Adaptive Strategies Employed by Clonal Plants in Heterogeneous Patches

: Heterogeneity is widespread in natural environments; as a result, connected clonal ramets often live in areas characterized by patches of different resources. Speciﬁcally, clonal plants are frequently affected by conditions of heterogeneous water stress. This raises the question of how clonal plants grow and reproduce in areas with patches of different resources. In this study, we investigated the adaptation mechanisms of clonal plants under heterogeneous environmental conditions. On the one hand, we bore in mind that phenotypic plasticity is abundantly exhibited in clonal plants. Clonal plants respond to water stress mainly through regulation of the size of individuals, the allocation of population biomass, and the number of daughter plants, as well as the extension ability and branching intensity of clonal organs, which directly affect reproduction and population stability in clonal plants. On the other hand, we also considered the physiological integration in clonal plants which has been shown in many studies. Ramets of clonal plants normally stay connected to each other through horizontal connectors (stolons or rhizomes). Communicated substances and resources such as water, mineral nutrition, photosynthetic products, and secondary metabolites are translocated between ramets; by such means, the plant relieves stress caused by heterogeneous patches. In this study, we sought to obtain scientiﬁc references to improve our understanding of how clonal plants in natural environments acclimate to stresses caused by soil heterogeneity.


Introduction
Clonality in plants is widespread, including in species that span heterogeneous environments [1,2]. Such environments affect many essential functional traits and physiological processes in plants and also determine population structures and evolutionary processes [3]. Clonality is an important characteristic in clonal plants; clonal biological individuals have the ability to produce potentially independent offspring through vegetative reproduction [4][5][6]. Continuous vegetative reproduction promotes persistence when sexual reproduction is limited or absent, leading to higher population stability and ecosystem resilience [7]. As shown in Figure 1, a seedling that initially germinates from zygotes produced by the sexual reproduction process is termed the mother plant or the ortet. Individuals produced by clonal growth are called ramets, and these are capable of independent existence [8]. Clonal plants produce a number of genetically identical ramets that connect to each other by means of spacers; these are stolon or rhizome internodes which cover a physical distance for a period of time [9][10][11]. The length, branching intensity, and branching angles of spacers determine the placement pattern of clonal ramets in space [12]. A number of studies have shown that clonal plants exhibit a considerable degree of phenotypic plasticity in heterogeneous stress conditions, and that physiological integration is triggered so that the risks from stress habitats are shared [13][14][15][16]. Compared with non-clonal plants, clonal plants occupy larger areas of horizontal space by expanding into new habitats at an overwhelming rate, so that connected clonal ramets usually lie in different horizontal patches [3,17]. There are many ways to classify clonal plants. For example, they can be classified as root-derived, shoot-derived, or combined root-derived and shoot-derived types, according to the origin of clonal organs. A classification into two types can also be made based on the branching of clonal organs, so that there are monopodial-branching clonal plants and sympodial-branching clonal plants. Finally, depending upon the degree of dispersion of the ramets, clonal plants may also be classified as guerilla-growth, phalanx-growth, or intermediate-growth forms [3,18]. independent existence [8]. Clonal plants produce a number of genetically identical ramets that connect to each other by means of spacers; these are stolon or rhizome internodes which cover a physical distance for a period of time [9][10][11]. The length, branching intensity, and branching angles of spacers determine the placement pattern of clonal ramets in space [12]. A number of studies have shown that clonal plants exhibit a considerable degree of phenotypic plasticity in heterogeneous stress conditions, and that physiological integration is triggered so that the risks from stress habitats are shared [13][14][15][16]. Compared with non-clonal plants, clonal plants occupy larger areas of horizontal space by expanding into new habitats at an overwhelming rate, so that connected clonal ramets usually lie in different horizontal patches [3,17]. There are many ways to classify clonal plants. For example, they can be classified as root-derived, shoot-derived, or combined root-derived and shoot-derived types, according to the origin of clonal organs. A classification into two types can also be made based on the branching of clonal organs, so that there are monopodial-branching clonal plants and sympodial-branching clonal plants. Finally, depending upon the degree of dispersion of the ramets, clonal plants may also be classified as guerilla-growth, phalanx-growth, or intermediate-growth forms [3,18]. Clonal plants are widely distributed among ecosystems, and they dominate in different kinds of habitats, helping to maintain ecosystem balance [6,14]. A host of common economic plants, such as food crops, forage crops, and medicinal plants, exhibit clonal growth habits, as do many harmful weeds. Indeed, the benefits or harms of such plants are directly related to their clonal growth habits. In addition. recent studies have demonstrated that some clonal plants may be suitable for repairing and restoring vulnerable habitats. These include (a) clonal plants in karst landform habitats, such as Ficus tikoua [20], Alchornea trewioides [21], and Drepanostachyum luodianense [22]; (b) clonal plants in deserts, such as Calamagrostis epigejos [23], Hedysarum leave [24], Hippophae rhamnoides L., Psammochloa villosa [25], and Stenocereus eruca (Cactaceae) [26]; (c) clonal plants in grasslands, such as Leymus chinensis [27,28], Zoysia japonica Steud [29,30], and Buchloe dactyloides [31]; (d) clonal plants in wetlands, such as Phragmites australis [32,33], Paspalum paspaloides [34], Acorus calamus L., and Spartina alterniflora Loisel [34]. Researchers have also been keen to study the invasion mechanism in invasive clonal plants such as Alternanthera philoxeroides [35], Solidago canadensis L. [36], Carpobrotus edulis [37], and Mikania micrantha H.B.K. [38]. Studies such as those just mentioned have primarily focused on the unique characteristics of clonal growth, the structure of new ramet development, and the degree of physiological integration between ramets [39]. In brief, it may be said that the strategies employed by Clonal plants are widely distributed among ecosystems, and they dominate in different kinds of habitats, helping to maintain ecosystem balance [6,14]. A host of common economic plants, such as food crops, forage crops, and medicinal plants, exhibit clonal growth habits, as do many harmful weeds. Indeed, the benefits or harms of such plants are directly related to their clonal growth habits. In addition. recent studies have demonstrated that some clonal plants may be suitable for repairing and restoring vulnerable habitats. These include (a) clonal plants in karst landform habitats, such as Ficus tikoua [20], Alchornea trewioides [21], and Drepanostachyum luodianense [22]; (b) clonal plants in deserts, such as Calamagrostis epigejos [23], Hedysarum leave [24], Hippophae rhamnoides L., Psammochloa villosa [25], and Stenocereus eruca (Cactaceae) [26]; (c) clonal plants in grasslands, such as Leymus chinensis [27,28], Zoysia japonica Steud [29,30], and Buchloe dactyloides [31]; (d) clonal plants in wetlands, such as Phragmites australis [32,33], Paspalum paspaloides [34], Acorus calamus L., and Spartina alterniflora Loisel [34]. Researchers have also been keen to study the invasion mechanism in invasive clonal plants such as Alternanthera philoxeroides [35], Solidago canadensis L. [36], Carpobrotus edulis [37], and Mikania micrantha H.B.K. [38]. Studies such as those just mentioned have primarily focused on the unique characteristics of clonal growth, the structure of new ramet development, and the degree of physiological integration between ramets [39]. In brief, it may be said that the strategies employed by clonal plants to adapt to abiotic stress surroundings have been extensively studied by researchers [40]. However, the present published literature is lacking in overviews of adaptive strategies employed by clonal plants in response to heterogeneous water stresses.
In nature, spatial-temporal heterogeneity is very pervasive in resource distribution and plant habitat conditions [13,41]. Heterogeneous water stress is a habitat change that affects clonal plants frequently; indeed, it is the most prominent factor negatively influencing plant growth and development [42]. It has been reported that chronic or severe stress leads to low adaptability of plant populations to their environment [43] because physiological activities in individual plants are badly affected. The effects of water loss and the negative impacts of xylem sap pressures during drought have been mitigated by exploiting a variety of interdependent and coordinated morphological, anatomical, and physiological traits [44,45]. Drought eventually reduces the rate of photosynthesis by negatively impacting mesophyll conductance, Rubisco activity, and ATP synthase activity in leaves [46,47]. Many plants are able to accumulate ethylene rapidly after waterlogging, so ethylene content has been suggested as a reliable indicator of the waterlogging perception mechanism in plants [48]. It is known that, in most plants, leaf chlorophyll content decreases under flood conditions. This phenomenon may be readily explained. Waterlogging induces the synthesis of a large amount of ethylene, leading to up-regulation of chlorophyllase gene expression, enzyme activity, and the promotion of plant chlorophyll decomposition [49,50]. Under flood conditions, the carboxylation ability of Rubisco is far less than its oxygenation reaction; this explains the significant decrease in Rubisco carboxylase activity which has been found in many species of plant after waterlogging [51,52].
Positive interactions can alleviate the impact of abiotic stress upon plant communities and may also help counter the effects of climatic extremes [53]. In order to adapt to adverse environments, many plants are able to detect and respond to changes in soil moisture [31,54,55]. Plant roots can secrete root chemicals after sensing changes in soil habitats, and thus regulate water-use efficiency. Under water stress, more abscisic acid (ABA) is produced in roots and transported to leaves via the xylem; this regulates stomatal movement and thus affects water status and water balance in the plant [56]. The increase in ABA content helps to maintain the membrane structure and also plays a key role in acclimation memory in plants suffering drought stress. Because the plants retain a memory of the drought stress, the epigenome changes rapidly and produces physiological responses related to the new adaptive structure whenever non-fatal drought stress occurs again [57,58]. Studies have also shown that plants are often subjected to hypoxia during waterlogging; under such conditions, the core hypoxia genes are awakened, enabling recovery from prolonged hypoxia [59]. Some plant roots can develop more aeration tissues, such as adventitious roots, which partially compensate for root growth inhibition when waterlogging occurs [42]. Interactions among physiological integration, plant construction growth, and plastic responses have also been reported by researchers [9]; however, the adaptation mechanisms used by plants in heterogeneous water environments have not yet been systematically clarified. In this paper, therefore, we review the adaptation mechanisms (phenotypic plasticity and physiological integration among clonal ramets) under conditions of heterogeneous water stress, based on research on clonal plants published in the literature.

