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.