Yasuro Kadono

Effects of aquatic macrophytes on water quality and phytoplankton communities in shallow lakes

We investigated aquatic macrophytes, water quality, and phytoplankton biomass and species composition in three shallow lakes with different levels of vegetation cover and nutrient concentration in Kushiro Moor, during August 2000. Trapa japonica can live in a wide range of nutrient levels. This species forms an environment with a steeper extinction of light, higher concentrations of dissolved organic carbon (DOC), lower concentrations of dissolved oxygen (DO) near the bottom, and lower concentrations of nitrate + nitrite and soluble reactive phosphorus (SRP) than other vegetation types. The pH was much higher in a Polygonum amphibium community, and the DO near the bottom did not decrease compared to a T. japonica community in the summer. The relationship between chlorophyll a and the limiting nutrient (total phosphorus (TP) when total nitrogen (TN) : TP is ≥10 and TN/10 when TN : TP is <10) significantly differed between lakes with and without submerged vegetation. The chlorophyll a concentrations at a given nutrient level were significantly lower in water with submerged macrophytes than in water without them. Correspondence analysis showed that the difference in phytoplankton community structure across sites was largely due to the presence or absence of submerged macrophytes, and the ordination of phytoplankton species in the lakes with submerged macrophytes is best explained by environmental gradients of TN, chlorophyll, pH and SRP.

Factors influencing seed production of Phragmites australis

Abstract We investigated the seed set rate of a common reed, Phragmites australis (Cav.) Trin. ex Steudel, in the Lake Biwa–Yodo River system in south-western Japan, and analyzed the factors that influenced seed production. In the 12 populations studied, the seed set rate ranged from 0.1 to 59.6% (mean 9.7%). Intensive studies in four populations showed that pollen stainability, used to indicate the pollen viability, and the percentage of germinating pollen were high, irrespective of ploidy levels, and seed production proved to be inhibited at any stage leading to ovule development. In a pollination experiment P. australis was shown to be at least partially self-incompatible, and the addition of pollen increased seed set rate. Resource limitation was not found. Pollen limitation brought about by the clonal structure of P. australis populations may be the most important factor decreasing seed production. The damage by a thrips, Chirothrips manicatus Haliday, was another factor of reduced seed set rate in some populations.

Molecular Adaptation of rbcL in the Heterophyllous Aquatic Plant Potamogeton

Background 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. Principal Findings 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. Conclusions/Significance 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.

Distribution and habitat of JapanesePotamogeton

The geographical distributions and habitats of JapanesePotamogeton were studied by extensive field survey throughout Japan.According to the distribution pattern, the species were classified into 3 groups: (1) the species widespread throughout Japan, (2) the ones distributed in northern Japan and (3) the ones distributed in central to south-western Japan.The habitats of each species were investigated in relation to water body type and pH, alkalinity, Ca++, Cl− and conductivity of water. Flow condition and the degree of water level fluctuation seemed to be important habitat factors affecting the occurrence of some species. Each species had its preference regarding the five chemical parameters of water. Especially, significance of alkalinity is discussed in relation to the inorganic carbon contents of water.Interspecific associations were examined. Floating-leaved species were shown to be mutually exclusive. Submerged species showed somewhat intricate associations. Also it was remarkable that, while in lentic environments 29 pairs with significant associations were revealed, only 2 pairs were significantly associated in lotic environments.

Floral morph composition and pollen limitation in the seed set of Nymphoides indica populations

We investigated the floral morph composition and seed set of the populations in a heterostylous aquatic plant, Nymphoides indica O. Kuntze (Menyanthaceae). The bias of floral morph ratio in the populations was negatively correlated with the seed set. In the populations with biased floral morph ratio, the pollen on the stigma of the common morphs were dominated by pollen of the same morph, resulting in reduced seed set. The addition of pollen of the opposite morph on the stigmas of the common morph resulted in an increase of fruit and seed set rates. The intermorph pollination rate and the fruit and seed set rates of Nymphoides indica were highest when two floral morphs were close in the populations. Various insect species including Lepidoptera, Diptera and Hymenoptera visited the flowers. However, geitonogamous pollination was common because of the short flying distance of the pollinators. These results indicate that the seed set of Nymphoides indica is limited by the shortage of compatible pollens and that the success of intermorph pollination is influenced by spatial distribution of the two floral morphs. As Nymphoides indica is endangered, conservation of the populations with high fecundity is proposed by us.

Inherited maternal effects on the drought tolerance of a natural hybrid aquatic plant, Potamogeton anguillanus

We tested whether maternal effects have led to the adaptive divergence of strains of the natural hybrid Potamogeton anguillanus, whose putative parents show contrastingly divergent ecologies. To examine the correlation between phenotypic characters and maternal types, we conducted drought experiments and DNA typing using nuclear and chloroplast genes. In the field, we investigated the distribution of the maternal type along the depth and the inshore-offshore gradient. Hybrids of P. malaianus mothers (M-hybrids) and those of P. perfoliatus mothers (P-hybrids) could not be distinguished morphologically under submerged conditions, but differed in drought tolerance. M-hybrids and P. malaianus formed more terrestrial shoots and exhibited higher survival than P-hybrids and P. perfoliatus in drought experiments. The distribution survey clarified that M-hybrids were dominant in shallow and inshore areas, whereas they were almost absent in deeper and offshore areas. These results indicate that the natural hybrid P. anguillanus differs in adaptive values depending on the maternal type. Bidirectional hybridization and heritable maternal effects may have played important roles in its phenotypic adaptation to local environmental conditions.

