Physiological and photosynthetic plasticity in the amphibious, freshwater plant, Littorella uniflora, during the transition from aquatic to dry terrestrial environments

The physiological and photosynthetic responses of Littorella uniflora (L.) Ascherson, an amphibious macrophyte of isoetid life form, to rapid and prolonged emersion onto dry land, was studied at a reservoir. Water relations were little affected in the short term, but declining water potential and turgor pressure indicated water stress after flowering. High leaf lacunal CO₂ concentrations suggested continued CO₂ uptake from sediments. In contrast, a switch from Crassulacean acid metabolism (CAM) to C₃ photosynthesis was indicated by much lower levels of ΔH⁺ (down minus dusk titratable acidity) and phosphoenolpyruvate carboxylase (PEPC) activity in new terrestrial leaves, 7–8‐fold higher activity of ribulose bisphosphate carboxylase oxygenase (Rubisco), and increased chlorophyll and soluble protein contents. Accumulated nitrate and amino acid pools were depleted, whereas storage of carbohydrates as soluble sugars, fructan and starch increased. Plant carbon and nitrogen isotope ratios (δ¹³C and δ¹⁵N) declined, perhaps reflecting changes in C fixation processes, N metabolism, and source C and N. In leaves of plants grown half‐emersed for an extended period, contrasting activities of PEPC and Rubisco were found in submersed and emersed portions. Overall, L. uniflora showed considerable phenotypic plasticity, yet seemed to remain poised for re‐submersion; these characteristics could be adaptive in the unpredictable water margin habitat.     

The ABA-mediated switch between submersed and emersed life-styles in aquatic macrophytes

Hydrophytes comprise aquatic macrophytes from various taxa that are able to sustain and to complete their lifecycle in a flooded environment. Their ancestors, however, underwent adaptive processes to withstand drought on land and became partially or completely independent of water for sexual reproduction. Interestingly, the step backwards into the high-density aquatic medium happened independently several times in numerous plant taxa. For flowering plants, this submersed life-style is especially difficult as they need to erect their floral organs above the water surface to be pollinated. Moreover, fresh-water plants evolved the adaptive mechanism of heterophylly, which enabled them to switch between a submersed and an emersed leaf morphology. The plant hormone abscisic acid (ABA) is a key factor of heterophylly induction in aquatic plants and is a major switch between a submersed and emersed life. The mechanisms of ABA signal perception and transduction appear to be conserved throughout the evolution of basal plants to angiosperms and from terrestrial to aquatic plants. This review summarizes the interplay of environmental factors that act through ABA to orchestrate adaptation of plants to their aquatic environment.     

Growth rates and morphological adaptations of aquatic and terrestrial forms of amphibious Littorella uniflora (L.) Aschers.

Morphological – anatomical features of the terrestrial and the aquatic life form of the rosette species Littorella uniflora, inhabiting nutrient poor soils of oligotrophic lakes, were investigated together with growth rates of both life forms and of transplants. Growth rates were the same for the two life forms. However, growth of transplanted plants was somewhat reduced by transition from one environment to another. This was especially true for aquatic plants, which may be stressed by desiccation when moved to the terrestrial environment. The morphological – anatomical differences between the life forms were small compared with many other amphibious species which produce highly specialized leaves and life forms in air and under water. It is suggested that the conservative leaf morphology of Littorella is a consequence of the high dependence on rhizospheric CO₂ of both the aquatic and the terrestrial form of Littorella, making production of leaves specialized for carbon uptake in one specific environment unnecessary.     

Factors affecting terrestrial movement in the amphibious mangrove rivulus (Kryptolebias marmoratus)

We hypothesised that the exploration tendency of the amphibious mangrove rivulus Kryptolebias marmoratus would be inhibited in the terrestrial environment because of constraints on terrestrial locomotion or orientation. Using a novel object test, we showed that the fish explored objects in the aquatic but not the terrestrial environment, supporting the existence of constraints on terrestrial exploration. In further tests of the effects of extrinsic factors on terrestrial movement between aquatic refuges, shallow water depth simulating desiccation risk and the presence of a conspecific simulating intraspecific competition increased emersion outside of refuges, while high water salinity had no effect. These extrinsic factors had little effect on terrestrial movement between different aquatic refuges, except possibly for the lowest water depth tested. A significant association observed between emersion activity and movement between aquatic refuges suggested that terrestrial movement in K. marmoratus might depend on the tendency of individuals to emerse.     

