Effects of root-zone temperature on nitrogen accumulation by non-bearing apple trees

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 containing 20 mg of N as NH4NO3, in which both ammonium and nitrate-N fractions were enriched in N-15 (10 atom%). The root system of each plant was pressurized (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, transpiration rate and stomatal conductance were lower for trees subjected to LRT 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 reductase detected at the lower root temperature, which increased the nitrate xylem sap concentration and diminished the rate of N uptake.