Phenotypic Plasticity of Clonal Plants in Response to Water Heterogeneity
In plants, plasticity is a persistent trait that affects the ability of plant populations to adapt to environmental changes [60][61][62]. In clonal plants, plasticity includes two aspects: clonal morphology and clonal growth. Phenotypic responses to a novel or extreme environment are initially plastic, with any genetic changes occurring only later. Some scholars have demonstrated that phenotypic plasticity is not a whole-plant response, but a property of individual meristems, leaves, branches, and roots, which is triggered by local environmental conditions [63]. In response to the stress caused by either excessive or scarce resources, the growing organs of clonal plants may exhibit changes in morphology, physiology, and biomass [64,65]; these can be seen as evolving traits targeted by natural selection. Clonal plants respond to water stress mainly through regulation of the size of individuals, the allocation of population biomass, and the number of daughter plants, as well as the extension ability and branching intensity of clonal organs, which directly affect reproduction and population stability in clonal plants [63,[66][67][68]. In previous studies, the phenotypic plasticity of clonal plants under abiotic stress has been assessed using indexes such as the number and area of basal leaves, the number and density of clonal ramets, spacer length and specific spacer length, branching intensity and angle, and biomass [13]. Whether clonal plants benefit from the plasticity depends on the extent of phenotypic plasticity and the conditions of the growth environment. Plasticity in resource uptake has been proven to be beneficial for plants in a wide range of environments [16]. Consequently, clonal plants can grow well in heterogeneous habitats, using ecological adaptation strategies for acquiring necessary resources formed during evolution [16]. So, what are the similarities and differences in the adaptation strategies of phenotypic plasticity in clonal plants when facing water heterogeneity? Compared with homogeneous water conditions, water heterogeneity exerts a greater influence on phenotypic plasticity in clonal plants. However, the ecological adaptation strategies of clonal plants are different in different habitats. These may be separately described as follows: (a) Clonal plants in sandy land. Ramets of Calamagrostis epigeios with optimal water supply condition were found to produce significantly more new rhizomes and more offspring (ramets), accumulate more shoot biomass, and allocate more biomass to their shoots, compared with those under a condition of insufficient water supply [23]. Clonal reproduction and the extension of clonal organs are therefore inhibited by heterogeneous water stress. (b) Clonal plants in grassland. When Zoysia japonica was subjected to heterogeneous water stress, the main stolon diameter of ramets was found to be significantly thicker in low water patches, but the growth and development of plants decreased significantly in high-water patches [69]. In another study, Calamagrostis pseudophragmites responded to water heterogeneity with increased growth of underground parts (rhizomes, roots, and buds) which promoted clonal reproduction and extension of clonal organs [17]. Because water heterogeneity is usually accompanied by heterogeneity of nutrients and other resources, the growth strategy of Calamagrostis pseudophragmites may be closely related to resource allocation. (c) Clonal plants in karst habitats. When Phyllostachys praecox was planted in a heterogeneous water environment, more substances were transported in the underground parts of the plant, and root biomass was significantly enhanced, so that the utilization rate of resources was increased. Contrarily, when soil water content was increased, substances tended to be used for the growth of ramets (overground part), while clonal rhizome lengths and root lengths decreased significantly [70]. Such morphological changes under conditions of water stress ensure higher competitiveness of clonal plants in light and air and enhance their dominance as species in a drought environment. (d) Invasive clonal plants that grow rapidly in many habitats. Clarifying the mechanism of biological invasion is a key step in predicting future invasion situations, and in mitigating the negative effects triggered by invasive clonal plants [37]. Many studies have demonstrated that clonal growth is an attribute of successful plant invasion [71,72]. The regeneration ability of small clonal fragments is also an important advantage of plant clonal growth. Furthermore, increased thickness of stolons and greater internode lengths can increase the survival rate of small-cloned fragments [38]. In one study, researchers found that the biomass of invasive species Carpobrotus Edulis (L.) increased after drought stress. Interestingly, this increase in biomass was mainly due to the production of above-ground structures which increased the spread along the soil surface, a finding with important implications for the likely success of this aggressive invader in colonizing new environments [37]. (e) Clonal plants of other habitats. Researchers found a positive correlation between the total biomass of Aegilops tauschii and soil moisture content. Any decrease in available moisture slows down growth and decreases the accumulation of biomass [67]. However, the ramets of Carex plants have been found to grow better in water-sufficient patches, as their roots grew longer [73]. It is beneficial for clonal plants to absorb as much water and nutrition as possible in areas of shallow soil. The adaptive strategies employed by plants from different habitats in response to heterogeneous water stress are summarized in Table 1.

