**6. Plasticity mechanisms**

There are four primary mechanisms in wild plant populations that regulate plasticity through adaptive responses [6]. The four plasticity mechanisms are physiological, developmental, cellular, and epigenetic responses [6]. Physiological plasticity describes all physiological responses associated with phenotypic traits and signaling networks [40, 71]. Developmental plasticity is associated with human or animal neural developmental, and plant embryonic development, in response to stress [6, 40]. Cellular plasticity describes adaptive responses within cells that are often associated with reducing reactive oxygen species accumulation through redox mechanisms [72]. Epigenetic plasticity describes changes to molecular mechanisms in response to abiotic stresses resulting in altered gene expression and function without changes in the DNA [71].

**45**

*Phenological Plasticity of Wild and Cultivated Plants DOI: http://dx.doi.org/10.5772/intechopen.85070*

Physiological plasticity is the most dynamic of plasticity mechanisms and is often involved in all other mechanisms of plasticity [40, 71]. Novel and emerging environments trigger many physiological responses such as carbon dioxide (CO2) assimilation, changes in chlorophyll content, water use efficiency, sugar sensing and photosynthesis [73]. Physiological changes correlate directly to plant fitness, and changes in plasticity determine how a plant responds to environmental stresses [73, 74]. Studying the association between a physiological phenotype and changes in gene expression within wild populations will make it is possible to identify and target genes that are responsible for adaptive responses, i.e., plasticity genes [24, 73, 74]. In this way plasticity genes and gene variants become a selective tool for understanding plasticity heritability dynamics, as well as identifying positively

Seed dormancy is an excellent example of physiological plasticity [73, 74] Seed dormancy prevents germination out of season, even under favorable conditions, and ensures species survival of natural catastrophes [16, 76]. Environmental cues such as light, temperature, and moisture impact the depth of seed dormancy and the length of time required for dormancy release [76]. In weeds, discontinuous or extensive seed dormancy ensures germination only in favorable conditions and confers environmental plasticity, or the ability to respond to changing biotic or

Discontinuous or extensive seed dormancy impacts environmental plasticity through variable emergence timing throughout a growing season [76].

Discontinuous seed dormancy is likely a major "weedy" characteristic contributing to physiological plasticity in many wild plants and weed populations [76]. Downy brome (*Bromus tectorum L.*) is an invasive grass weed in both natural and agricultural environments which produces seeds with discontinuous seed dormancy [77–80]. New downy brome seedlings have the capacity to emerge in any season; early and late in the fall, before and after cool season crops or native grasses emerge, and even in the spring [80]. Differences in emergence timing in downy brome may be due to differences in dormancy status and may occur because of phenotypic and genotypic variation within a single population cluster, the presence of multiple population clusters within a single location, and the viability of seed in the seed bank [80]. The successful invasion of wild plant populations is measured by the number of individuals in a population, reproductive output, the range of habitats occupied, and the ability for survival and adapt in new environments through time [16]. Therefore, downy brome is an excellent example of a model colonizing species as it allocates most of the developmental time to seed production [16]. Downy brome increases the chances for survival of future generations, by maximizing contributions to seed banks [76]. Physiological plasticity mechanisms like prolific seed production, discontinuous seed dormancy, and variable germination in weeds increase the likelihood of outcompeting wild and cultivated plant species in native

There is currently very little information about the specific genes or molecular mechanisms regulating dormancy or dormancy loss in many weeds or wild plant species [77]. Gaps in molecular information slow the progress for understanding the impact of wild plasticity on adaptability [1, 16]. However, detailed physiological observations and translational research are useful tools. These are powerful tools for studying the mechanisms that drive physiological plasticity in the seed and throughout all plant life stages, in natural and agricultural environments, and in

**6.1 Physiological plasticity**

adapted populations [24, 73–75].

abiotic environmental factors [16].

and non-native environments [76].

both wild and cultivated plant populations [1, 16, 82–86].
