Defoliation responses of little bluestem at the organism and population level

Abstract

Little bluestem (Schizachyrium scoparium (Michx.) Nash var. frequens F. T. Hubb) is a highly polymorphic perennial grass of North America. The studies described here deal with various effects of defoliation on little bluestem at the organism and population level. The intent was to (1) clarify the effects of differential defoliation on root initiation and root and leaf elongation of dependent tillers, (2) evaluate the effects of defoliation on four genotypes of little bluestem differing in plant height and tillering rate, (3) evaluate the long-term effects of defoliation on populations as measured by morphological and allozymic comparisons of long-term grazed and nongrazed populations and (4) evaluate the extent of apomixis in the genus Schizachyrium. It was found that the portion of the grass-shoot system defoliated has a pronounced effect on subsequent growth of dependent tillers. Uniform defoliation of all plant shoots decreased the rate of root elongation and increased the rate of leaf elongation of dependent tillers in comparison to untreated plants. These effects were not as pronounced when only a parent-shoot or dependent tiller was defoliated. Genotypes with a high rate of tillering supported 4-7 leaf stage tillers with significantly wider leaves, fewer and shorter roots, and less shoot regrowth after defoliation. The tall genotypes supported tillers with significantly fewer roots, but with greater root elongation after defoliation. These results demonstrated the significant amount of genetic variability in growth form and response to defoliation which can arise from a single sexual generation of this highly outcrossing species.