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

The relationship of the nodule organism to its host plant has been much discussed, some authors regarding it as an instance of true symbiosis, while others regard the organism as a parasite to which the host plant offers a certain resistance. Failures to obtain inoculation of legumes with strains of nodule bacteria belonging to a different inoculation group may be regarded as examples of such resistance. Even where nodules are formed, the fixation of nitrogen and the benefit derived by the host plant varies according to the strain of the nodule organism concerned (Stevens (1) and Wright (2) ). Some strains, while producing nodules, cause no increase in growth or nitrogen content in the host plant (3) and (4). It is uncertain whether the resistance of the host plant prevents the normal functioning of such strains, or whether they are actively parasitic on the nodule tissue. Strains of the nodule organism thus differ in their relationship to the host plant. The behaviour of a single strain in the tissues may also be altered by the condition of the host plant. Thus, when Vicia faba is grown in a boron-deficient solution, the conducting tissue develops abnormally, so that the vascular supply to the nodules is either absent or incomplete. In such nodules the bacteria fix but little nitrogen and destroy the host cells in which they lie, although the same strain in healthy plants behaves normally and fixes appreciable amounts of nitrogen (Brenchley and Thornton (5) ). It is thus possible experimentally to alter the relationship between the host plant and the bacteria, so that a strain of the latter which is normally beneficial to its host becomes actively parasitic. It was suggested that, in normal nodules, the bacteria derive their energy material from the carbohydrates conveyed to them along the vessels, but that in boron-deficient plants they are to a large extent deprived of their carbohydrates, owing to failure of the vascular supply, and derive energy by attacking the host protoplasm. If this hypothesis be correct, it should be possible to induce the change from symbiosis to parasitism by cutting off the carbohydrate supply in other ways, for example by keeping the plants in darkness.
RESP-664