Delayed germination cause fitness costs in offspring under intraspecific competition
While the number of flowers produced by girdled and non-girdled E. annuus was similar at the end of their lifetime, flower development in girdled E. annuus was approximately 2 weeks slower than that in non-girdled E. annuus in the field (Fig. 2d). As most E. annuus seeds do not undergo dormancy, delays in floral production are likely to cause delays in offspring germination in newly disturbed areas. Therefore, we sought to determine the consequences of delayed germination in the context of intraspecific competition using a greenhouse competition experiment. One E. annuus plant germinated (Early Germination; EG) in a pot, and another E. annuus plant germinated (Delayed Germination; DG) at an adjacent site in the same pot 2 weeks later (Fig. 3a). Competition between the EG and DG plants was strong enough to inhibit the normal growth of DG compared to that in EG plants. The rosette sizes of the EG plants were larger than those of the DG plants (Fig. 3b), indicating a higher growth potential in many biennial plants, including E. annuus . Indeed, the DG plants produced no axillary branches, whereas the EG plants produced approximately five branches throughout the observation period (Fig. 3c, d). Importantly, the supremacy of growth in EG plants led to a larger number of flowers than that in DG plants, denoting the fitness cost of delayed germination (Fig 3e). Poor growth of DG plants compared to that of EG plants was also observed in total biomass measurements (Fig. 3f).
Several variables, such as winter dormancy, changing day length, and biotic stresses, were not reflected in the greenhouse experiment. To account for naturally occurring conditions, we further conducted a field competition experiment. E. annuus plants in the vegetative phase were transplanted to the field with a gap of 2 weeks in germination (EG and DG) at a close distance in which the canopies of EG and DG plants overlapped (Fig. 4a). We monitored the plants to track their growth and flower production during intraspecific competition. As expected, the initial gap in rosette size between the EG and DG plants persisted throughout winter and subsequently extended to the spring season (Fig. 4b). This gap implies a difference in the resources used for growth, which was represented by the number of successfully flowered plants in the subsequent spring season (Fig. 4c). The DG plants bolted less than the EG plants and produced fewer branches than the EG plants (Fig. 4d). These defects in growth in DG plants resulted in a significantly lower number of flowers (Fig. 4e) than in EG plants at the end of the flowering season of E. annuus . The number of flowers produced by DG plants was not only smaller than the flower number of EG plants at the corresponding time points but also smaller than that at 2 weeks before (the corresponding stage). Because the flower diameters of DG and EG plants were not significantly different (Fig. 4f), we concluded that DG plants produced fewer seeds than EG plants. The total biomass of the DG plants was also lower than that of the EG plants (Fig. 4g). Interestingly, DG plants were more susceptible to herbivore attacks than EG plants before overwintering (Fig. 4h), implying an additional cost of delayed germination under natural conditions. Collectively, intraspecific competition caused serious cost in DG plants in natural environments as well as in the controlled greenhouse experiment.