LUCERNE CULTIVATION

Hawera Star, Volume LIV, 5 January 1935, Page 16

LUCERNE CULTIVATION

INNOCULATION OF SEED STUDY IN BACTERIA INGRESS INTO PLANT ROOTLETS. Eor many centuries farmers have ijealised that [leguminous plants oilrich the soil for succeeding crops. As long ago as the middle of last century the belief was 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 afteiwards the special faculty of nitrogenfixation possessed by these plants was recognised to be due to the n(ti\ ities of the nodule bacteria. The bacterium is typically a short, rod-shaped organism. AY hen young it appears uniform in structure, but after it has aged for a few (lays its contents become aggregated into several granules. These granules. are eventually liberated into the soil as minute round bodies bearing on their surface an equipment of hairlike flagella, through tho lashing action of which the organism swims freely in tho soil mosturq. This active form is known as a coccus. Tt is of primary importance to tho lucerne seedling that there should be many cocci close at hand when the seedling's root emerges from the seed. For it is as cocci"that the bacteria gain ingress into the plant’s rootlets. AA hethei a coccus achieves infection or not, however, the flagella are soon lost and the coccus then elongates into a rodshaped bacterium, similar to that fiom which it originated. The bacterial life cycle is thus completed and a second generation brought into being, ad within a very short time. The process of infecting the plant is interesting. The cocc-i by swimming find their wav 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 tho bacteria invade . tho 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 the reinfection of mature plants it occurs at a comparably earlv stage in newly developing rootlets.* On entering the hair-t:p the bacteria multiply rapidly and spread downwards as a mass within the hair until they reach and pass beyond the junction of lmir and rootlet into the tissues of the rootlet itself. Efere their nresenee so stimulates the colls in the* invaded region, that the latter divide rapidly, and a small swelling results—the beginning of a nodule. As the process continues the bacteiia pass between and eventually into the dividing cells until the nodule attains full size. There always remains, however, a cover of non-infected cells forming the surface-layer of the nodule. The nitrogen utilised in the fixation process reaches the bacteria within the nodule l>y diffusion from the air of the soil through the nodule tissues. Good soil aeration is thus a necessity for maximum fixation. The greatest nitrogen-fixing activity occurs about the time that the lucerne plant comes into flower. After this period some of the nodules decay away, and their bacteria are released into the soil. Here they remain over the winter i ready to renew activity as spring approaches. A fresh sot of nodules is formed early in spring on the original plant, as the result of infection of the new rootlets then developing. In effect, therefore, provided soil conditions remain suitable, the nodule population on a plant is normally re-» stored year after year.