ANALYSIS OF THE POST‐INFECTION GROWTH STIMULUS IN ORCHID MYCORRHIZA

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Abstract
Summary: The sequence of symbiotic infection of Dactylorhiza purpurella protocorms shows that commencing shortly after infection there is a linear increase in length and breadth whereas growth of uninfected protocorms is negligible.From the time when external hyphae make physical contact with epidermal hairs of the host the processes of penetration, infection of the cortex and formation of the first pelotons occupy approximately 29 hours. There is a linear relationship between the number of pelotons and time but the size of protocorms is not directly related to the number of pelotons. Evidence suggests that stimulation of growth can take place before intracellular lysis of the endophyte occurs.In the absence of external nutrients infecting hyphae are rapidly lysed and little or no growth stimulus occurs. The hypothesis is posed that ‘digestion’ of the endophyte is a defence reaction not a pre‐requisite for a growth stimulus and that nutrients are transferred across the living fungus‐host interface and stimulate growth.

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