克隆整合提高淹水胁迫下狗牙根根部的活性氧清除能力

虽然国内外已开展大量关于克隆整合影响植物抗逆生理的研究,但迄今未见克隆整合是否会影响逆境下不同分株清除活性氧过程的报道。以河岸带适生克隆植物狗牙根(Cynodon dactylon)为例,研究克隆植物的抗氧化生理响应,检测了狗牙根在先端淹水/不淹水、先端与基端匍匐茎连接/切断两个因素的交互作用下的根部主要抗氧化酶:超氧化岐化酶(Superoxide dismutase, SOD)、抗坏血酸过氧化物酶(Ascorbate peroxidase, APX)、过氧化氢酶(Catalase, CAT)的活力以及生物量的变化。结果显示,淹水环境中狗牙根先端的生物量和根部SOD酶活力在匍匐茎连接处理下显著高于切断处理组,同一处理的生物量以及根部APX、CAT酶活力总体上表现出不同程度的提高趋势;与受淹先端连接的基端分株根部抗氧化酶活力均低于切断处理组,且SOD和CAT受连接处理影响显著;淹水和切断处理显著降低先端分株的生物量,但对基端和克隆片段影响不明显。这表明淹水胁迫下克隆整合提高了其根部活性氧清除能力,显著改善了先端分株的表现。     

Causes and consequences of sectoriality in the clonal herb Glechoma hederacea

The causes of sectoriality and consequences for clone behaviour are examined using data from the stoloniferous herb Glechoma hederacea. The proximal causes of physiological integration patterns are investigated using anatomical studies, acid fuchsin dye to reveal patterns of xylem continuity between ramets, and ¹⁴C as a label to reveal quantitative photoassimilate translocation patterns in the phloem. Dye movement in the xylem was acropetal and sectorial, and the sectoriality was determined by phyllotaxy. Patterns of ¹⁴C-labelled photoassimilate allocation were qualitatively similar to those of xylem based resources, although there was some basipetal movement of photoassimilate. The patterns of physiological integration and independence between ramets are shown to be governed by rules which depend on vascular continuity and discontinuity between ramets. Physiological support to stolon apices results in acquisition of relative branch autonomy (branches become semi-autonomous integrated physiological units, IPUs).This paper evaluates whether observed physiological integration patterns may be modified by altering normal source-sink relationships or by modifying environmental conditions. An experiment using different defoliation intensities, and different defoliation patterns at the same overall intensity, demonstrated that the precise positions of leaves removed from a clone had unique consequences for its subsequent development. Individual ramets of a given clone may be located in microhabitats of differing quality. An experiment in which competition was either present or absent throughout the space occupied by the clone, or patchy in distribution, showed that G. hederacea did not respond to competition at the whole clone level. Instead, connected stolons (IPUs) responded independently to local competition. Sectoriality may promote the restriction of lethal, localised environmental factors within the affected IPU. A study investigating the uptake and translocation of zinc by clones revealed that quantified patterns of zinc distribution resembled patterns of ¹⁴C movement in the phloem, and that there was no significant transport of zinc from one stolon to another.Although sectorial patterns of resource movement in G. hederacea can be modified in the short term, in the long-term, physiological integration may not allow this species to integrate the effects of environmental heterogeneity. A mobile clonal species with a high growth rate and relatively short-lived ramets, such as G. hederacea, is likely to benefit from a semi-autonomous response to patch quality at the level of the stolon, since the alternative of widespread intra-clonal support may increase the residence time of the clone in unfavourable pathches.     

Clonal plants as cooperative systems: Benefits in heterogeneous environments

All natural environments are spatially and temporally heterogeneous. Consequently, their ability to provide essential resources for the growth of plants is variable. Modular plant species produce repeated basic structures which, in the case of clonal species, are called ramets. Ramets belonging to the same clone are distributed throughout the environment in space and time, and therefore they may be located in sites which differ in resource-providing quality. The connections between ramets may allow resources to be shared, enabling the clone to behave as a cooperative system. As a result of such physiological integration, ramets can survive in conditions where there is lethal shortage of a resource because they are connected to, and supported by, ramets located in conditions where there is ample supply of the same resource. Physiological integration between connected ramets presents opportunities for heterogeneous environments to be exploited to an extent that is only just becoming apparent. As heterogeneity is ubiquitous in natural environments, it may be expected that plants, as relatively immobile organisms, will have evolved the capacity to cope with it by making appropriate localized morphological and/or physiological plastic responses. Recent studies suggest that such responses not only enable clonal species to cope with environmental heterogeneity, but that under some circumstances they can benefit more from environments which are heterogeneous rather than homogeneous, even when both types of environment contain the same amount of resources. Studies on Glechoma hederacea (Lamiaceae) that illustrate this phenomenon are described.     

