Discussion
In comparison to the broad evidence supporting the benefits of tolerance responses of plants under attack, what factors hinder fitness compensation by tolerance response are unclear. Measuring fitness using appropriate parameters is crucial for assessing the adaptive value of a trait in an ecologically relevant manner . Traits or phenomena can affect plant fitness at various stages of the plant life cycle . Although plant growth sufficiently represents fitness in some cases, the production and/or germination of seeds is considered proxies closer to actual fitness. In this study, we showed that restoring the number of flowers did not fully compensate for the fitness loss caused by girdling beetle attacks, under intraspecific competition in offspring generation.
Plants under herbivore attack actively increase their photosynthetic performance to compensate for damage . E. annuus also showed increased chlorophyll content, which could represent enhanced photosynthetic performance after girdling by P. rufiventris . Moreover, insect attacks enhance branching, which results in increased reproductive organ production . Although girdling behavior caused a serious loss in flower number due to the loss of shoot apical meristem in the short term, increased axillary branch emergence in girdledE. annuus restored the number of flowers within 7 weeks of girdling. At the end of the flowering season, girdled E. annuusproduced not significantly different numbers or sizes of flowers compared to those in non-girdled E. annuus .
Our attention was drawn to the 2 weeks of delay in flower production in girdled E. annuus plants. Plant-insect interactions often result in phenological shifts in plants, and the scale of the shift varies from hours to months in the context of defense. The phenological shift is trans-generationally transmitted to make a ‘mismatch’ with herbivory . However, the delay in flower production in girdled E. annuus , which is likely to persist through seed germination due to lack of seed dormancy, caused a dramatic cost in the intraspecific competition of offspring rather than benefits from the mismatch. The DG plants, which were referred to be produced by girdled E. annuus plants, showed smaller biomass and produced fewer flowers than did the EG plants, which were referred to be produced by non-girdled E. annuus plants.
The consequences of competition between DG and EG plants were similar in the greenhouse and field experiments. Our field competition experiment was designed to test whether the competition-associated cost of delay in germination is also exhibited in the field, which has different conditions from those of greenhouse experiments . For instance, rosettes tolerate cold stress during winter and transition into a reproductive form in consecutive springs using accumulated resources . Consistent with the greenhouse results, DG plants produced smaller rosettes, lower biomass, and fewer flowers than EG plants in the field. Moreover, herbivore damage was more severe in DG plants than in EG plants during the rosette stage. As plant ontogeny significantly affects the degree of defense and the early stages are particularly vulnerable to herbivory , it can be suspected that the growth defect by the intraspecific competition caused weaker defense in DG plants, and even worsened the inferiority of DG plants over EG plants in the subsequent spring.
Notably, the growth and reproductive trends of the EG and DG plants were opposite in the two different field experiments performed with and without biotic interactions. Excluding intraspecific competition and herbivory led to similar rosette sizes in EG and DG plants. Furthermore, DG plants produced more branches and, thereby, more flowers than EG plants in the following year. The observed data may have resulted from artifacts originated from discrepancies between plant developmental stages and environmental conditions. The mismatch between resource demand determined by plant ontogeny and resource availability in the environment could cause fitness costs . Moreover, the ontogeny of plants significantly affects the recruitment of plants at the community level . While the exact mechanism underlying the low fitness of EG plants are elusive, our field assay without competition may not fully reflect the relevant phenology of E. annuus and thereby resulted in fitness defect in EG plants. Nevertheless, when EG and DG plants were planted closely and exposed to natural herbivory, as in the field, those potential defects were diminished and EG plants were superior to DG plants with respect to growth and development, as discussed previously. These imply that the biotic interaction might have a more major impact on the fitness of E. annuus compared to abiotic conditions.
Ruderal species, including E. annuus , adopt an opportunistic strategy of occupying an empty niche caused by disturbance . Disturbances occur in various ways in ecosystems, and their timing is unpredictable. Therefore, the long seed dispersal period of pioneer species might be adapted to increase the chance of invading disturbed populations , and the girdling-induced delay in flower production might increase the cost of losing access to the disturbed population. Plant-herbivore interaction exerts selection on both organisms and the effects are transmitted over a long period . Although our study was conducted under the assumption that tolerance-associated delay in flower production affects intraspecific competition and herbivory in the subsequent generation, evolutionary changes in plants and herbivore rooting for such phenomena needs to be studied in the future. Moreover, interspecific competition which was not considered in the current study but might strongly affect the fitness of tolerating plants, deserves further investigation.
Our data emphasize that the restoration of floral production in plants under herbivory does not fully compensate for fitness in the context of tolerance responses. However, the current results do not reflect all aspects of plant fitness. Although girdled E. annuus plants started to produce flowers 2 weeks later than non-girdled plants, flower and seed production continued for more than 8 weeks. The wide range of seed dispersal periods may diminish the initial gap in germination time between seeds produced by girdled and non-girdled plants.
In this study, we describe an overlooked fitness cost that occurs during tolerance responses to insect attacks. Although the tolerance response enabled attacked plants to restore the number of flowers, the restoration caused 2 weeks of delay in the germination of offspring. Delayed germination was found to cause critical defects in offspring under intraspecific competition. In other words, the fitness loss of being attacked by girdling behavior is inflicted on the offspring produced by girdled plants, even with a tolerance response. We conclude that reproductive tolerance which is considered to result in similar fitness between damaged and undamaged plants, actually may not fully compensate for the fitness depending on competition and herbivory. Furthermore, we suggest future research on plant defense, especially those involving phenological shifts, to take into account the effect of the shift in offspring to more accurately measure the adaptive value of plant defense traits.