New roots of Malus domestica Borkh MM106 apple rootstock were divided into two categories, 1) feeder roots and 2) extension roots based on morphology and their ability to take up NH4
+, were studied. The roots were harvested in August from 1-year-old potted plants growing under natural conditions in Corvallis, Ore. Extension roots were thicker and longer than feeder roots. Average diameter and length were 0.89 and 45.29 mm for extension roots and 0.27 and 5.36 mm for feeder roots. Root special length (cm/g FW) and surface area (cm2/g FW) were 11.94 and 33.17 for extension roots and 108.97 and 93.38 for feeder roots. Maximum uptake rate, Imax, Km, and root absorption power, α (α = Imax•1/Km), for NH4
+ absorption were 6.875, 0.721, and 9.48 for extension roots and 4.32, 0.276, and 15.63 for feeder roots. Feeder roots had stronger affinity to NH4
+ (low Km) and higher NH4
+ absorption power (high α value) than extension roots. The feeder roots were better able to uptake NH4
+ at lower external solution concentrations than extension roots according to the nutrient depletion curve, which indicates feeder roots being more efficient than extension roots in nutrient absorption when NH4
+ availability was low.