Plant clonality: Biology and diversity

The current approaches to the study of clonal plants are reviewed. Most studies concentrate at the level of the ramet and clonal fragment exploring the “microscopic” view of clonal plants, dealing with the translocation of resources, clonal integration, plasticity of growth etc. The information gained, by this approach can be used in the understanding of higher levels of organization within the clonal system either with the help of spatially explicit modelling techniques, or by using means and distributions of size within a population instead of studying individual ramets separately. Plant scientists use the term clone with two meanings, viz. (a) a set of physiologically connected, but potentially independent ramets, and (b) a set of genetically identical, but potentially physically separated individuals. The overlap of these terms differs between individual plant species, depending on the extent of physical separation of the ramets and the degree of physiological integration between the ramets; the lower the frequency of ramet separation, the closer are the physiological and genetic concepts of the clone. Three critical areas seem to be neglected in clonal plant research: (a) the interrelationship between hierarchical levels in clonal plants, (b) the particular spatial structure of their environment, and (c) the importance of clonal plants in different ecological communities.     

Effects of clonal integration on plant plasticity in Fragaria chiloensis

The ability of clonal plants to transport substances between ramets located in different microsites also allows them to modify the plastic responses of individual ramets to local environmental conditions. By equalising concentrations of substances between ramets, physiological integration might decrease responses to local conditions. However, integration has also been observed to increase plasticity and induce novel plastic responses in ramets. To ask how integration modifies plant plasticity in the clonal herb, Fragaria chiloensis, ramets were given either low light and high nitrogen or high light and low nitrogen, simulating a pattern of resource patchiness in their native habitat. Ramets in contrasting light/nitrogen treatments were either connected or single. Effects of light/nitrogen and connection were measured at three levels of morphological organisation, the organ, the ramet, and the clonal fragment. Connection between ramets reduced or had no effect on plastic responses in leaf size at the level of the plant organ. This suggested that integration dampened certain plastic responses. Connection induced a new plastic response at the level of the clonal fragment, an increase in allocation to vegetative reproduction in patches of low light and high nitrogen. It is concluded that clonal integration can have different effects on plant plasticity at different levels of plant organisation. It appears that, at least in this species, integration can increase plasticity at the level of the clonal fragment and concentrate vegetative reproduction in particular microsite types.     

Physiological integration enhanced the tolerance of Cynodon dactylon to flooding

Many flooding‐tolerant species are clonal plants; however, the effects of physiological integration on plant responses to flooding have received limited attention. We hypothesise that flooding can trigger changes in metabolism of carbohydrates and ROS (reactive oxygen species) in clonal plants, and that physiological integration can ameliorate the adverse effects of stress, subsequently restoring the growth of flooded ramets. In the present study, we conducted a factorial experiment combining flooding to apical ramets and stolon severing (preventing physiological integration) between apical and basal ramets of Cynodon dactylon, which is a stoloniferous perennial grass with considerable flooding tolerance. Flooding‐induced responses including decreased root biomass, accumulation of soluble sugar and starch, as well as increased activity of superoxide dismutase (SOD) and ascorbate peroxidase (APX) in apical ramets. Physiological integration relieved growth inhibition, carbohydrate accumulation and induction of antioxidant enzyme activity in stressed ramets, as expected, without any observable cost in unstressed ramets. We speculate that relief of flooding stress in clonal plants may rely on oxidising power and electron acceptors transferred between ramets through physiological integration.     

Does clonal integration improve competitive ability? A test using aspen (Populus tremuloides [Salicaceae]) invasion into prairie

Many clonal plants consist of many connected individual ramets, allowing them to share water and nutrients via physiological integration. Integration among ramets may also improve the ability of clonal plants to tolerate abiotic stress or improve the competitive ability of individual ramets. Here I use a field experiment to determine whether clonal integration improves ramet performance for a widespread clonal tree species invading into native prairie. Aspen (Populus tremuloides) dominates the southern treeline in western Canada, has long-lived belowground connections between mother and daughter ramets, and reproduces vegetatively via resprouting rhizomes after disturbance. I applied two competition treatments (neighbors present or absent) and two clonal integration treatments (belowground rhizomes between mother and daughter ramets either severed or left intact) to 12 replicate Populus daughter ramets at each of three sites. Neighbors improved the survivorship of Populus ramets by 25-35% after 2 yr, but decreased growth by ～20%. Clonal integration tended to improve ramet survival and growth, but these trends were often not significant. Clonal integration did not alter the effects of competition from neighboring vegetation, suggesting that connections between ramets do not necessarily improve the competitive ability of Populus invading into native prairie.     

