Fang-Li Luo

Shifting effects of physiological integration on performance of a clonal plant during submergence and de-submergence

BACKGROUND AND AIMS Submergence and de-submergence are common phenomena encountered by riparian plants due to water level fluctuations, but little is known about the role of physiological integration in clonal plants (resource sharing between interconnected ramets) in their adaptation to such events. Using Alternanthera philoxeroides (alligator weed) as an example, this study tested the hypotheses that physiological integration will improve growth and photosynthetic capacity of submerged ramets during submergence and will promote their recovery following de-submergence. METHODS Connected clones of A. philoxeroides, each consisting of two ramet systems and a stolon internode connecting them, were grown under control (both ramet systems untreated), half-submerged (one ramet system submerged and the other not submerged), fully submerged (both ramet systems submerged), half-shaded (one ramet system shaded and the other not shaded) and full-shaded (both ramet systems shaded) conditions for 30 d and then de-submerged/de-shaded for 20 d. The submerged plants were also shaded to very low light intensities, mimicking typical conditions in turbid floodwater. KEY RESULTS After 30 d of submergence, connections between submerged and non-submerged ramets significantly increased growth and carbohydrate accumulation of the submerged ramets, but decreased the growth of the non-submerged ramets. After 20 d of de-submergence, connections did not significantly affect the growth of either de-submerged or non-submerged ramets, but de-submerged ramets had high soluble sugar concentrations, suggesting high metabolic activities. The shift from significant effects of integration on both submerged and non-submerged ramets during the submergence period to little effect during the de-submergence period was due to the quick recovery of growth and photosynthesis. The effects of physiological integration were not found to be any stronger under submergence/de-submergence than under shading/de-shading. CONCLUSIONS The results indicate that it is not just the beneficial effects of physiological integration that are crucial to the survival of riparian clonal plants during periods of submergence, but also the ability to recover growth and photosynthesis rapidly after de-submergence, which thus allows them to spread.

Effects of Spatial Patch Arrangement and Scale of Covarying Resources on Growth and Intraspecific Competition of a Clonal Plant

Spatial heterogeneity in two co-variable resources such as light and water availability is common and can affect the growth of clonal plants. Several studies have tested effects of spatial heterogeneity in the supply of a single resource on competitive interactions of plants, but none has examined those of heterogeneous distribution of two co-variable resources. In a greenhouse experiment, we grew one (without intraspecific competition) or nine isolated ramets (with competition) of a rhizomatous herb Iris japonica under a homogeneous environment and four heterogeneous environments differing in patch arrangement (reciprocal and parallel patchiness of light and soil water) and patch scale (large and small patches of light and water). Intraspecific competition significantly decreased the growth of I. japonica, but at the whole container level there were no significant interaction effects of competition by spatial heterogeneity or significant effect of heterogeneity on competitive intensity. Irrespective of competition, the growth of I. japonica in the high and the low water patches did not differ significantly in the homogeneous treatments, but it was significantly larger in the high than in the low water patches in the heterogeneous treatments with large patches. For the heterogeneous treatments with small patches, the growth of I. japonica was significantly larger in the high than in the low water patches in the presence of competition, but such an effect was not significant in the absence of competition. Furthermore, patch arrangement and patch scale significantly affected competitive intensity at the patch level. Therefore, spatial heterogeneity in light and water supply can alter intraspecific competition at the patch level and such effects depend on patch arrangement and patch scale.