Dispersal and distribution in Leopoldinia (Palmae)

The Amazon palm genus Leopoldinia exemplifies the transition from a terrestrial to an amphibious life-style, accompanied by a switch from synzoochorous dispersal to hydrochory. Leopoldinia piassaba produces heavy fruits with a fleshy, sweet mesocarp and is presumed to be dispersed by terrestrial animals. This species enters aquatic habitats only so far as these are submersed for a short while. The other two species, L. major and L. pulchra , resist long-lasting submersion and have much smaller, buoyant fruits, which are dispersed by water and are produced in proportionately larger guantities. They are eaten and mostly destroyed, but occasionally probably also dispersed by fish. L. pulchra , the species with the smallest fruits, has acquired the widest geographical distribution.     

Clonal integration supports the expansion from terrestrial to aquatic environments of the amphibious stoloniferous herb Alternanthera philoxeroides

Effects of clonal integration on land plants have been extensively studied, but little is known about the role in amphibious plants that expand from terrestrial to aquatic conditions. We simulated expansion from terrestrial to aquatic habitats in the amphibious stoloniferous alien invasive alligator weed ( Alternanthera philoxeroides ) by growing basal ramets of clonal fragments in soils connected (allowing integration) or disconnected (preventing integration) to the apical ramets of the same fragments submerged in water to a depth of 0, 5, 10 or 15 cm. Clonal integration significantly increased growth and clonal reproduction of the apical ramets, but decreased both of these characteristics in basal ramets. Consequently, integration did not affect the performance of whole clonal fragments. We propose that alligator weed possesses a double-edged mechanism during population expansion: apical ramets in aquatic habitats can increase growth through connected basal parts in terrestrial habitats; however, once stolon connections with apical ramets are lost by external disturbance, the basal ramets in terrestrial habitats increase stolon and ramet production for rapid spreading. This may contribute greatly to the invasiveness of alligator weed and also make it very adaptable to habitats with heavy disturbance and/or highly heterogeneous resource supply.     

Comparative pollen morphology and ultrastructure of the Callitrichaceae

The Callitrichaceae are an aquatic family of dicots that include the single, geographically cosmopolitan genus Callitriche. Callitriche contains 40-50 terrestrial, amphibious, and obligately submersed species, and it is the only known genus in the plant kingdom with co-occurring aerial and hydrophilous pollination syndromes. Pollen morphology and ultrastructure were described for 13 Callitriche species using scanning electron and transmission electron microscopy. Representative taxa of each growth form were examined; these included three terrestrial species (C. deflexa, C. peploides, and C. nuttallii), nine amphibious species (C. brutia, C. cophocarpa, C. cophocarpa-stagnalis hybrid, C. cribrosa, C. hamulata, C. heterophylla var. heterophylla, C. lusitanica, C. marginata, and C. trochlearis), and one obligately submersed species (C. truncata). Of the amphibious taxa, C. heterophylla var. heterophylla and C. trochlearis had internal geitonogamy, a type of internal self-fertilization. Pollen from all taxa was spheroidal, small, intectate, and lacked well-defined apertures. Taxa primarily differed with respect to exine thickness, surface ornamentation, and the presence or absence of aperture-like regions. The pollen of terrestrial species, as well as that of C. marginata, had well-developed exines with thick sculptured and basal layers. In general, amphibious taxa produced pollen with distinct, but thinner, exines than that of terrestrial taxa. Pollen of the amphibious taxa with internal geitonogamy had a thicker basal layer than species without internal geitonogamy, whereas the overall exine was reduced in C. hamulata and absent in C. brutia and C. lusitanica. Pollen of the obligately submersed C. truncata also lacked an exine. These palynological data were correlated with growth habits and related pollination biologies, as well as with phylogenetic interpretations of Callitrichaceae. Exine reduction or loss has evolved at least twice in the family, and it is associated with aneuploid reduction in chromosome number.     

