Effects of CO2enrichment on growth and root15
uptake rate of loblolly pine and ponderosa pine seedlings
We examined changes in root growth and15NH4/+uptake capacity of loblolly pine (Pinus taeda L.) and ponderosa pine (Pinus ponderosa Douglas, Ex Laws.) seedlings that were grown in pots in a phytotron at CO2partial pressures of 35 or 70 Pa with NH4/+as the sole N source. Kinetics of15N-labeled NH4/+uptake were determined in excised roots, whereas total NH4/+uptake and uptake rates were determined in intact root systems following a 48-h labeling of intact seedlings with15N. In both species, the elevated CO2treatment caused a significant downregulation of15NH4/+uptake capacity in excised roots as a result of a severe inhibition of the maximum rate of root15NH4/+uptake (V(max)). Rates of15NH4/+uptake in intact roots were, however, unaffected by CO2treatment and were on average 4-to 10-fold less than the V(max) in excised roots, suggesting that15NH4/+absorption from the soil was not limited by the kinetics of root15NH4/+uptake. Despite the lack of a CO2effect on intact root absorption rates,15NH4/+uptake on a per plant basis was enhanced at high CO2concentrations in both species, with the relative increase being markedly higher in ponderosa pine than in loblolly pine. High CO2concentration increased total15NH4/+uptake and the fraction of total biomass allocated to fine roots (< 2 mm in diameter) to a similar relative extent. We suggest that the increased uptake on a per plant basis in response to CO2enrichment is largely the result of a compensatory increase in root absorbing surfaces.
Bassirirad, H; Griffin, KL; Strain, BR; Reynolds, JF
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