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Phenotypic plasticity, or the ability of an organism to alter its phenotype in response to changes in the environment, strongly influences the ecological success of individual plants (Bradshaw, 1965). That being so, the relationships between the pattern and magnitude of plasticity and components of fitness in plants have been examined under various conditions of abiotic and biotic environmental heterogeneity (Sultan, 2000; Miner et al., 2005; Pigliucci, 2005; Valladares et al., 2007). However, little is known of the processes and mechanisms by which phenotypic plasticity occurs in response to different environmental conditions (Ackerly & Sultan, 2006). To understand the adaptive evolution of phenotypic plasticity in natural environments, it is crucial to investigate the mechanisms by which plants sense environmental signals and how those signals influence the plant phenotype (Schmitt et al., 2003).
All plants have a developmental system, which differs from that of animals: they grow through the repeated production of modular units (leaf, stem and branch) throughout their lifetime. Given this developmental system, the study of meristem plasticity can play a vital role in improving our understanding of phenotypic plasticity in plants, for the following reasons. First, the plasticity of the whole plant is a combination of the plasticity within modules plus the plasticity in production of different types of module, each of which is derived from the growth and development of a single meristem (de Kroon et al., 2005). Second, given that the fate and activity of meristems play central roles in plant developmental processes (Leyser & Day, 2003), investigation of the responses of meristems to the environment can make a contribution to the field of ‘eco-devo’, which aims to combine an understanding of the mechanism of phenotypic expression with its consequences for the success of plants in their habitats and evolutionary changes. Finally, the rate of plant growth depends on the supply of new cells produced by meristems; hence, an investigation of meristem plasticity is essential for understanding how the rate of plant growth changes in response to changes in the environment.
Grass shoots include shoot apical meristems and leaf meristems (Fig. 1a). The shoot apical meristem, which is located at the shoot apex, initiates leaf primordia and its activity is characterized by the rate at which new leaves appear. The leaf meristem is located at the leaf base and drives the growth and development of an initiated leaf by means of the production of new cells. As shown in Fig. 1b, the initial cells at the basal end of the leaf meristem enter into the cell cycle, undergo several rounds of division, and exit from the meristematic zone. It is thus evident that a number of different factors contribute to the activity of the leaf meristem: the period during which initial cells continue to enter the cell cycle; the time the cells take to complete a single cell cycle; and the number of cell cycles that the meristematic cells undergo before leaving the meristematic zone. These factors correspond to the duration of cell production, the single cell cycle time and the number of dividing cells, respectively. Quantification of these parameters is essential for the investigation of relationships between meristem plasticity and plant growth. For the developmental system of a leaf of grass, in which the width of the leaf is first determined along the circumference of the shoot apical meristem, followed by elongation along the longitudinal axis (Beemster & Masle, 1996; Sugiyama, 2005a), these cell parameters can be quantified by measuring the rate of leaf elongation and observing the length of the cells along the longitudinal axis (Fiorani et al., 2000; Sugiyama, 2005b).
Figure 1. (a) The two types of meristem in a grass shoot. The shoot apical meristem (SAM) is located at the shoot base, and the leaf meristem (LM) is located at the leaf base. Leaf primordia (LP), which are initiated on the SAM, proliferate first along the transverse axis and encircle the subapical region of the SAM, and then elongate along the longitudinal axis. (b) Cell division parameters that represent leaf meristem activity can be estimated from the cell length profile. The cell division zone is defined as the region within which cell length decreases. The initial cells of the meristem enter into the cell cycle at the basal end of the meristematic zone, undergo several rounds of the cell cycle, which consists of G1, S, G2 and M stages, and exit from the cell cycle at the distal end of the zone. These processes are governed by: (1) how long the initial cells continue to enter into the cell cycle; (2) how fast the cells complete a single cell cycle; and (3) how many cell cycles the meristematic cells complete before leaving the division zone. These factors are represented by the duration of cell production, the average time of a single cell cycle and the size of the division zone or the number of dividing cells, respectively.
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Soil nutrients and light can each serve either as specific signals that trigger a particular response from a plant or as a resource for the plants to use. The former represents a signal-mediated response and the latter a resource-mediated response. For example, when plants increase their height under shaded conditions by using phytochrome molecules, they are showing a signal-mediated response (Smith, 2000; Schmitt et al., 2003) and when plants intercept incident light and convert it into assimilated carbon in a photosynthetic pathway, they are showing a resource-mediated response. It is important to distinguish between the two types of response mechanism, because they give rise to contrasting views of the ecological roles of phenotypic plasticity (Sultan, 1995; Kurashige & Callahan, 2007). The signal-mediated response can be viewed as a future-oriented response that triggers adaptive plasticity to minimize future deleterious effects of the environment, such as shading by neighbouring plants (Lambers et al., 1998; Terashima et al., 2006). By contrast, the resource-mediated response is an opportunistic, present-oriented response of plants to current resources, the effect of which is inevitable growth response (Sultan, 1995). Signal-mediated responses are expected to result in changes in plasticity that are asymmetric between light and soil nutrients, because signals are specific to a particular environment. By contrast, resource-mediated responses are expected to give rise to symmetric changes in plasticity, because carbon assimilation rate is strongly influenced by both light and soil nutrients as main resources of plants.
In this study, we specifically addressed the following three issues by examining the plasticity of meristem activity in response to light and soil nutrients in fast- and slow-growing Festuca grass species. First, we investigated whether the cell parameters that represent meristem activity showed signal- or resource-mediated responses to soil nutrients and light. Second, we examined the manner in which the cell parameters influenced the plasticity of leaf size (length, width and area) and relative growth rate (RGR). Given that the RGR of a vegetative shoot depends on the supply of new cells produced by meristems (sink activity), as well as the supply of carbohydrate from mature leaves (source activity), we examined how the plasticity of RGR is influenced by the plasticity of the meristem and the specific leaf area (SLA), which is closely associated with the rate of photosynthesis per unit leaf mass (Reich et al., 1998). Third, we explored interspecific differences in the magnitude of phenotypic plasticity. Among the four species of Festuca studied, F. arundinacea Schreb. and F. pratensis Hudson are fast-growing species, whereas F. rubra L. and F. ovina L. are slow-growing species (Sugiyama, 2005c). We examined whether the fast-growing species showed a greater magnitude of plasticity with respect to RGR and leaf size than the slow-growing species. In addition, we determined which cell parameters were responsible for interspecific differences in the magnitude of plasticity and derived the implications of our findings for the evolution of phenotypic plasticity.