Herbivory and growth in terrestrial and aquatic populations of amphibious stream plants

1. Many amphibious plant species grow in the transition between terrestrial and submerged vegetation in small lowland streams. We determined biomass development, leaf turnover rate and invertebrate herbivory during summer in terrestrial and aquatic populations of three amphibious species to evaluate advantages and disadvantages of aerial and submerged life.
2. Terrestrial populations had higher area shoot density, biomass and leaf production than aquatic populations, while leaf turnover rate and longevity were the same. Terrestrial populations experienced lower percentage grazing loss of leaf production (average 1.2–5.1%) than aquatic populations (2.9–17.3%), while the same plant dry mass was consumed per unit ground area.
3. Grazing loss increased linearly with leaf age apart from the youngest leaf stages. Grazing loss during the lifetime of leaves was therefore 2.4–3.1 times higher than mean apparent loss to standing leaves of all ages. The results imply that variation in density of grazers relative to plant production can account for differences in grazing impact between terrestrial and aquatic populations, and that fast leaf turnover keeps apparent grazing damage down.
4. We conclude that the ability of amphibious plants to grow submerged permits them to expand their niche and escape intense competition on land, but the stream does not provide a refugium against grazing and constrains plant production compared with the terrestrial habitat.     

Solitary plants do as well as clumped ones in Silene uniflora (Caryophyllaceae)

Many studies have been done on seed production as a function of population density in wild plants Most of them show a higher seed-set per flower of densely growing plants In this study, in a gynodioecious Silene uniflora (Caryophyllaceae) population on the Baltic island of Oland, high plant density did not increase fruit-set or seed-set Instead, the total number of seeds set was slightly higher for widely scattered plants as a result of a somewhat lower number of ovules per flower in densely growing plants Individuals had nothing to gain by growing close together, because there was no reproductive parameter which could compensate for the low ovule number per flower of closely growing plants Plant size determined the total seed production of individuals, but did not correlate with any of the fitness components studied per flower basis The different gender morphs showed the same pattern, which is discussed as a comparison with pollen limitation in self-incompatible and self-incompatible plants     

Nutrient availability and nutrient use efficiency in plants growing in the transition zone between land and water

The transition zone between terrestrial and freshwater habitats is highly dynamic, with large variability in environmental characteristics. Here, we investigate how these characteristics influence the nutritional status and performance of plant life forms inhabiting this zone. Specifically, we hypothesised that: (i) tissue nutrient content differs among submerged, amphibious and terrestrial species, with higher content in submerged species; and (ii) PNUE gradually increases from submerged over amphibious to terrestrial species, reflecting differences in the availability of N and P relative to inorganic C across the land–water ecotone. We found that tissue nutrient content was generally higher in submerged species and C:N and C:P ratios indicated that content was limiting for growth for ca. 20% of plant individuals, particularly those belonging to amphibious and terrestrial species groups. As predicted, the PNUE increased from submerged over amphibious to terrestrial species. We suggest that this pattern reflects that amphibious and terrestrial species allocate proportionally more nutrients into processes of importance for photosynthesis at saturating CO₂ availability, i.e. enzymes involved in substrate regeneration, compared to submerged species that are acclimated to lower availability of CO₂ in the aquatic environment. Our results indicate that enhanced nutrient loading may affect relative abundance of the three species groups in the land–water ecotone of stream ecosystems. Thus, species of amphibious and terrestrial species groups are likely to benefit more from enhanced nutrient availability in terms of faster growth compared to aquatic species, and that this can be detrimental to aquatic species growing in the land–water ecotone, e.g. Ranunculus and Callitriche.     

Flowering phenology,floral display and reproductive success in dioecious,Aralia nudicaulis L. (Araliaceae)

Summary Aralia nudicaulis L. is a dioecious, perennial, herbaceous plant that is commonly found in the understory vegetation throughout the boreal forest of North America. Female remets have fewer flowers per inflorescence, initiate flowering earlier, and reach peak flowering before male ramets. The consequences of the asynchrony in flowering between the sexes on pollination and seed set were examined during a two-year study. In both years there was significant variation in seed set associated with the flowering times of individual female ramets. In 1983, seed production was highest in the middle of the flowering season. In 1984, seed production was greatest in the later stages of flowering. Variation in seed set was not attributed to lack of pollination in 1983. In 1984, pollination limited seed set per flower during peak flowering. However, seed production never reached the potential five seeds per flower, suggesting that resource limitation was the most important factor affecting fecundity in both years. The asynchronous pattern of flowering is suggested to be the result of the different inflorescence sizes between the sexes.     

