SPEED OF DEVELOPMENT OF RADIATE AND NON‐RADIATE PLANTS OF SENECIO VULGARISL. FROM HABITATS SUBJECT TO DIFFERENT DEGREES OF WEEDING PRESSURE

Summary In a cultivation experiment, plants of both radiate and non-radiate Senecio vulgaris L. (S. vulgaris ssp. vulgaris var. hibernicus Syme and var. vulgaris) from intensively weeded habitats develop more quickly from sowing to first fruiting than plants from less intensively-weeded habitats. Within the same habitat, non-radiate plants are quicker from sowing to first fruiting than radiate plants. This shortening of the life-cycle is achieved by a faster development from sowing to the formation of the first capitulum buds, resulting in shorter plants with fewer leaves on the main axis. In contrast, the development from anthesis to first fruiting takes longer in quick-developing plants, and the number of achenes on the first fruiting capitulum as well as the potential reproductive output is smaller. This is explicable in terms of reduced availability of resources resulting from the smaller number of leaves in the quick-developing plants. The possible origin of the two populations of radiate S. vulgaris is discussed.