Effects of CO2 enrichment on growth and root 15NH4+ uptake rate of loblolly pine and ponderosa pine seedlings

Open Access

1 November 1996

journal article
research article
Published by Oxford University Press (OUP) in Tree Physiology

Vol. 16 (11-12) , 957-962
https://doi.org/10.1093/treephys/16.11-12.957

Abstract

We examined changes in root growth and ¹⁵NH₄⁺ 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 CO₂ partial pressures of 35 or 70 Pa with NH₄⁺ as the sole N source. Kinetics of ¹⁵N-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 with ¹⁵N. In both species, the elevated CO₂ treatment caused a significant downregulation of ¹⁵NH₄⁺ uptake capacity in excised roots as a result of a severe inhibition of the maximum rate of root ¹⁵NH₄⁺ uptake (V_max). Rates of ¹⁵NH₄⁺ uptake in intact roots were, however, unaffected by CO₂ treatment and were on average 4- to 10-fold less than the V_max in excised roots, suggesting that ¹⁵NH₄⁺ absorption from the soil was not limited by the kinetics of root ¹⁵NH₄⁺ uptake. Despite the lack of a CO₂ effect on intact root absorption rates, ¹⁵NH₄⁺ uptake on a per plant basis was enhanced at high CO₂ concentrations in both species, with the relative increase being markedly higher in ponderosa pine than in loblolly pine. High CO₂ concentration increased total ¹⁵NH₄⁺ 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 CO₂ enrichment is largely the result of a compensatory increase in root absorbing surfaces.

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