Calamagrostis epigejos Guerilla
The growth characteristics of ramet rhizome, the number of new ramets, and biomass allocation were all changed. [23] Grassland clonal plant Zoysia japonica Guerilla Content levels of CAT (catalase), proline, and MDA (malondialdehyde) were significantly increased. [74] Karst clonal plant Lolium perenne L. Phalanx The plant responded to stress by increasing specific root length and specific root area in deep-soil areas, and by reducing root tissue density. [75]

Drepanostachyum luodianense Phalanx
The degree of membrane lipid peroxidation was reduced by increasing the content of osmotic adjustment substances, and by increasing coordination between protective enzymes. [76] Wetland clonal plant

Typha orientalis Guerrilla
Plant height, number of ramets, interval diameter and length, and biomass accumulation were significantly increased. [34]

Bolboschoenus planiculmis Intermediate
Plant biomass, rhizome length, bulb number, total leaf length, plant height, spacer length, and root-shoot ratio were changed. [77] Other clonal plant

Physiological Integration in Clonal Plants in Response to Water Heterogeneity
Physiological integration, as an important strategy used by clonal plants to adapt to their habitat, has been the subject of considerable attention by researchers. Physiological integration means that ramets of clonal plants normally stay connected through horizontal connectors (stolons or rhizomes). However, communication substances and resources have been shown to be translocated between ramets; these include water, mineral nutrition, photosynthetic products, and secondary metabolites [1,11,78]. For this paper, we designed a diagram to illustrate physiological responses in clonal plants under heterogeneous water stress, as shown in Figure 2. The source-sink relationship, water potential, and branch structure are primary factors determining clonal physiological integration [80][81][82]. Sharing of resources between ramets continues until young ramets are self-sufficient [83], and the transfer pattern of shared resources is related to growth season and tiller age [27]. The resource-sharing ability varies greatly between species and within species [84]. In general, if the total of resources in plants is limited, increased resource allocation to a specific organ may lead to reduced resource allocation for another organ [39,85]. For instance, the authors of one study found that investment in root production decreased while above-ground biomass increased under conditions of long-term water stress; this increase in biomass was mainly due to the production of above-ground structures, increasing the spread along the soil surface [37], with ramets transporting photosynthates to support their growth. In another study, it was shown that clonal integration promoted biomass partitioning both for apical ramets growing in conditions of low water availability and basal ramets growing in conditions of high-water availability [86]. This behavior provides the potential for rapid clonal growth when conditions improve; it also enhances the tolerance of clonal populations and increases the growth and reproduction of clonal plants [87][88][89][90].
spread along the soil surface [37], with ramets transporting photosynthates to support their growth. In another study, it was shown that clonal integration promoted biomass partitioning both for apical ramets growing in conditions of low water availability and basal ramets growing in conditions of high-water availability [86]. This behavior provides the potential for rapid clonal growth when conditions improve; it also enhances the tolerance of clonal populations and increases the growth and reproduction of clonal plants [87][88][89][90]. Physiological integration alleviates the stress caused by water deficiency. Researchers have reported improved survival rates in water-stressed offspring ramets that remained connected to their parent plants [56,92]. Connected ramets have been shown to exhibit stronger resistance responses (e.g., higher antioxidant enzyme activity and proline content, lower O2 − productivity, and MDA