日本の水草研究: その現状と課題

[論文] Articl

タガラシの冬一年草生活史とその発芽特性

Ranunculus sceleratus L. is a winter annual species that has been common in rice fields and ditches in Japan. Recently, however, its populations have decreased, with extinctions of local populations. To understand the characteristics of winter annual life cycle of this species and the causes of its recent population decline, we studiedgermination of seeds collected from two populations（Kobe and Tajima）. Cold and hot pre-treatments had no significant effect on germination of the Kobe seeds, but the hot treatment seemed to break the dormancy of the Tajima seeds. The optimal temperatures for the germination of seeds from both populations were 20 and 15℃ alternation（day/night）, and no germination was observed at higher temperature combinations such as 30 and 25℃. The seeds thus appear to stay in conditional dormancy during the summer and start to germinate at lower temperatures in the autumn. The germination characteristics of the species are adaptive for germination on bare ground after rice harvesting. Recent changes in rice-field environments, such as drying in winter and succession in fallowed rice fields, have caused extinction of local populations of R. sceleratus.タガラシ（キンポウゲ科）は，水田や水路に普通に生育する冬一年草である。しかし，近年，乾田化の進んだ水田を中心に生育地が急減し，地方版レッドデータブックでは絶滅危惧種に挙げる例もみられる。本研究は，ヨーロッパでは夏一年草と報告されるタガラシが，なぜ日本では冬一年草の生活史をとるのか，そして近年の減少の要因は何かを，その発芽特性から解明しようとしたものである。種子は兵庫県神戸市（以下，神戸）と豊岡市（但馬）の2カ所の水田から採集した。神戸の種子は低温処理（4℃）と高温処理（30℃）の有無にかかわらず，明条件の変温条件下（20/15℃）ではよく発芽した。但馬の種子では低温処理は二次休眠を誘導し，逆に高温処理は休眠を打破する傾向が見られた。暗条件では神戸の種子はわずかに発芽したが，但馬の種子は全く発芽しなかった。発芽は好気条件，嫌気条件で両産地とも差はなかった。恒温条件下（20℃）では，低温処理，高温処理を施した場合を含め，いずれの条件下でも全く発芽は見られなかった。また発芽適温を調べた結果，発芽率は20/15℃ の変温条件下で最も高く30/25℃ のような高温下ではまったく発芽しなかった。以上の結果より，タガラシの種子は，キンポウゲ科に一般的な後熟を完了した後，夏の高温下では休眠を強いられ，秋になって気温が低下するとともに発芽を開始すると推測された。また変温条件と光が発芽を促進したことから，タガラシの種子は稲刈り後の裸地環境での発芽に適応していると考えられた。近年の乾田化は，湿生植物であるタガラシにとって好ましくないだけでなく，稲刈り後の藁敷きや休耕による植生遷移の進行によって植被率が高まることもタガラシの発芽適地を消失させ，各地の集団が消滅する原因になっていると考えられた

Effects of land use on trophic states and multi-taxonomic diversity in Japanese farm ponds

金沢大学環日本海域環境研究センターFarm ponds are among the most biodiverse anthropogenic freshwater habitats because of their small size, shallow water depth, and aquatic vegetation. Land-use changes, such as converting riparian vegetation to human use or changing the management practices of farm ponds, are assumed to be major factors that change such ecosystems from a clear-water state to a turbid state, leading to deterioration of water quality and biodiversity in such ponds. Using the database of a large-scale pond survey, we evaluated the effects of surrounding land use (landscape factors and modern pond management practices), fish abundance, and other environmental variables on total phosphorus concentration and taxonomic richness patterns of six biological indicators associated with changes in the trophic state. Local- and landscape-level vegetation structure associated with land use and total fish abundance were among the factors influencing the total phosphorus concentration of farm ponds, a main driver of trophic state changes. In addition, a transition from a clear-water state to a turbid state was associated with lower taxonomic richness of aquatic plants, macroinvertebrates, and adult Odonata, and a higher taxonomic richness of phytoplankton and fish. Based on these results, we discuss potential land-use and pond management strategies for conserving and/or restoring the water quality and biodiversity of farm ponds through maintenance of a clear-water state. © 2017 Elsevier B.V.Embargo Period 24 month

釧路湿原3湖沼の水生植物の現状

[論文] Articl

Molecular Adaptation of rbcL in the Heterophyllous Aquatic Plant Potamogeton

Heterophyllous aquatic plants show marked phenotypic plasticity. They adapt to environmental changes by producing different leaf types: submerged, floating and terrestrial leaves. By contrast, homophyllous plants produce only submerged leaves and grow entirely underwater. Heterophylly and submerged homophylly evolved under selective pressure modifying the species-specific optima for photosynthesis, but little is known about the evolutionary outcome of habit. Recent evolutionary analyses suggested that rbcL, a chloroplast gene that encodes a catalytic subunit of RuBisCO, evolves under positive selection in most land plant lineages. To examine the adaptive evolutionary process linked to heterophylly or homophylly, we analyzed positive selection in the rbcL sequences of ecologically diverse aquatic plants, Japanese Potamogeton.Phylogenetic and maximum likelihood analyses of codon substitution models indicated that Potamogeton rbcL has evolved under positive Darwinian selection. The positive selection has operated specifically in heterophyllous lineages but not in homophyllous ones in the branch-site models. This suggests that the selective pressure on this chloroplast gene was higher for heterophyllous lineages than for homophyllous lineages. The replacement of 12 amino acids occurred at structurally important sites in the quaternary structure of RbcL, two of which (residue 225 and 281) were identified as potentially under positive selection.Our analysis did not show an exact relationship between the amino acid replacements and heterophylly or homophylly but revealed that lineage-specific positive selection acted on the Potamogeton rbcL. The contrasting ecological conditions between heterophyllous and homophyllous plants have imposed different selective pressures on the photosynthetic system. The increased amino acid replacement in RbcL may reflect the continuous fine-tuning of RuBisCO under varying ecological conditions