Nitrogen addition increases intraspecific competition in the invasive wetland plant Alternanthera philoxeroides, but not in its native congener Alternanthera sessilis

Nitrogen is often released in pulses with different frequencies, and N supply pulses may affect growth, reproduction, and biomass allocation of plants. However, few studies have examined how N supply pulses affect intraspecific competition of clonal plants and whether such an effect depends on the N supply amount. We grew one (no competition) or 12 ramets (with intraspecific competition) of both an invasive clonal plant Alternanthera philoxeroides and its native congener Alternanthera sessilis in five different N treatments: control (no N addition), low/high amount with low/high frequencies (pulses). Nitrogen addition significantly increased the growth of both species, while intraspecific competition decreased it. Nitrogen addition significantly increased intraspecific competitive intensity of A. philoxeroides as measured by the log response ratio of growth traits, but did not affect that of A. sessilis. Despite the N supply amount, N pulses had little effect on the growth and thus intraspecific competition of the two species. Therefore, increasing N deposition may change population structure and dynamics and the invasion succession of A. philoxeroides, but changes in N pulses may not.

Impacts of sediment type on the performance and composition of submerged macrophyte communities

To restore deteriorated lake ecosystems, it is important to identify environmental factors that influence submerged macrophyte communities. While sediment is a critical environmental factor for submerged macrophytes and many studies have examined effects of sediment type on the growth of individual submerged macrophytes, very few have tested how sediment type affects the growth and species composition of submerged macrophyte communities. We constructed submerged macrophyte communities containing four co-occurring submerged macrophytes (Hydrilla verticillata, Myriophyllum spicatum, Ceratophyllum demersum and Chara fragilis) and subjected them to three sediment treatments, i.e., clay, a mixture of clay and quartz sand at a volume ratio of 1:1 and a mixture at a volume ratio of 1:4. Compared to the clay, the 1:1 mixture treatment greatly increased overall biomass, number of shoot nodes and shoot length of the community, but decreased its diversity. This was because it substantially promoted the growth of H. verticillata within the community, making it the most abundant species in the mixture sediment, but decreased that of M. spicatum and C. demersum. The sediment type had no significant effects on the growth of C. fragilis. As a primary nutrient source for plant growth, sediment type can have differential effects on various submerged macrophyte species and 1:1 mixture treatment could enhance the performance of the communities, increasing the overall biomass, number of shoot nodes and shoot length by 39.03%, 150.13% and 9.94%, respectively, compared to the clay treatment. Thus, measures should be taken to mediate the sediment condition to restore submerged macrophyte communities with different dominant species.

Herbivory-induced maternal effects on growth and defense traits in the clonal species Alternanthera philoxeroides

Plants have evolved a variety of defense traits against foliar herbivory, including the production of primary and secondary metabolites, the allocation of chemical compounds, and morphological plasticity. Using two vegetative generations of the invasive clonal species Alternanthera philoxeroides, we investigated the effects of maternal and offspring herbivory by Planococcus minor on the integrative defense strategy of plants. Herbivory severely inhibited leaf, stolon and root growth, as well as the production of primary metabolites (soluble sugars, starch, and total non-structural carbohydrates in stolons), and decreased average leaf area and specific leaf area of the second-generation A. philoxeroides. The changes in growth measures of the first-generation A. philoxeroides with herbivory were consistent with that of the second generation. By contrast, herbivory basically did not affect the concentration of non-structural carbohydrate compounds in the roots, and even increased the concentrations of N and total phenols in taproots. Furthermore, herbivory-induced maternal effects also reduced the growth of the second-generation plants. The results suggest that A. philoxeroides is capable of adapting to herbivory by P. minor, mainly via the allocation of available resources in belowground organs, and that the herbivory effect can persist across vegetative generations. These features may potentially improve the regeneration and tolerance of A. philoxeroides after a short-term herbivory.

