A study of ageing of spruce [Picea sitchensis (Bong.) Carr.] ectomycorrhizas. I. Morphological and cellular changes in mycorrhizas formed by Tylospora fibrillosa (Burt.) Donk and Paxillus involutus (Batsch. ex Fr.) Fr

An age sequence of mycorrhizas formed by Paxillus involutus (Batsch. ex Fr.) Fr. and Tylospora fibrillosa (Burt.) Donk was sampled from 6–9‐month‐old seedlings of Picea sitchensis (Bong.) Carr. grown on peat in root observation chambers. Information about the ageing process was obtained from co‐ordinated studies of mycorrhizal morphology, anatomy, ultrastructure and cell vitality (indicated by FDA‐staining). Newly produced mycorrhizas were pale and turgid over most of their length with obvious extramatrical mycelium. Most mycorrhizas over 50 d old had a light turgid apical portion and a darkened wrinkled proximal portion: extramatrical mycelium was less obvious. Many remained in this state until the sampling stopped at day 142, but some were completely darkened before day 50. The morphological appearance of a mycorrhiza was not a good indicator of its chronological age. Most mycorrhizas displayed periodic busts of growth which added portions of young turgid cortex to an ageing axis. The percentage of cortical cells showing nuclei in 1 μm sections of the apical 1 mm declined from 40–50 % in mycorrhizas < 25 d old to 5–15 % in mycorrhizas 110–140 d old. Cortical cells became more vacuolate as they aged and the number of organelles appeared to decline. Senescence proceeded from the outer to the inner cortex and from proximal to distal regions. Degenerate cortical cells were present in the Hartig net zone of mycorrhizas over 25 days old. Cortical cell degeneration preceded, but was closely followed by, degeneration of adjacent Hartig net. The FDA study supported the general pattern and timing of cell death interpreted from the morphological and ultrastructural study. Cortical/Hartig net fluorescence declined markedly in mycorrhizas over 70–85 d old. The stele was the last tissue in the mycorrhiza to degenerate. It is suggested that a mycorrhiza ceases to function in nutrient and water uptake when no living cortical/Hartig net interface remains. In this study a few mycorrhizas became non‐functional by day 31. For most mycorrhizas the major decline in function took place after 85 days. This estimate is in broad agreement with the estimates of mycorrhizal life span obtained from biomass studies in the field.