Seed-Setting in subterranean clover (Trifolium Subterraneum L.). II. Strain-environment interactions in single plants

Abstract

The relationship between seed-setting (expressed as mature seeds per inflorescence), strain, and environment was investigated in subterranean clover, with special reference to burr burial. Single plants of a range of maturity types were grown in two environments, one cool and moist, the other hot and dry but with frequent watering of the plant roots. In the mild environment, early-maturing strains buried a high proportion of burrs, and late-maturing strains relatively few. Under the more severe conditions, late-maturing types buried the majority of their burrs. Seed-setting in buried burrs was generally good, and relatively constant in all the strains and the two environments tested. The capacity to set seed above the ground varied widely amongst strains, and within each strain was greater in the mild environment. The results indicate that seed-setting is influenced strongly by the microenvironment of inflorescences and developing burrs, and that strains differ in sensitivity to such microenvironment. Burr burial is apparently a protective mechanism against unfavourable factors (probably low relative humidity and high temperature) of the atmospheric environment. Under the more severe environmental conditions, there was a positive correlation between seed-setting above the ground and percentage leaf in the plant tops. It is presumed that this was due partly to amelioration of the microenvironrnent by the foliage. The negative relationship between efficiency of seed-setting above the ground and proportion of burrs buried is discussed in terms of natural selection. The microenvironmeilt, and the differential response of strains, affected not only the number of seeds per inflorescence, but also the weight and viability of the seeds produced. The concept of a threshold embryo weight for viability is discussed.