1. Introduction
Root system architecture refers to the morphological traits and branching patterns of root system in soil matrix, which play a prominent role in exploring soil space and acquiring resources (Lynch, 1995; Laboski et al., 1998; Tracy et al., 2015). The root morphological traits are closely related to their efficiency in obtaining water and nutrients from the soil, exploring soil space, and ability to resist environmental stress (Markesteijn and Poorter, 2009; Freschet et al., 2017; Weemstra et al., 2021b). Branching patterns are often described by topological index (TI), and different branching patterns generally represent the internal competition patterns of root system and their adaptability to different soil habitats (Oppelt et al., 2001; Spanos et al., 2008). As a consequence, root system architecture has a profound impact on the growth and development of plant individuals, which is the basis for them to adapt to constantly changing environmental conditions (Alvarez-Flores et al., 2014; Hogan et al., 2020).
Interspecific and intraspecific variation of traits is the cornerstone for coexistence of different plant species and construction of stable plant community (Violle et al., 2012; Weemstra et al., 2021a). Research on variations of plant functional traits at different ecological scales has found that interspecific and intraspecific variations are important indicators of plant response and adaptation to environmental changes, as well as resource competition strategies (Wright et al., 2004; Bu et al., 2017). Although interspecific variation has gained more attention in ecological research based on functional traits, increasing empirically published evidence demonstrated that intraspecific variation is an ecological indicator that cannot be ignored because of representation of plant response to environmental changes and phenotypic plasticity (Albert et al., 2010a; Siefert et al., 2015; Defrenne et al., 2019). However, the published studies have focused more on the interspecific variation of root morphological traits (Weemstra et al., 2016; Erktan et al., 2018; Carmona et al., 2021), neglecting the important indicative role of intraspecific variation in traits based underground ecology research.
The phylogenetic relationship of species is an important genetic factor that affects the variation of root system architecture traits (Hogan et al., 2020), and this impact may be stronger than environmental factors including climate change and mycorrhizal status, although they have been considered important factors affecting root system architecture variation (Maherali, 2017; Valverde‐Barrantes et al., 2017; Lozano et al., 2020). The root trait phylogenetic conservatism (RTPC) hypothesis suggests that differences between root traits in related species may be smaller compared to phylogenetic structures with weak leaf traits, thereby exhibiting strong phylogenetic conservatism (Valverde-Barrantes et al., 2014; Liu et al., 2019). Research on morphological traits of fine root on a global scale suggested that specific root length (SRL) , root diameter (RD) , and other root system architecture traits of woody plants are limited by species evolutionary history, so that demonstrate similarity in root traits among related species (Kong et al.2014; Valverde‐Barrantes et al. 2017; Ma et al. 2018; Zhou et al. 2018). However, the diversity of root system functions and the complexity of soil environment may lead to the impact of species evolutionary history on root system architecture traits that is not consistent with the expectations of the RTPC hypothesis (Kramer-Walter et al., 2016; Wang et al., 2018). Consequently, it is necessary to conduct more empirical research to verify whether phylogenetic relationships have a significant impact on the formation and development of root system architecture.
Plants can respond to potential environmental stress by changing organ morphological traits and the proportion of biomass in each organ (Bouma et al., 2001; Poorter et al., 2012; Freschet et al., 2018; Zhou et al., 2019). By balancing the biomass and morphology of the organs responsible for resource acquisition, coexisting species can achieve a balance between aboveground and underground resource acquisition (Freschet et al., 2015a). Specifically, the adaptive changes in root system architecture determine the foraging characteristics and the ways in which underground resources are acquired and conserved (Guo et al., 2008; Alvarez-Flores et al., 2014; Hogan et al., 2020), which directly affect the material accumulation and morphogenesis of the aboveground parts of plant (Dannowski and Block, 2005). Conversely, the development and expansion of roots in soil depend on the carbon fixed by photosynthesis in plant leaves (Willaume and Pagès, 2011). Therefore, plant functional traits are potential covariates that explain biomass allocation, and there may be synergies or trade-offs between them (Yin et al., 2019). It is worth noting that this relationship may depend on phylogenetic relationships, which can be demonstrated through correlation analysis of Phylogenetic Independent Contrast (PIC) values (Felsenstein, 1985; Paradis and Schliep, 2019).
Annual ephemeral plants are an important component of desert early spring vegetation in northern Xinjiang, China (Mao and Zhang, 1994). They are unique plant group with a distinctive life history, which utilize winter snow melt water and relatively sufficient precipitation in spring to quickly germinate and grow in early spring (Zhang et al., 2020). As a consequence, they can quickly complete their life cycle before the onset of a dry and hot summer climate(Mao and Zhang, 1994; Wang et al., 2021). Through long-term adaptive evolution, this plant group has formed an ecological strategy suitable for harsh desert environments (Lan and Zhang, 2008; Shi et al., 2006). The most published researches have focused on the adaptive characteristics of the aboveground parts of annual ephemerals (Cheng and Tan, 2009; Xiao et al., 2014; Lu et al., 2015; Mamut et al., 2018), with relatively few studies on root systems. In addition, published empirical experiments mainly focus on the impact of environmental factors on the growth and biomass allocation patterns of annual ephemerals (Cheng et al., 2006; Mamut et al., 2019; Qiu et al., 2007; Zhang et al., 2020), with little attention paid to the ecological adaptation of root system architecture of annual ephemerals to the desert environment in the genetic context.
Therefore, this study attempts to solve the following scientific problems by studying the root system architecture traits of 47 annual ephemerals. i) What are the variation patterns of root architecture traits in annual ephemeral species? and ii) are they influenced by the phylogenetic relationship of the species? iii) How do annual ephemerals adapt to desert environments through coordination or trade-offs between root system architecture traits and biomass allocation?