Impact of enhanced ultraviolet-B radiation on flower, pollen, and nectar production

Intensified ultraviolet-B radiation or UV-B (wavelengths between 280 and 320 nm) can delay flowering and diminish lifetime flower production in a few plants. Here we studied the effects of enhanced UV-B on floral traits crucial to pollination and pollinator reproduction. We observed simultaneous flowering responses of a new crop plant, Limnanthes alba (Limnathaceae), and a wildflower, Phacelia campanularia (Hydrophyllaceae), to five lifetime UV-B dosages ranging between 2.74 and 15.93 kJ·m·d. Floral traits known to link plant pollination with bee host preference, host fidelity and larval development were measured. Intensified UV-B had no overall effect on nectar and pollen production of L. alba and P. campanularia flowers. A quadratic relationship between UV-B and nectar sugar production occurred in P. campanularia and showed that even subambient UV-B dosages can be deleterious for a floral trait. Other floral responses to UV-B were more dramatic and idiosyncratic. As UV-B dosage increased, L. alba plants were less likely to flower, but suffered no delays in flowering or reductions to lifetime flower production for those that did flower. Conversely, an equal proportion of P. campanularia plants flowered under all UV-B treatments, but these same plants experienced delayed onset to bloom and produced fewer flowers at greater UV-B intensities. Therefore, intensified UV-B elicits idiosyncratic responses in flowering phenology and flower production from these two annual plants. Diurnal patterns in nectar and pollen production strongly coincided with fluctuating humidity and only weakly with UV-B dosage. Overall, our results indicated that intensified UV-B can alter some flowering traits that impinge upon plant competition for pollinator services, as well as plant and pollinator reproductive success.     

Seasonal effects of sun exposure and emersion on intertidal seaweed physiology: Fluctuations in antioxidant contents, photosynthetic pigments and photosynthetic efficiency in the red alga Porphyra umbilicalis Kützing (Rhodophyta, Bangiales)

There is a great deal of speculation regarding the physiological and biochemical mechanisms that give certain seaweed species the ability to colonize the intertidal zone. Frequent exposure to ambient temperatures and high irradiance levels in addition to dehydration during tidal emersion generates acute physiological stress. The ability of seaweeds like Porphya to overcome these challenges and survive in such a harsh environment has been linked to elevated reactive oxygen metabolism. The current study focused on measuring seasonal changes in antioxidant enzymes plus alterations in pigment contents and photosynthetic efficiency of P. umbilicalis plants found growing in the uppermost intertidal zone.Our results suggest that P. umbilicalis exhibits increased antioxidant metabolism, which could contribute to its success in colonizing such a stressful habitat. Elevated levels of glutathione reductase GTR, catalase and carotenoid contents during emersion suggested heightened protection against reactive oxygen species ROS damage is a necessary attribute for species in the upper intertidal regions. This hypothesis was further strengthened by the finding that the greatest antioxidant increases were observed during summer months when irradiance levels and temperatures were at their peak. Winter emersion did not elicit the same physiological response, as antioxidant levels were similar in submersed and emersed plants.For the most part, photosynthetic pigments were largely affected by sun exposure and less by emersion stress. Shaded blades maintained higher concentrations of photosynthetic pigments compared to sun exposed thalli concurring with established research. Photosynthetic efficiency measurements indicated emersion and not sun exposure was the greater facilitator of photoinhibitory damage and ROS generation at PSII. The findings of this field study strengthen previous assertions that protection via elevated antioxidant metabolism and increased PSII repair are involved in providing relief from the acute environmental stresses in the intertidal zone.     

Effects of salinity and clonal integration on the amphibiousplant Paspalum paspaloides: growth,photosynthesis and tissue ion regulation

Aims Clonal integration can increase performance of clonal plants suffering from environmental stress, and clonal plants in many wetlands commonly face stress of flooding accompanied by salinity. However, few studies have tested roles of clonal integration in amphibious plants expanding from terrestrial to aquatic saline habitats.     

