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 hydroponics
with 0.5 mM N as NO3- or NH4+ in a growth chamber. Root relative growth
rate (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 herbaceous
and for most woody species. Root respiration and the activity and
capacity of the alternative pathway (AP) showed no differences between
the two N-sources. Both activity and capacity of the AP were very low
and the cytochrome pathway was fully engaged. Specific respiratory costs
were calculated for maintenance, 6.3 nmol O-2 g(-1) s(-1), for nitrate
uptake, 0.99 mol O-2 mol N-1, and for growth 5.4 mmol O-2 g(-1). These
values are similar to those found for slow-growing herbaceous species.
The major part of total root respiration, over 64%, was used for
maintenance processes, and a much smaller fraction for growth and
nitrate uptake.