The influence of the host plant in inducing parasitism in lucerne and clover nodules

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Abstract
When inoculated lucerne seedlings are placed in the dark, formation of fresh nodules is soon stopped and nodules already formed soon cease to grow in size. This is associated with, and probably due to, cessation of cell division throughout the root. In such nodules, the bacteria become parasitic on the host tissues, destroying the cytoplasm and nuclei of infected cells, and invading cell walls and intercellular spaces. In old nodules on lucerne and clover plants growing in the light, the bacteria behave similarly. The swollen forms in the cytoplasm, after destruction of the host nucleus, break up into granules which eventually disappear, while coccoid rod forms from the old infection threads invade the cell wall and intercellular spaces; causing nodule tissue to disintegrate. It is suggested that in both these cases the change in bacterial behavior is due to lack of carbohydrate and that, where this is limited, the bacteria derive their energy from the host tissue. This hypothesis implies that in normal nodule tissue some other factor, perhaps air supply, limits the bacterial population and thus prevents attack of the host tissues. Observations on lucerne seedlings growing in agar show that where the air supply is inadequate, the nodules do not function normally, although these conditions do not cause the bacteria to injure the nodule tissue, the carbohydrate supply not being the factor limiting their growth.

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