Effects of patch contrast and arrangement on benefits of clonal integration in a rhizomatous clonal plant

The availabilities of light and soil water resources usually spatially co-vary in natural habitats, and the spatial pattern of such co-variation may affect the benefits of physiological integration between connected ramets of clonal plants. In a greenhouse experiment, we grew connected or disconnected ramet pairs [consisting of a proximal (relatively old) and a distal (relative young) ramet] of a rhizomatous herb Iris japonica in four heterogeneous environments differing in patch arrangement (reciprocal vs. parallel patchiness of light and soil water) and patch contrast (high vs. low contrast of light and water). Biomass of the proximal part, distal part and clonal fragment of I. japonica were all significantly greater in the intact than in the severed treatment, in the parallel than in the reciprocal patchiness treatment and in the high than in the low contrast treatment, but the effect of severing the connection between ramet pairs did not depend on patch arrangement or contrast. Severing the connection decreased number of ramets of the distal part and the clonal fragment in the parallel patchiness arrangement, but not in the reciprocal patchiness arrangement. Therefore, the spatial arrangement of resource patches can alter the effects of clonal integration on asexual reproduction in I. japonica.

Direct and legacy effects of herbivory on growth and physiology of a clonal plant

The ability to tolerate novel herbivores is widely considered to influence plant invasion success. For clonal plants that have reduced capacity to evolve in response to novel herbivores, legacy effects of herbivory on parental plants might be translated to offspring ramets, resulting in pre-adaptation to tolerate herbivory for new vegetative growth. Using the invasive clonal plant Alternanthera philoxeroides, we first exposed plants to herbivory by Planococcus minor, a widespread and generalist piercing-sucking insect. Herbivory decreased above- and below-ground plant biomass by approximately 50% with a concomitant 134% increase in root N concentration but no changes in concentrations of soluble sugars, starch or non-structural carbohydrates related to herbivory tolerance. Offspring ramets were then exposed to herbivory by three different herbivore species: (1) P. minor, (2) the specialist leaf-beetle Agasicles hygrophila, and (3) the stenophagous tortoise-beetle Cassida piperata. There was no evidence of interactive effects between herbivory on parental plants and herbivory on offspring plants on growth, biomass allocation patterns, or physiological responses, suggesting that pre-adaptation to herbivory did not occur in A. philoxeroides with these herbivores. There were, however, species-specific herbivore tolerance responses. In the offspring generation, herbivory by A. hygrophila strongly suppressed growth and biomass allocation, but patterns were generally weaker for other herbivores. Tolerance effects could be explained by stimulatory effects of grazing by C. piperata and P. minor on taproot biomass along with idiosyncratic increases of starch and non-structural carbohydrate concentration in some storage organs. Our results highlight the importance of A. hygrophila in controlling the aboveground spread of A. philoxeroides. However, herbivory by other species was largely tolerated and accompanied by increased allocation to underground storage organs and altered physiological reserves, both of which could allow this invasive plant to tolerate herbivory and successfully invade new areas in the face of new herbivore pressure.

Effects of salinity and clonal integration on the amphibious plant 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. Methods Basal (older) ramets of clonal fragments of Paspalum paspaloides were grown in soil to simulate terrestrial habitats, whereas their apical (younger) ramets were placed at the surface of saline water containing 0, 50, 150 and 250 mmol l−1 NaCl to mimic different salinity levels in aquatic habitats. Stolons connecting the apical and basal ramets were either intact (connected) to allow clonal integration or severed (disconnected) to prevent integration. Important Findings Increasing salinity level significantly decreased the growth of the apical ramets of P. paspaloides, and such effects on the leaf growth were much higher without than with stolon connection after 60-day treatment. Meanwhile, leaf and total mass ratios of the connected to the disconnected apical ramets were higher at high than at low saline treatments. Correspondingly, Fv/Fm and F/Fm′ of the apical ramets were higher with than without stolon connection in highly saline treatments. The results suggest that clonal integration can benefit the spread of apical ramets from terrestrial soil into saline water, and that the positive effects increase with increasing salinity. However, clonal integration did not significantly affect the growth of the whole fragments. Due to clonal integration, Na+ could be translocated from the apical to the basal ramets to alleviate ion toxicity in apical ramets. Our results suggest that clonal integration benefits the expansion of P. paspaloides from terrestrial to aquatic saline habitats via maintained photosynthetic capacities and changed biomass allocation pattern.