The potential of irradiation and induced polyploidy for improving St. Augustinegrass Stenotaphrum secundatum (Walt.) Kuntze

Abstract

St. Augustinegrass, Stenotaphrum secundatum is a warm-season perennial species of the tribe Paniceae. The species is classified as a sod-forming grass and is characterized by its stoloniferous growth habit and absence of rhizomes. Compared to turf species in common use it represents an extreme in texture due to its coarseness, but it is widely used and produces one of the more attractive lawns. Accessions presently used for turf purposes are propagated vegetatively and extensive areas are quite uniform. This may account for widespread incidence of the disease and insect problems with this species. The present study was undertaken to evaluate the potential value of irradiation and induced polyploidy as possible methods to use in a breeding program for S. secundatum. Seed obtained from common St. Augustinegrass lawns were irradiated with gamma rays and fission neutrons. Smears of pollen mother cells of representative M₁ plants revealed translocations and the resulting multivalent associations. Data obtained from studies of M₂ plants (progeny from M₁) included leaf length and width, internode length and diameter, quality estimations, and weekly measurements of growth. Leaf length and width and stem characters were highly correlated. However, irradiation had no apparent effect on relationships of these characters. Germinating seedlings and actively growing stolons were used in studying methods of inducing polyploidy. The common diploid accession, three triploids and one tetraploid were utilized in this study. No new ploidy levels were recovered from the treated material. It is assumed that this species is quite tolerant to colchicine treatments. The polyploid accessions and two diploid accessions were used in a study of environmental response. Of the characters evaluated, maximum leaf length was the only one which exhibited a significantly different response to environment. However, the small number of polyploid entries restricts the scope of the inferences one may draw from the data. Studies leading to the development of technique which, at present, appears suitable for screening S. secundatum seedlings for tolerance to infection by Rhizoctonia solani are included. The technique included use of potato-dextrose agar cultures of the fungus which were diluted with water and macerated into a slurry. The seed were placed on the slurry and the seedlings were evaluated after two weeks in an environment suitable for seedling and fungus growth. The condition of the root was the basis of selection. Using this technique, approximately 17,000 seedlings were screened and 99 selected as being conspicuously superior in root development. Overall implications of the data and possible alternative approaches are discussed.