Implications of a long-term, pollinator-mediated selection on floral traits in a generalist herb

Background and Aims

The phenotypic selection of a diverse insect assemblage was studied on a generalist plant species (Paeonia broteroi) in ten flowering seasons, with tests for whether visitor preferences for plants with larger flowers eventually translated into significant differences among plants in visitation rates, seed production, seed mass, seed germination and seedling survival.

Methods

Selection gradients were used to assess if selection on flower size contributed to explain differences in visitation rates, seed production and seed mass. First, independent analyses were carried out for each season; then for the ten season as a whole. Seedling emergence and survival were assessed by generalized linear models.

Key Results

Directional selection was found on flower size through visitation rates and seed production, and stabilizing selection through seed mass. Thus, larger flowers were more visited, and produced more, but lighter seeds, than smaller flowers. The results suggest a conflicting selection on flower size through seed number and size. Floral integration found in the study populations was larger than that in populations of a distant region. Finally, seed size did not influence seedling emergence and survival; thus, any advantages of seed size may be constrained under natural conditions before plants become reproductive individuals.

Conclusions

Plants with larger flowers may be benefited by producing more lighter seeds than fewer heavier ones, as they may contribute disproportionately to the seed bank, and have better chances that any descendant could eventually recruit. However, it seems unlikely that differences in flower size and integration found among populations in different regions could have been originated by rapid evolutionary change. First, because of the conflicting selection described; second, because of the remarkably low seedling survival found under natural conditions. Consequently, the influence of pollinator selection alone does not seem to explain differences in flower size and integration.Key words: Paeonia broteroi, long-term selection, conflicting selection, flower size, seed production, generalist pollination     

Factors affecting fruit and seed production of Paeonia ostii “Feng Dan”, an economically important oil tree

Many factors may affect reproduction of animal-pollinated species. In this study, the effects of pollen limitation, attractive traits (flower number, plant height and flower width) and flowering phenological traits (flowering onset, duration and synchrony) on female reproduction, as well as the patterns of variation in fruit and seed production within plants, were investigated in Paeonia ostii “Feng Dan” over two flowering seasons (2018 and 2019). Fruit set was very high (90%), and pollen supplementation did not increase fruit and seed production in either year, indicating no pollen limitation. Fruit set, ovule number per fruit and mean individual seed weight per fruit were not affected by any of the six attractive and phenological traits in either year, whereas seed number per fruit was related to the three attractive traits in one or both years. Seed number per plant was positively affected by the three attractive traits and best explained by flower number in both years, but the effect of each of the three phenological traits on seed number per plant differed between years. Within plants, the fruit set, ovule number, seed set and seed number per fruit declined from early- to late-opening flowers, presumably because of resource preemption, but the mean individual seed weight did not vary across the flowering sequence. Our study shows that attractive traits of Paeonia ostii “Feng Dan” are more important than flowering phenological traits in the prediction of total seed production per plant.     

Clonal Structure,Seed Set,and Self-Pollination Rate in Mass-Flowering Bamboo Species during Off-Year Flowering Events

Bamboos are typical examples of highly synchronized semelparous species. Their mass-flowering events occur at supra-annual intervals but they sometimes flower on a small scale in off-years. If some bamboo ramets (culms) of a genet flower and die in off-years, whereas other culms of the same genet do not flower synchronously, the genet can still survive blooming in an off-year and could participate in the next mass-flowering event. At genet level, the effect might be similar to that achieved by synchronously reproducing iteroparous plants. In addition, if multiple genets flower simultaneously in off-years, cross-pollination will be promoted. However, it is not known whether all the culms in a genet flower synchronously and whether multiple genets flower in off-years. We determined the clonal structure of three temperate dwarf bamboo species, i.e., Sasa senanensis, S. kurilensis, and S. palmata, at 24 off-year flowering sites and the surrounding areas in northern Japan using seven microsatellite markers. We also estimated seed set at seven of the sites and self-pollination rates at five sites to determine off-year reproductive success. Next, we investigated whether seed sets at the culm level were related to flowering area and/or number of flowering genets, using generalized linear mixed-effect models (GLMMs). Multiple genets flowered at 9/24 flowering sites. We found that 40/96 of the genets identified had some flowering culms. Non-flowering culms were present in 24/40 flowering genets. Seed set was in the range 2.2%–12.5% and the self-pollination rate was 96.3%. In the best GLMM, seed set increased with flowering area. Seeds were produced in off-years, but cross-pollination was rare in off-years. We suggest that some dwarf bamboos may exhibit iteroparity or imperfectly synchronized semelparity at the genet level, a characteristic similar to that of other reproductively synchronous plants. We also found synchronous flowering of a few genets even in off-years.     

