CAWTHRON INSTITUTE BULLETIN NO. 10

Nelson Evening Mail, Volume LXVI, 11 August 1934, Page 5

CAWTHRON INSTITUTE BULLETIN NO. 10

l!y K. M. Curtis, M.A., D. 1.0., 1 Chief <#f the Mycology Departuieiil.

THE SYMBIOTIC BACTERIA OF LUCERNE AND SEED-INOCULATION

For many centuries farmers have realised that leguminous plants enrich

sl a nee prepared from seawtod, lias now ljut'onie fairly general. In some

tin* soil for succeeding crops. As long i ago as the middle of last century the belief vcas already held that it was the nodules on their roots that enabled legumes to bring about this process, and about the same time bacteria were discovered to be regularly present in the nodules. Not long afterwards the special faculty of nitrogen-fixation posseted by these plants was recognised to be due to the activities of the nodule bacteria. THE BACTERIA ' The bacterium is typically a short, rod-shaped organism. When young it appears uniform in structure, but after it lias aged for a few days its contents become aggregated into several granules. These granules are-eventu-ally liberated into the soil as minute round bodies bearing on their surface an equipment of liair-like flagella, through the lashing action of which the organism swims freely in the soil moisture. This active form is known as a cocus. It is of primary importance to the lucerne seedling that there should be many cocci close at band when the seedling’s root emerges from tlie seed. For it is as cocci that the bacteria gain ingress into the plant s rootlets. Whether a coccus achieves infection or not, however, the flagella are soon lost and the coccus then elongates into a rod-sliaped bacterium, similar to that from which it originated. ’[’lie bacterial life cycle is thus i completed and a second generation brought into being, all within a very short time. FORMATION OF THE NODULE The process of infecting the plant is interesting. The cocci by swimming find their way to the tips of the fine root-hairs that clothe the youngest roots and rootlets. Owing to the effect of a secretion from the bacteria the root-hairs curl over at their tip, whereupon the bacteria invade the hair through this quite limited, crumpled region. In lucerne seedlings infection takes place about the time when the first true leaf begins to expand, while in tlie re-infection of mature plants it occurs at a comparably early stage in newly developing rootlets.' On entering the hair-tip the bacteria multiply rapidly and spread downwards as a mass within the hair, until they reach and pass beyond the junction of hair and rootlet into the tissues of tlie rootlet itself. Here their presence so stimulates the cells in the invaded region that the latter divide rapidly and a small swelling results—the beginning of a nodule. As the process continues the bacteria pass between and eventually into the dividing cells until the nodule attains full size. There always remains, however, a cu\ei of non-infected cells forming the sur-face-layer of the nodule. The nitrogen utilised in the fixation process reaches the bacteria within the nodule by diffusion from the air of the soil through the nodule tissues. Good soil-aeration is thus a necessity for maximum fixation. The greatest nitro-gen-fixing activity occurs about the time that tlie lucerne plant comes into flower. After this period some of tlie nodules decay away, and their bacteria are released into the soil. Here they remain over the winter ready to renew activity as spring approaches. A fresh set of nodules is formed early in spring on tlie original plant, as the result of infection of the new rootlets then developing. In efieet therefore, provided soil conditions remain suitable, tlie nodule population on a plant is normally restored year after year. ALTERNATIVE METHODS OF INOCULATION The first experimental inoculation of soil of which we have record was made in ]SB7, and took tlie form of spreading over new land soil that had previously borne a" good leguminous crop. To the German, Nobbe, in 1890, however belongs tlie credit of originating t the more dependable process of directly j inoculating tlie seed with pure cul- ' tures of tlie bacteria, rather than of relying on chance-inoculation from tlie soil. Success however is still frequently obtained by the soil-spreading method provided certain conditions are operative. The inoculating soil must lie within a short distance of the land to he treated, otherwise the cartage of the necessary 809 to 500 pounds of soil per acre is too expensive to he practicable. For some time before its removal it should be kept free from weeds lest they he introduced on to the lucerne area. The distribution of the inoculated soil must he even and, finally, the whole process of spreading and harrowing-in must he carried out in dull light. For exposure to bright sunshine during the drying like-

