There is a substantial theoretical and empirical literature on the optimal timing and nature of the switch from vegetative to reproductive growth in annual plants. We used the GA-deficient rosette mutant of rapid-cycling Brassica rapa to examine the effects of flowering time and competition on fecundity. We manipulated flowering by growing plants under low light intensities and applying GA at different times. We modeled competition and the invasion of a population by a flowering time mutant by growing focal plants with or without neighbors that flowered at the same or at different times from the focal plants. Without neighbors, there were no significant differences between early- and late-flowering focal plants in final height or fecundity. Early-flowering plants were not affected by competition with neighbors flowering at the same time or later, but late-flowering plants were strongly affected by competition with neighbors flowering at the same time or earlier. We conclude that under the conditions of our experiments, which included assured pollination during a definite end to the reproductive season, early flowering is always favored. This result is consistent with most empirical studies and contrary to some theoretical predictions.

Key words: annual plants, Brassica rapa, flowering time