content) to induce resistance [74,93]. Photosynthesis is a physiological process in which plants are sensitive to environmental changes. Net photosynthetic rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs) are the main parameters reflecting the photosynthetic capacity of plants [86]. The authors of [74] found that levels of chlorophyll content, the chlorophyll a/b ratio, and the photosynthetic parameters (Pn, Gs) of offspring ramets were not reduced when parents were subject to well-watered conditions and offspring ramets were under drought conditions [74]. These interesting results suggested that the physiological support provided by parent plants enables their offspring ramets to maintain high photosynthetic efficiencies. This itself demonstrates the benefits of physiological integration in terms of feedback regulation of photosynthesis. However, if the intensity of environmental stress or external interference reaches levels which exceed the individual's tolerance and regulation ability, the risk of death to the basal plant cannot be overcome, and life in the basal plant is finally extinguished [94]. In recent years, to better study physiological integration in clonal plants, the isotope tracer method has been used as an accurate direct method [82] while cutting off roots and stems, defoliation, and shading are indirect methods that have also been widely used [10,24,31,95]. The adaptations of some clonal plants to conditions of water heterogeneity by means of physiological integration-and the specific research methods used to investigate this adaptation-are summarized in Table 2. Physiological integration alleviates the stress caused by water deficiency. Researchers have reported improved survival rates in water-stressed offspring ramets that remained connected to their parent plants [56,92]. Connected ramets have been shown to exhibit stronger resistance responses (e.g., higher antioxidant enzyme activity and proline content, lower O 2 − productivity, and MDA content) to induce resistance [74,93]. Photosynthesis is a physiological process in which plants are sensitive to environmental changes. Net photosynthetic rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs) are the main parameters reflecting the photosynthetic capacity of plants [86]. The authors of [74] found that levels of chlorophyll content, the chlorophyll a/b ratio, and the photosynthetic parameters (Pn, Gs) of offspring ramets were not reduced when parents were subject to well-watered conditions and offspring ramets were under drought conditions [74]. These interesting results suggested that the physiological support provided by parent plants enables their offspring ramets to maintain high photosynthetic efficiencies. This itself demonstrates the benefits of physiological integration in terms of feedback regulation of photosynthesis. However, if the intensity of environmental stress or external interference reaches levels which exceed the individual's tolerance and regulation ability, the risk of death to the basal plant cannot be overcome, and life in the basal plant is finally extinguished [94]. In recent years, to better study physiological integration in clonal plants, the isotope tracer method has been used as an accurate direct method [82] while cutting off roots and stems, defoliation, and shading are indirect methods that have also been widely used [10,24,31,95]. The adaptations of some clonal plants to conditions of water heterogeneity by means of physiological integration-and the specific research methods used to investigate this adaptation-are summarized in Table 2. The daughter plants did not sacrifice their own growth when transporting the photosynthetic substance to the stressed mother plants. [24] Acid fuchsin dye application Water Acopetal/ basipetal Depending upon the ramet level, the donor ramet delivered different water proportions to the recipient ramet. [23] Mikania micrantha Herb Connecting/ severing stem Water/ photosynthate Relatively stable photosynthesis and specific leaf areas were maintained through clonal integration, but the resource allocation of ramets was changed. [10]

