Influence of Emergence Date and Strain on Phenology, Seed Production, and Germination of Thlaspi arvense L.

Plants of Thlaspi arvense L. flowered over a long period of time during summer. Plants of the early-flowering strain (EF) that emerged in the fall and overwintered flowered first, followed by overwintered plants of the late-flowering strain (LF). About 1 mo later, plants of both EF and LF strains that germinated in the spring began to flower Spring-emerging plants grew taller and had more branches but fewer lateral shoots than plants that had overwintered. In addition, the spring-emerging plants produced larger numbers of smaller seeds with no difference in total seed dry weight. LF plants grew taller than those of the EF strain. Under field conditions, seeds from these plant sources had their highest germination rates the first fall. In subsequent years, germination rates were greatest in spring with a secondary peak in the fall and little germination during the summer. Also, seeds from LF plants germinated at a faster rate than did those of the EF strain over a 3-yr period. Strain and germination period had significant effects on T. arvense, increasing the variability in phenology and morphology among populations, and likely increasing the species' tolerance to a wide range of environmental conditions. Variability in time to flower within strains appears to be under polygenic control.