The persistence and productivity of subterranean clover in southern Australia with special reference to rate of development in different cultivars

Subterranean clover, Trifolium subterraneum L. (commonly referred to as sub clover) has long been recognised as the key to improvement of annual pastures in southern Australia. Although the precise acreage of sub clover is difficult to determine, Donald (1970) has suggested that it may have been sown on as much as 80% of the present estimated area of 50 million acres (20 million ha) 0f sown pasture in this region (see Fig.1). This area (especially the western part of it) has a Mediterranean-type climate with mild wet winters and hot, dry summers. The total annual rainfall varies from about 8 inches (200 mm) to 40 inches (1000 mm) and the length of the growing season (the period during which rainfall relative to evaporation is sufficient to support plant growth) ranges from less than 4 months in the low rainfall areas of Western Australia to 10-11 months in some parts of south-eastern Australia. The species of which the pastures in this area are composed, are predominantly annuals. They become re-established each year following the autumn break, and flower and set seed during the spring prior to over-summering in the seed phase. The autumn break is on average later and the onset of the dry summer period earlier as the annual rainfall becomes smaller, but, in any one location, the year to year variation in the length of the growing period is considerable. In isolated areas reasonable constancy in length of growing period is achieved by autumn and spring irrigation. Pastures may be permanent, becoming re-established each year over an indefinite period, or temporary, in which case a pasture phase of one to several years alternates with a cropping phase, mainly cereals, of 1-3 years. Sub clover has proved to be well adapted to the conditions prevailing in southern Australia and it has played a key role in the pastures because of its capacity to fix atmosphere nitrogen. This together with liberal dressings of superphosphate which have been the rule has greatly improved the fertility status of the soils, thereby increasing the productivity of the pastures themselves and the crops grown in sequence with them. That sub clover had agricultural potential was first realised by Mr A. W. Howard in 1889 in the Mt. Barker district of South Australia, and his efforts to publicise it in the face of public indifference, have been well documented (Hill 1936; Davies 1951; Morley 1961; Symon 1961). There was a period of little progress. Then, through the efforts of many research workers, came an advance in knowledge of the plant and how it could best be used, and, with this, a tremendous increase in its use. There still exists opportunities for its greater use in new areas in southern Australia and for better use in some existing areas. This will require additional research to provide new cultivars and a better understanding of the interaction between genotype and environment. Matching of the genotype with the environment and the importance of this in determining the persistence and productivity of the species will be the main theme of this thesis. The thesis begins with a literature review in which variation within the species and how this is implicated in its widespread use in agriculture are the underlying considerations. This is followed by a report of work concerning the effects of various factors of the environment on the developmental physiology of the plant