Restricted seed contribution and clonal dominance in a free-floating aquatic plant Utricularia australis R. Br. in southwestern Japan

Utricularia australis R. Br. is an aquatic angiosperm species common in natural and irrigation ponds in temperate regions. This species reproduces both sexually and vegetatively, but in southwestern Japan the occurrence of male-sterile populations, in which plants produce no pollen and propagate only vegetatively, has been recorded. We studied the reproductive contribution of seeds in normal pollen-producing populations using isozyme analyses, a pollination experiment under culture and field observations. Although seedlings obtained from controlled mating indicated segregation of isozyme, polymorphism of the isozyme genotype was detected mainly among populations, but rarely within each pond population. This suggested clonal dominance and rarity of seed or seedling survival in natural populations. In the pollination experiment, the mean seed set ratio in cross-pollination between plants of the same isozyme genotype (7.6%) did not differ significantly from self-pollination (7.6%), but was lower than cross-pollination between plants of different genotypes (45.7%). The low ratio in crossing between the same genotype plants was ascribed to the clonality of the parents. In general, these results corresponded with the low ratios in seed setting observed in natural populations (7.9–13.7%). All the male-sterile populations we surveyed showed the same genotype, thus male sterility in the study area was considered to have the same origin.

The genetic structure of populations of the vulnerable aquatic macrophyte Ranunculus nipponicus (Ranunculaceae)

Ranunculus nipponicus (Makino) Nakai is a vulnerable aquatic macrophyte in the Kinki district, which is the southernmost distribution of this species in Japan. The genetic diversity and structure within and among eleven extant populations were assessed using the inter-simple sequence repeats (ISSR) polymerase chain reaction in association with combinations of propagation pattern (clonal and/or seeds) and genotypic geographical structure. In total, 53 bands were amplified, of which 18 (34%) were polymorphic. Analysis of the ISSR bands identified 46 genotypes among 81 individuals from one stream population and 72 distinct genotypes among 147 individuals in the Kinki district. An unweighted pair group method with arithmetic mean (UPGMA) dendrogram showed some unity among upstream and downstream subpopulations within one stream and eleven populations. The Shannon index of genetic diversity was 0.109 for one stream population and 0.313 for total genetic diversity, suggesting relatively high genetic diversity. Analysis of molecular variance (AMOVA) revealed that 84.1% of the total genetic diversity occurred among populations and the remaining diversity (15.9%) occurred within populations. Significant genetic differentiation occurred among populations in the Kinki district. These results suggest that conservation of each population is important for maintaining genetic diversity of R. nipponicus in this district.

Sexual reproduction under fluctuating water levels in an amphibious plant Schoenoplectus lineolatus (Cyperaceae) : a waiting strategy?

Sexual reproduction of an amphibious macrophyte Schoenoplectus lineolatus (Fr. et Stav.) T. Koyama (Cyperaceae) was observed in three irrigation ponds with different water regimes in Hyogo Prefecture, western Japan. In each pond, inflorescence formation, flowering (anthesis), and seed set were recorded at three water depths (shallow, middle, and deep). Inflorescence formation was observed under submerged conditions in the three ponds. In Oku-ike Pond, anthesis was observed when the water level dropped in August and early September. Flowers opened after the emergence of the inflorescence into the air. In contrast, anthesis did not occur, despite inflorescence formation, in the other two ponds with no water level drop or a short period of water level drop. Submerged inflorescences could not set seeds unless pollination had already occurred in the air. Finally, inflorescence formation under submerged conditions was considered to be a waiting strategy for water drawdown in the irrigation ponds where marked water level decreases occur from summer to autumn. Anthesis can quickly start soon after the emergence of inflorescences.

Growth and reproductive characteristics of an aquatic macrophyte Ottelia alismoides (L.) Pers. (Hydrocharitaceae)

Life history and reproductive traits of a submerged aquatic weed, Ottelia alismoides (L.) Pers., were studied in irrigation ponds in southwestern Japan. In addition, growth and seed production were investigated in relation to nutrient condition and water depth. The size of plants was very variable but reproductive allocation was nearly constant irrespective of the plant size at reproductive stage, amounting to more than 30% at maximum. Bagging experiments confirmed autogamy of the species. Ottelia alismoides grew best in nutrient-rich water with mud substratum. The optimal water depth for the growth of the species was approximately 50 cm compared with 20 and 90 cm. However, the reproductive allocation and seed set rate did not change much in different nutrient and water depth conditions. The seeds were produced by cleistogamous flowers in a depth of 90 cm. Morphological and ecological plasticity revealed in the present study was considered to be adapted to unstable environments such as rice fields and irrigation ponds liable to water-level fluctuation. The conditions required for the restoration of the populations of O. alismoides are discussed based on the present results.