Lobelia siphilitica plants that escape herbivory in time also have reduced latex production

Flowering phenology is an important determinant of a plant's reproductive success. Both assortative mating and niche construction can result in the evolution of correlations between phenology and other reproductive, functional, and life history traits. Correlations between phenology and herbivore defence traits are particularly likely because the timing of flowering can allow a plant to escape herbivory. To test whether herbivore escape and defence are correlated, we estimated phenotypic and genetic correlations between flowering phenology and latex production in greenhouse-grown Lobelia siphilitica L. (Lobeliaceae). Lobelia siphilitica plants that flower later escape herbivory by a specialist pre-dispersal seed predator, and thus should invest fewer resources in defence. Consistent with this prediction, we found that later flowering was phenotypically and genetically correlated with reduced latex production. To test whether herbivore escape and latex production were costly, we also measured four fitness correlates. Flowering phenology was negatively genetically correlated with three out of four fitness estimates, suggesting that herbivore escape can be costly. In contrast, we did not find evidence for costs of latex production. Generally, our results suggest that herbivore escape and defence traits will not evolve independently in L. siphilitica.     

Density-dependent flowering phenology,outcrossing, and reproduction in Impatiens capensis

Summary We investigated the effects of plant density on cleistogamous (CL) and chasmogamous (CH) flowering phenology and seed production in a natural Impatiens capensis population, by censusing individually marked plants at experimentally reduced and natural densities. CL flowering was earlier at natural density. This plastic density response may have resulted from a stress-related threshold for CL flowering; slower growing plants at natural density flowered earlier. Although apparently triggered by slow early growth, early CL flowering also involved an additional cost for later growth rate. In contrast, CH flowering was unrelated to relative growth rate, but apparently required a size threshold. Experimental density reduction resulted in earlier CH flowering and a dramatic increase in the percentage of plants producing CH flowers. Individual CL and CH flowering duration and flower production were greater at reduced density. These density-dependent effects caused differences between treatments in the shape and location of population flowering phenology curves. Moreover, the percentage of CH seeds produced per individual was much higher at reduced density. At natural density total seed production per plant was lower and more hierarchical than at lower density, suggesting that dominance and suppression shape jewelweed fitness distributions.     

Large plant size counteracts early seed predation during the extended flowering season of a Silene uniflora (Caryophyllaceae) population

A population of the alvar race of the perennial herb, Silene uniflora (Caryophyllaceae), which grows on thin soils in open alvar habitats on the Baltic island of Oland SE Sweden, was found to have an extended and more or less bimodal flowering phenology Large individuals produced flowers during both periods, while small, and presumably young, individuals only produced flowers in either of the two periods, or in part of both In the early flowering period plants were heavily infested by the seed-predatory larvae of a noctuid moth, Hadena confusa , but in the late flowering period only a small proportion of the fruits was attacked by the seed predators The proportion of flowers developing into fruits was consistently high throughout the season Both the number of seeds per capsule and the mass of seeds decreased over the flowering season However, the germination success of early and late seeds did not differ Thus, although differing in number of seeds, both early and late flowers contributed to individual reproductive success Large individuals started to flower early in the season and despite their high loss of seeds in the early part of the summer they contributed a larger number of seeds to the seed pool than smaller and later-flowering individuals Although selection was acting to favour later flowering during a year with high early seed predation, consistency of date of flower initiation and of relative predation impact on individuals across years was low suggesting that recurrent selection by seed predators is weak Seed predation, although heavy, is therefore judged to be unlikely to cause a significant evolutionary response on flowering phenology in this plant