Non-bearing, potted, apple plants were subjected to root temperature of 8 /- I(LRT) or 23 +/- 1 degrees C (HRT) and irrigated with 100 mi of water co
ntaining 20 mg of N as NH4NO3, in which both ammonium and nitrate-N fractio
ns were enriched in N-15 (10 atom%). The root system of each plant was pres
surized (325 kPa) at day 1, 2, 4, and 8 after N-15 application to evaluate
the transport of nitrogen derived from fertilizer (NDFF) through the xylem.
This technique was used to investigate N absorption. LRT reduced the rate
of N uptake the day after N-15 application relative to HRT. Two, 4 and 8 d
after fertilization, the rate of exudation and consequently the uptake rate
was similar for LRT and HRT. The total amount of N removed by plants after
8 d was not affected by root treatments. Carbon dioxide assimilation, tran
spiration rate and stomatal conductance were lower for trees subjected to L
RT during the time course of the experiment. We suggest that the delay in N
absorption was related mostly to the lower activity of root nitrate reduct
ase detected at the lower root temperature, which increased the nitrate xyl
em sap concentration and diminished the rate of N uptake.