Root respiration, growth and nitrogen uptake of Quercus suber seedlings

We tested the influence of two N-sources, NO3- and NH4+, on root growthand respiration of Quercus suber L. Seedlings were grown on hydroponicswith 0.5 mM N as NO3- or NH4+ in a growth chamber. Root relative growthrate (RGR) of NO3--fed seedlings ranged from 9 to 34 mg g(-1) day(-1).These values are lower than those described for slow-growing herbaceousand for most woody species. Root respiration and the activity andcapacity of the alternative pathway (AP) showed no differences betweenthe two N-sources. Both activity and capacity of the AP were very lowand the cytochrome pathway was fully engaged. Specific respiratory costswere calculated for maintenance, 6.3 nmol O-2 g(-1) s(-1), for nitrateuptake, 0.99 mol O-2 mol N-1, and for growth 5.4 mmol O-2 g(-1). Thesevalues are similar to those found for slow-growing herbaceous species.The major part of total root respiration, over 64%, was used formaintenance processes, and a much smaller fraction for growth andnitrate uptake.