ROOT RESPIRATION, GROWTH AND NITROGEN UPTAKE OF QUERCUS-SUBER SEEDLINGS

We tested the influence of two N-sources, NO3- and NH4+, on root growt h and respiration of Quercus suber L. Seedlings were grown on hydropon ics with 0.5 mM N as NO3- or NH4+ in a growth chamber. Root relative g rowth rate (RGR) of NO3--fed seedlings ranged from 9 to 34 mg g(-1) da y(-1). These values are lower than those described for slow-growing he rbaceous and for most woody species. Root respiration and the activity and capacity of the alternative pathway (AP) showed no differences be tween the two N-sources. Both activity and capacity of the AP were ver y low and the cytochrome pathway was fully engaged. Specific respirato ry costs were calculated for maintenance, 6.3 nmol O-2 g(-1) s(-1), fo r 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 herbaceo us species. The major part of total root respiration, over 64%, was us ed for maintenance processes, and a much smaller fraction for growth a nd nitrate uptake.