4.2 The plasticity of morphological and physiological traits
Functional traits can be divided into morphological traits and physiological traits (Liu & Ma, 2015). Strong plasticity of variability was observed in whole plant traits, especially in the morphological traits, and coefficient of variation was the accepted method used to standardize variability among traits with fundamentally different units (Acasuso-Rivero et al., 2019). Morphological traits involve the shape and structure of each organ, responses of individuals to environmental changes based on internal genes, and adaptations of plants to environmental heterogeneity (Hu et al., 2008). In our research, the leaf area, stem pitch number, height, and shoot basal diameter were attributed to morphological traits. All these traits had high coefficients of variation, which were higher than 20%, indicating that morphological traits are sensitive under the stress of environmental factors. Higher phenotypic plasticity contributed to P. australisbeing able to adapt to a wide range of niches under salinity stress in the Yellow River Delta.
Physiological traits are related to biochemical processes and reproduction (Zhang & Luo, 2004). Some researchers showed that physiological traits were more susceptible to soil properties with regard to morphological traits (Schneider et al., 2017). Compared with morphological traits, the coefficient of variation values of physiological traits contained the SPAD, leaf thickness, SLA, and leaf water content in our research and were lower than 20%. The lower coefficients of variation indicated relatively lower plasticity and stable intrinsic properties of P. australis under various environmental stressors. SLA and leaf thickness, although shown as morphological traits (Smith & Knapp, 2001), were regarded as physiological traits in some research (Zhang & Luo, 2004; Wright et al., 2004). They remain relatively stable under specific environments, and were not only influenced by soil factors, but also influenced by factors including leaf tissue characteristics (such as the epidermis cells and mesophyll cells) and chemical composition.
In the Yellow River Delta, the mode of P. australis morphological traits is an integrated system in growth, but physiological traits have different mode to reflect the environmental stress. Grouping plants by functional trait types rather than by organs could help us more accurately understand the process of plant growth and the response to environmental stress. It’s worth noting that while plasticity is not equal to fitness (Acasuso-Rivero et al., 2019), the relationship between them is still worth studying in the Yellow River Delta under environmental stress.