Nutrient inflows into apple roots. II. Nitrate uptake rates measured on intact roots of mature trees under field conditions

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Abstract
The rates of uptake of nitrate‐N per unit length; surface area and volume of root were measured in solution depletion experiments conducted in a root laboratory, using intact roots of two 4.5‐year‐old apple trees (Discovery/M.9 and Worcester Pearmain/M.9) at two different depths in the soil profile.In Discovery/M.9, NO3− uptake rate per unit root was constant over the 20‐200 mmol m−3 range of solution concentration. In Worcester/M.9, the uptake rate per unit root over the 200‐150 mmol m−3 range (corresponding to a ‘lag’ phase) was lower than that over 150‐20 mmol m−3. The uptake rates after the lag phase at depths of 46 and 104 cm were ca. 1.3 and 5.0 times greater than those in Discovery/M.9 at the 46 and 110 cm depths, respectively. The concentration below which net uptake was zero was ca. 1 mmolm−3.In Discovery/M.9, the uptake rate per unit root at the 46cm depth was about 2.8 times that at 110 cm whereas in Worcester/M.9, the uptake rates at 46cm depth were about 1.8 and 1.4 times lower than those at 104cm over the solution concentration ranges 200‐150 and 150‐20 mmol m−3, respectively.Only small differences were observed in uptake rates per unit root between 1400‐1700 h, 2400‐0400 h, and 0700‐1100 h.For successive 5°C‐increments in root temperature between 5 and 25° C, the nitrate uptake rate per unit root increased by 130, 10, 30 and 5%, respectively. A major change in the activation energy for nitrate uptake was observed at a transition temperature located between 5°and 10°C.