Influx, efflux and net uptake of nitrate in Quercus suber seedlings

Nitrate influx, efflux and net nitrate uptake were measured for the slow-gr owing Quercus suber L. (cork-oak) to estimate the N-uptake efficiency of it s seedlings when grown with free access to nitrate. We hypothesise that nit rate influx, an energetically costly process, is not very efficiently contr olled so as to avoid losses through efflux, because Q. suber has relatively high respiratory costs for ion uptake. Q. suber seedlings were grown in a growth room in hydroponics with 1 mM NO3-. Seedlings were labelled with (NO 3-)-N-15 in nutrient solution for 5 min to measure influx and for 2 h for n et uptake. Efflux was calculated as the difference between influx and net u ptake. Measurements were made in the morning, afternoon and night. The site of nitrate reduction was estimated from the ratio of NO3- to amino acids i n the xylem sap; the observed ratio indicated that nitrate reduction occurr ed predominantly in the roots. Nitrate influx was always much higher than n et acquisition and both tended to be lower at night. High efflux occurred b oth during the day and at night, although the proportion of (NO3-)-N-15 tak en up that was loss through efflux was proportionally higher during the nig ht. Efflux was a significant fraction of influx. We concluded that the acqu isition system is energetically inefficient under the conditions tested.