Tatsuya Fukuda’s research while affiliated with Tokyo City University and other places

We conducted comparative analyses using an open-top chamber (OTC) to reduce wind stress to clarify the impact of decreased wind stress on the invasive species Bidens pilosa L. (Asteraceae), which ranks among the worst 100 species on the Invasive Alien Species List in Japan. Morphological analyses revealed that the number and size of leaves in the OTC group were significantly higher than those in the control group (wind). There was also a significantly higher investment in stems in the former than in the latter. No significant differences were observed in root dry mass; however, the resource allocation ratio to the roots was significantly higher in the wind group than in the OTC group. Although the total seed mass was greater in the OTC group, there were no significant differences in the ratio of resource allocation to seeds between the groups, and no significant differences were observed in the mass of each seed. However, the number of seeds was significantly higher in the OTC group. Adaptive changes in the leaves, stems, and roots to avoid and/or resist wind were reflected in differences in the number of seeds. In addition, a decrease in wind stress contributed to an increase in the number of seeds in B. pilosa. Such mechanisms are likely widespread because B. pilosa is often highly abundant in urban systems.

Adaptation of Farfugium japonicum (L.) Kitam. var. japonicum (Asteraceae) to the strong wind environment of coastal areas has been shown to reduce lamina size and shorten petioles; however, their effects on other traits of this species remain unknown. Our morphological analyses showed that shortening of the scape of this species is correlated with shortening of the petiole in coastal areas. The results suggested that when the height of the scapes became higher than that of the petioles, the wind stress on the scapes became stronger and their growth was suppressed. Therefore, the populations in coastal areas with strong winds had significantly shorter scapes than inland populations, and the height of petioles and scapes in the coastal populations were correlated. Further mechanical analysis by three-point bending tests revealed that the scapes had higher strength than the petioles. This species is evergreen and can produce new leaves regardless of the season, even if it loses its leaves by strong winds; however, because scapes only develop above ground for a limited period of the year, the loss of the scapes by strong winds has a significant impact on reproduction in that year. Therefore, even though the scapes were stronger than the petiole, shortening the scapes plays an important role in reducing strong wind stress in coastal areas.

The riparian zone, found alongside rivers and streams, is a unique habitat characterized by its vulnerability to sudden floods following intense rainfall. To cope with these challenging conditions, a specific group of plants with linear and lanceolate lamina have adapted to thrive in these environments. Despite their unique ability to withstand the forceful water flow, the specific adaptive characteristics of the petioles, which support the lamina remain unknown. Our morphological, anatomical, and mechanical analyses on the petioles of Osmunda lancea (Osmundaceae) along the river and an inland sister species of O. japonica revealed that the petioles of O. lancea had a larger cell volume in subepidermal cortex and were more flexible than those of O. japonica.

Setaria viridis var. minor and Setaria × pycnocoma occur sympatrically in various places in Japan, despite their different morphological and mechanical traits. Morphological and mechanical analyses were performed to determine whether the putative hybrid S. × pycnocoma possessed the morphological and mechanical characteristics of its ancestral species, S. viridis var. minor and Setaria italica . Our results indicated that S. italica had the highest panicle weight and longest culm with the lowest culm mechanical strength and S. viridis var. minor had the lightest panicle weight and shortest culm with the highest culm mechanical strength. S . × pycnocoma had a wide range of culm strengths, including mechanical properties of both ancestral species, which suggested that the intermediate mechanical characteristics of this species had been gained by repeated introgression within themselves.

Farfugium japonicum (L.) Kitam. var. japonicum grows mainly in the coastal areas of Japan. Meteorological recording data from natural habitats were used to investigate the factors associated with the laminas and petioles of radical leaves of F. japonicum var. japonicum to avoid or resist higher wind stress. Our morphological and mechanical results indicated that petiole length and petiole cross-sectional area had a weak correlation with wind speed and breaking strength, and the petiole second area moment of inertia did not differ significantly among populations. However, both lamina area and petiole length per petiole cross-sectional area decreased with increasing wind speed, indicating that F. japonicum var. japonicum resisted or avoided an increase in wind speed outdoors by reducing the lamina area and petiole length per petiole cross-sectional area without qualitative changes in their petioles. The results of this study indicated that densely distributed recording stations of the Automated Meteorological Data Acquisition System (AMeDAS) by the Japan Meteorological Agency can be used for environmental adaptation studies of plants in the field using nearby plant populations.

