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?