Buchloe dactyloides Herb
Acid fuchsin application Water Acopetal The chlorophyll fluorescence characteristics of the ortet were improved, increasing the ability of ramets to absorb nutrients. The distribution ratios of water transport were different in different structures of Zoysia japonica. [28]

Conclusions
In clonal plants, physiological responses to drought or waterlogging are characterized by regulation of the endogenous protective enzyme system, endogenous hormone level, and carbohydrate metabolism. This can be understood as an adaptation to habitat stress. ABA content increases when water is deficient, regulating stomatal movement, and leading to a decrease in carbon dioxide content; this results in an energy surplus, which leads to photooxidative stress and causes antioxidant defense of the plant organism. Moreover, drought can result in lower levels of chlorophyll content; this slows down photosynthesis and triggers resistance to drought as a short-term stress condition. In addition, the activity of endogenous protective enzymes increases, helping to inhibit the accumulation of reactive oxygen species (ROS), and thereby enhancing the drought tolerance of the plant organism. Under conditions of waterlogging, Ca 2+ and ROS signal pathways are awakened, and the formation of aerenchyma is also induced. Furthermore, the activities of SOD, APX, GPX, GST, CAT, and other enzymes all increase; however, respiration and ATP production decrease, sucrose and starch metabolism in tissues is regulated, and the ability to resist drought or waterlogging stress is triggered (Figure 3). Water heterogeneity is widespread in natural environments and exerts profound effects on growth and reproduction in plants, especially clonal plants. Under heterogeneous conditions, physiological integration in clonal plants may be triggered by a series of reactions, including responses to water reallocation and photosynthetic products. Finally, corresponding changes in morphology also occur, and these can be understood as a survival countermeasure in adapting to heterogeneous stress conditions. In short, this review provides a theoretical basis for understanding the adaptive mechanism in clonal plants to conditions of heterogeneity in natural environments. integration in clonal plants may be triggered by a series of reactions, including responses to water reallocation and photosynthetic products. Finally, corresponding changes in morphology also occur, and these can be understood as a survival countermeasure in adapting to heterogeneous stress conditions. In short, this review provides a theoretical basis for understanding the adaptive mechanism in clonal plants to conditions of heterogeneity in natural environments.

Conflicts of Interest:
The authors declare no conflict of interest.