductance supported higher rates of leaf transpiration. As soil water availability systematically decreased during the imposed dry-down period, seedlings with higher stem hydraulic conductance decreased stomatal conductance immediately, while those with lower stem hydraulic conductance maintained moderate stomatal conductance for several days. This may have resulted from their higher fine root hydraulic conductance and higher k_r/k_s, allowing fine roots to take up more water. Specific root length (SRL) was higher under well-watered conditions and may have facilitated greater water uptake by the fine roots. However, this effect was weakened by a high N supply. Meanwhile, high N fertilization decreased SRL, but elevated CO₂ lowered the negative effect of high N on SRL, especially on higher order roots. These results indicate that different environmental factors cause different acclimation responses in roots and shoots.