Despite the widespread recognition that root hydraulic conductivity (Lp) is a major determining factor of whole plant water loss and/or status, there is very little information on how it responds to rising levels of atmospheric CO2. Studies of plant water status responses to high CO2 often use stomatal conductance (gs) as a critical mechanism but it is currently unknown if Lp is coordinated or decoupled from shoot conductances. Therefore, in this study we examined Lp of four contrasting conifers to CO2 enrichment. Because Lp has been shown to be highly sensitive to availability and/or N-form we also examined if the potential CO2 effects are mediated by nitrate-N or ammonium-N nutrition. One year-old seedlings of Colorado spruce (Picea pungens), Douglas-fir (Pseudotsuga menziesii), Frasier-fir (Abies fraseri) and Norway spruce (Picea abies) were grown at near ambient (400 ppm) or elevated (600 or 800 ppm) CO2 concentrations under identical environmental regimes. Plants were fertilized using a modified Hoagland solution differing in N-form: nitrate-N, ammonium-N or ammonium nitrate as control. After six months of CO2 and N-form treatments, Lp was measured using intact detopped root systems and a pressure chamber.
Results/Conclusions
CO2 enrichment significantly affected Lp but this effect was highly species-specific. For example, across N treatments, Lp increased by an average of 61% in Colorado spruce and Norway spruce while it decreased by 25% in Douglas-fir and Frasier-fir with CO2 enrichment. In general, there was no consistent effect of inorganic N-form on Lp. However, the CO2 induced changes in Lp was dependent on N-form. For example, in Norway spruce increased CO2 from 400 to 800 ppm resulted in 44% increase in Lp in control and nitrate-fed plants but not in ammonium-fed plants. Using a covariate analysis we examined Lp responses to CO2 enrichment simultaneously with changes in total leaf area. The results indicate that in Colorado spruce and Douglas-fir Lp responses to changes in CO2 do scale to changes in leaf area, but not in Norway spruce and Frasier-fir. Therefore, Lp responses to high CO2 and the coordination with canopy demand for water are species-dependent. To our knowledge, this is the first study that examines Lp responses of multiple species to CO2 enrichment under different N-forms. The results show a strong effect of CO2 on Lp which appears to be partially decoupled from canopy conductivity. This marked effect of CO2 on root water transport properties is highly regulated by inorganic N-form.