Lysimachia mauritiana Lam. (Primulaceae), a biennial maritime plant, is widely distributed in East Asia, the Philippines, Micronesia, Polynesia, and the Indian Ocean. Because of this species’ wide distribution in Japan, we hypothesized that it could germinate and grow even in the bare coastal areas damaged by the tsunami caused by the Great East Japan Earthquake. Thus, we aimed to evaluate the seed germination of L. mauritiana under different sowing depths, temperatures, and salinity soil conditions. The highest germination rate was obtained by sowing L. mauritiana seeds near the soil surface, with germination rate decreasing as the seeding depth increased. Lysimachia mauritiana germinated even at relatively low temperatures. Moreover, we found that L. mauritiana seeds could germinate in less than 1% salt water and in salt-accumulated soil, using soil soaked in 5% or 10% salt water. Our results therefore suggest that L. mauritiana seeds could germinate by sowing on the bare soil surfaces damaged by the tsunami.

Alien, introduced, and non-native species expand their distribution through the development of various transportation networks. In Japan, Ligustrum lucidum Aiton (Oleaceae) was introduced in the mid-1800s for use as roadside trees, but has escaped from the planting area and spread everywhere. This species also invades coastal areas and has adapted to develop its characteristic features under various stress conditions. To clarify the adaptive features of L. lucidum in coastal areas, we conducted morphological and anatomical analyses. We found that the stomatal size of L. lucidum was smaller in the coastal populations than in inland populations, which is similar to the coastal adaptation pattern of L. japonicum Thunb., a closely related native species. Our results suggest that strong selective pressure on coastal area conditions could force their leaves to avoid excessive transpiration, and such an adaptation pattern of L. lucidum could expand its distribution to various coastal areas.

Serpentine soils consist of broadly skewed elemental profiles, including abundant toxic metals and low nutrient content in drought-prone, patchily distributed substrates; therefore, they are one of the most challenging settings for plant life. In this study, a comparative study was conducted using serpentine and inland populations of Ligustrum japonicum Thunb. (Oleaceae) to determine morphological and anatomical differences between the same species growing in the serpentine and inland areas. Longitudinal leaf sections indicated that serpentine populations had slightly thicker leaves than inland populations, contributing to the increased heights of adaxial and abaxial epidermal cells and palisade and spongy tissues. Moreover, the serpentine population had smaller stomata than the inland populations. These results suggest that the strong selective pressure under serpentine soil conditions could force leaves to restore water and avoid excessive transpiration.

To clarify the process of plant adaptation to coastal areas, we conducted morphological and anatomical analyses of Eurya japonica Thunb. (Pentaphylacaceae). There was no significant difference in leaf shape between the inland and coastal populations, although the leaves in coastal populations tended to be thicker. However, our anatomical analysis revealed significant differences in stomatal size and adaxial and abaxial epidermal cell sizes. The smaller stomata of the coastal population of this species were effective in reducing transpiration during gas exchange. Furthermore, the expansion of epidermal cells could be an adaptive strategy to retain water in leaves in coastal environments.

Canavalia lineata (Thunb.) DC. (Fabaceae) is a perennial trailing vine that flowers in the summer, and its seeds are sea-dispersed. It grows under various environmental conditions in coastal areas of the temperate and subtropical regions of Asia. Plant species with a wide distribution tend to differentiate in different environmental conditions; therefore, we conducted this study to find whether C. lineata has regional differentiation in seed germination and seedling emergence. Seeds of C. lineata collected from different areas of Japan were used to compare the seed-floating ability, the effect of low temperature on seed dormancy (0, 2, and 8 months) before germination, and the influence of temperature (4 °C, 15 °C, 20 °C, and 25 °C) on seed germination. Seed-floating tests indicated that many C. lineata seeds can float on the sea without losing their germination ability, and there was no associated regional differentiation. In addition, our results showed that this species could germinate without being exposed to low temperatures, but the timing of germination in such cases was different from that in the seeds that experienced low temperatures. The optimal temperature for C. lineata germination was 20 °C or higher. These analyses did not reveal any regional differentiation. Our results suggest that sea-dispersed seeds of C. lineata that do not lose germination ability for a long period may germinate in relatively warm areas regardless of the presence of low temperature conditions. The study also indicates that as germination of this species requires a relatively high temperature; thus, their distribution has not expanded to the north of Japan.