eases the lucerne seed is sent foi inoculation to institutions handling this type of work. Imt as a rule agar cultures are forwarded to the farmer. > who then does the inoculation himself. SEED INOCULATION ! The method devised by Thornton of i the Rothamsted Experimental Station . in England, and now adopted in most British Dominions, is for the bacterial mass contained in one culture tube to be mixed with i pint of skim milk to , which a little calcium phosphate has been added. This quantity should be • sufficient to treat 15 pounds of seed, i The inoculation is simply a matter of 1 thoroughly wetting the seed with the • mixture, and then spreading it im--1 mediately in a thin layer to dry. All that is done, therefore, is to ensure . that the bacteria, by being carried on ; the seed-coat, will be close to the seedling’s root when it bursts through i the coat at germination. The drying of the seed after treatment should be ! thorough, otherwise it will clog the drill. It should be sown within a week after inoculation. Owing to the effect : of the milk on the surface of the seed i coat, treated seed when properly dried , passes more freely through the drill than untreated seed, and due allowance should be made lest too thick , sowing results. The addition of the calcium phosphate to the milk is ini - portaut, for this chemical stimulates the formation of the essential swimming cocci and increases their power of migration, with the result that nodules are produced in larger numbers and at deeper levels than when the phosphate is omitted. The members of the pea family, in relation to their bacterial colleagues, fall into more than a dozen natural groups, each characterised by its bacteria being able to form nodules on other members of the group, but not as a rule on members of other groups. The lucerne group includes the lucernes (Medicago sativa and M. falcata), bur clover (M. hispida), yellow trefoil (M. lupulina), and the white and yellow sweet-clovers (Melilotus alba and Melilotus officinalis). On the other hand all the true clovers (crimson, white, alsvke and mammoth red) and the cow-clovers belong to another group, and their bacteria will not form nodules on lucerne. (.'O.NMTIONS PROMOTING NODULE i DEVELOPMENT Certain soil conditions favour a free development of nodules. The soil must be possessed of sufficient moisture, at least when infection normally occurs, to allow of free migration of the bacteria by swimming. >Soil temperature has a direct bearing on the well-being of the bacteria, the optmium both for the bacteria and for the size of the resultant nodule being about 75 degrees F. An acid soil is harmful alike to the germination of the lucerne seed, to nodule formation and to the free growth of the plant. On the whole, calcium salts may be said to stimulate nodule formation. Phosphates tend to increase root development in the lucerne plant, stimulate multiplication of the bacteria in the soil and, as already mentioned, induce the appearance of the motile cocci. Applications of nitrate, on the other hand, require consideration. Light dressings of this , fertiliser on soils of somewhat low fertility assist early development in the seedling, and are said to induce I multiplication of the bacteria. Where j lucerne is sown with a nurse crop, care 1 must be exercised in the use of nitrogenous fertilisers on account of the smothering effect that may result from too free growth in the latter. Fields that may not be improved by inoculation are those winch at some time in the recent past have been under lucerne and produced good stands at that time. Fields that have borne an indifferent crop of lucerne in the past may show appreciable improvement if the seed is inoculated on resowing the field, or if suitably inoculated soil is introduced without resowing. For though nodule bacteria may be present in numbers in such soil, the condition of the first crop would indicate that they may not | be of vigorous strain. Under suitable conditions, where adequate precautions have been taken with regard to lime treatment, the bacterial inoculation of lucerne seed ensures rapid and vigorous development of the plant resulting in heavier yields and better quality of crop. As announced previously, bacterial cultures for the inoculation of lucerne seed will be available to farmers of the Nelson provincial district from the Mycology Department of the Cawthron Institute from the beginning of October, before which time the sowing •of lucerne in the Nelson district is not recommended.

ly to ensue in the process of distribution limy destroy a considerable proportion of the bacteria. I Seed treatment on the other hand is simple, cheap and incurs less likelihood of accidental failure on the part of the coccus to make contact with the root-hair. Numbers of culture media have been tested for their suitability as vehicles on which to distribute the bacteria, to farmers. Tubes of inocu- ! hited soil are still used for this purpose in parts of Europe and America, but the use of agar, a gelatin-like sub-