Clonal integration in Fragaria vesca growing in metal-polluted soils: parents face penalties for establishing their offspring in unsuitable environments

Clonal plants often establish descendent ramets in sites with contrasting presence of favourable and unfavourable factors. Connections between ramets allow translocation of essential resources from established ramets to developing ramets and, as consequence, integration confers net benefits to ramets growing under unfavourable conditions. Therefore, integrated ramets may survive in habitat patches that would be lethal to independent ramets or non-clonal plants. This experiment aimed to investigate the physiological and morphological responses of the clonal plant Fragaria vesca growing in heterogeneous substrate with patches of contrasting quality (i.e. uncontaminated or heavy-metal-contaminated). We observed that parents reduced their photosynthetic efficiencies and growth as consequence of maintaining their offspring. This cost did not affect survival of the parents. Physiological integration brings about benefits to offspring ramets growing both at uncontaminated and heavy-metal-contaminated soils. The benefits of integration were detected in both physiological and morphological traits, enhancing the survivorship of offspring ramets in the Cu-polluted soils. We conclude that integration improves the performance of developing ramets of F. vesca growing in heavy-metal-contaminated habitats, allowing clone systems to overcome the establishment risks and maintain their presence in these less favourable sites.     

Arbuscular mycorrhizal fungi reduce effects of physiological integration in Trifolium repens

Background and Aims

One of the special properties of clonal plants is the capacity for physiological integration, which can increase plant performance through mechanisms such as resource sharing and co-ordinated phenotypic plasticity when plants grow in microsites with contrasting resource availabilities. However, many clonal plants are colonized by arbuscular mycorrhizal fungi (AMF). Since AMF are likely to reduce contrasts in effective resource levels, they could also reduce these effects of clonal integration on plasticity and performance in heterogeneous environments.

Methods

To test this hypothesis, pairs of connected and disconnected ramets of the stoloniferous herb Trifolium repens were grown. One ramet in a pair was given high light and low nutrients while the other ramet was given high nutrients and low light. The pairs were inoculated with zero, one or five species of AMF.

Key Results

Pairs of ramets grown without AMF developed division of labour and benefited from resource sharing, as indicated by effects of connection on allocation to roots, accumulation of mass, and ramet production. Inoculation with five species of AMF significantly reduced these effects of connection, both by inhibiting them in ramets given high nutrients and inducing them in ramets given high light. Inoculation with one species of AMF also reduced some effects of connection, but generally to a lesser degree.

Conclusions

The results show that AMF can significantly modify the effects of clonal integration on the plasticity and performance of clonal plants in heterogeneous environments. In particular, AMF may partly replace the effects and benefits of clonal integration in low-nutrient habitats, possibly more so where species richness of AMF is high. This provides the first test of interaction between colonization by AMF and physiological integration in a clonal plant, and a new example of how biotic and abiotic factors could interact to determine the ecological importance of clonal growth.Key words: Arbuscular mycorrhizal fungi, biomass allocation, clonal plant, division of labour, environmental heterogeneity, light availability, nutrients, white clover     

Clonal integration in Ranunculus reptans: by‐product or adaptation?

We studied fitness consequences of clonal integration in 27 genotypes of the stoloniferous herb Ranunculus reptans in a spatially heterogeneous light environment. We grew 216 pairs of connected ramets (eight per genotype) with mother ramets in light and daughter ramets in shade. In half of the pairs we severed the stolon connection between the two ramets at the beginning of the experiment. During the experiment, 52.7% of the ramet pairs with originally intact connection physically disintegrated. We detected significant variation among genotypes in this regard. Survival of planted ramets was 13.3% higher for originally connected pairs. Moreover, there was significant variation among genotypes in survival, in the difference in survival between plant parts developing from mother and daughter ramets, and in the effect of integration on this difference. In surviving plants connection between ramets decreased size differences between mother and daughter parts. Variation among genotypes was significant in growth and reproduction and marginally significant in the effect of physiological integration on growth and reproduction. Connected daughter ramets had longer leaves and internodes than daughters in severed pairs indicating that integration stimulated plant foraging in both the vertical and the horizontal plane. Observed effects of integration on fitness components in combination with genetic variation in maintenance and effects of connection indicate that clonal integration in R. reptans has the capability to evolve, and therefore suggest that clonal integration is adaptive. If genetic variation in integration is common, future studies on clonal integration should always use defined genetic material and many clones to allow extrapolation of results to population and wider levels.