Morphological Studies of the Roots of Red Pine, Pinus Resinosa. Ii. Fungal Colonization of Roots and the Development of Mycorrhizae

Observations are reported on the distribution and appearance of long‐ and short‐root apices of nursery seedlings and plantation trees. Only emerging lateral roots over 50% of the diameter of the mother roots become long‐roots, and only from 10 to 21% attain this initial size. Short‐roots have rounded apices with small, sharply delimited, hemispherical or lens‐shaped meristematic zones. Long‐roots have pointed apices with paraboloid‐shaped meristematic zones showing light‐absorbing zones near the apex as well as near the base of the paraboloid. In small‐diameter long‐roots, lateral short‐roots tend to be uniformly distributed throughout the increment, but in large‐diameter roots they are closer together at the two ends of the increment. Lateral roots may be missing completely in the largest‐diameter pioneer roots. Both long‐ and short‐roots are inhabited by various species of fungi, mycorrhizal and non‐mycorrhizal. The patterns of fungal distribution in long‐roots vary from the proximal to the distal ends of the seasonal growth increments, depending on root growth and morphology. Mycorrhizal‐forming fungal species appear to predominate during the growing season but non‐mycorrhizal species may become prominent during the dormant season. The reactivation of apical and lateral meristems following dormancy occurs somewhat in advance of growth resumption by the fungi. However, delay in growth of the fungi is usually not long enough to permit emerging laterals to escape infection from the developing Hartig net in the mother root. Very large first‐order laterals of 3–0 seedlings and large pioneer roots in the plantations may ultimately acquire a sufficiently rapid growth rate to escape the fungus. When this occurs lateral roots may be initiated from a fungal‐free zone in the mother root and may become infected later. A very sparse Hartig net suffices for a strong invasion of emerging laterals, because hyphal growth is stimulated in the vicinity of a developing lateral.