Population differentiation and germination ecology in Stellaria media (L.) Vill.

Population differentiation in Stellaria media was studied with regard to life-cycle strategy and germination ecology. Two populations were identified in the study area, growing side by side, W1 and W2. The life-span of population W1 is much shorter than that of population W2, especially under summer conditions: 1-2 months versus 4-6 months; the time to flowering differs accordingly. Germination properties of seed produced under summer, winter, and field conditions were studied. Fresh seeds produced at 20° C showed good germination (ca. 75%) over a broad range of temperatures in the case of population W1, but seeds of population W2 showed appreciable germination only at high temperatures (ca. 30%). Seeds produced at 7° C showed very little (population W2) or no germination at all (population W1). To simulate seasonal changes in temperature, a "comprehensive" germination scheme was developed which enabled the response of hydrated seeds to two temperature cycles (cold-warm-cold and warm-cold-warm) to be tested. The two populations reacted differently. At the end of the cycles, only a few seeds of population W1, but about half of the seeds of population W2, remained dormant. The data obtained were used to study the effects of differential germination ecology on the dynamics of the two populations. In a reconstruction of the courses of development, the populations were shown to possess different strategies. Population W1 builds up a uniform seed stock, and population W2 a phenotypically diverse seed reserve. The implications for population dynamics are discussed.