Relation of RSA with drought
Fine and coarse roots have certain morphological traits which are responsible for having significant impact on surface area and length of roots and control productivity under drought stress. Root diameter, specific root length (SRL), root surface area per dry mass and root tissue density are some of the important morphological traits that control productivity in water scarcity (Fitter 2002). Roots with small diameter and increased SRL absorbs water more efficiently by minimizing the apoplastic barrier for water entry into the xylem, thereby increasing root hydraulic conductivity (Comas et al., 2013). The cortex formation aerenchyma is generally responsible for increasing SRL (Zhu et al., 2010). However, it has been observed in rice that aerenchyma restricts flow of water through root cortex reducing water uptake in drought stress conditions (X. Yang et al., 2012). Root hairs are more effective for uptake of nutrients and water (Suzuki et al., 2003). Root hairs maintain the hydraulic conductivity between soil and root, maximize the contact of roots with soil by minimizing surface resistance and leading to increased absorption by roots (Wasson et al. 2012). Segal et al. (2008) reported that barley mutants which lacked root hairs showed less water uptake despite having lots of branched roots.
In response to drought, plants encourage elongation of primary roots by inhibiting branching of lateral roots. More vertically structured roots are present naturally in plants that are well adapted to drought, for example in sorghum (V. Singh et al., 2010). Direction of root growth is also critical for determining effectiveness of root system. Under normal conditions, primary roots grow in response to gravity axis whereas lateral roots prefer horizontal growth while lateral roots have particular angle of emergence which is known as gravitropic set-point angle (GSA) that directs growth of lateral roots away from primary roots. Under water stress GSA undergoes changes in a way that allows enhanced uptake of nutrient and water conditions by producing longer and deeper roots (Lynch, 2013). Similarly primary roots also modulate its growth to extract maximum water from surroundings (Takahashi et al., 2009; Iwata et al., 2013).
It has been reported that response of RSA to drought varies considerably at intra- and interspecies level (Kou et al., 2022). Ability of plant to survive and maintain its yield under water scarce conditions is regarded as the plant plasticity response which confers drought tolerance to it (Gao et al., 2015; Suseela et al., 2020). Zhou et al (2018) conducted a meta-analysis based on root traits and concluded that drought drastically affect root length and its density. It enhanced diameter of roots and decreased shoot-to-root biomass ratio under drought stress.
Root architecture is dependent on plant response to the soil water and its distribution, in response to which plants make growth adjustments. Plants generally show plasticity in the distribution pattern of root, especially in deep-rooted species for example Zea mays andHelianthus annus. Plants with low root length density (per unit volume of soil) are mainly preferred in areas where water is available in shallow layers of soil whereas for deep layers plants with high root length density are considered more efficient in breeding programs (J. Lynch, 2013; Wasson et al., 2012). Breeding for larger xylem vessels is another good strategy for increasing axial hydraulic conductivity allowing roots to grow at greater depths of soil (Wasson et al., 2012). Hydraulic conductivity is somewhat poor in many woody plants as root-xylem has higher susceptibility to cavitation (Pockman & Sperry, 2000;Comas et al., 2013). During drought conditions root pressure play significant role in repairing embolized xylem by removing water vapor and air from xylem conduits and provides important area of concern for breeders to increase drought tolerance (J. Sperry et al., 2003; J. Sperry, 2011).
It has been proposed that root traits helps in reducing the root system cost (including greater formation of aerenchyma, smaller root diameter, reduced area of living tissue, increased size of cortical cell, fewer nodal roots, root cortical senescence) and aid in promoting elongation of roots, hence increased capture of resources and deeper exploration of soil (Lynch 2013; Fonta et al., 2022). Steeper angle of root growth is known to improve drought tolerance, for example, in rice, deeper rooting 1 (dro1 ) gene which is responsible for influencing angle of root growth has been used in breeding programs and cloning experiments extensively (Uga et al. 2013; Kitomi et al. 2015; Kou et al., 2022). The number and size of metaxylem also affects drought tolerance and water use efficiency in plants. Smaller metaxylem vessels maintains less axial hydraulic conductance in roots conserving soil water while also keeping root tips hydrated (J. P. Lynch et al., 2014). Smaller metaxylem vessels increase resistance to cavitation also (Guet et al., 2015; J. S. Sperry & Saliendra, 1994). Various studies have proven the role of smaller metaxylem vessels in roots, for instance, in wheat, under drought conditions presence of smaller metaxylem vessels in seminal roots of seedlings resulted in improved yield (Richards and Passioura 1989). In maize, genotypes that had decreased metaxylem area in nodal roots and reduced axial hydraulic conductance performed better under water stress than the ones which did not display this plasticity (Klein et al., 2020). Also in rice, it has been reported that drought-tolerant cultivars had increased number of xylem vessels under water scarcity than drought susceptible cultivars (Abd Allah et al., 2010). De Bauw et al., (2019) conducted an experiment and reported that in theIndica rice variety “Mudgo”, xylem vessel number and diameter increased in response to drought while in variety NERICA4, xylem vessel diameter increased but vessel number decreased with respect to water stress. Even along the root axis phenotyping variations exist in drought tolerant varieties of wheat, for instance, near the root tip fewer, larger metaxylem vessels are usually found whereas near the root base smaller metaxylem vessels are present (Kadam et al., 2015; Wasson et al., 2012). It has also been observed that at the base of lateral and nodal roots metaxylem constriction is present in rice (Hazman & Brown, 2018; Vejchasarn et al., 2016) which leads to reduced axial hydraulic conductivity which benefits rice to control the movement of water to shoot and ensuring enough water availability in root tissues, especially growing tips.
Fonta et al., (2022) studied the response of RSA and anatomy with respect to drought, where they used two rice cultivars IR64 which is high yielding, drought susceptible, Indica culivar, and Azucena, low yielding, moderately drought tolerant, Japonica cultivar with deep rooting system. They observed that number of nodal roots decreased significantly in both the cultivars under water stress which led to increased conservation of resources for promoting root elongation and hence resulting in deeper root systems. L- type lateral roots (long, thick lateral roots) showed prominent growth at depth under drought conditions and contributed towards deeper soil exploration. Root’s ability to exploit water from the soil also depends on maintaining its fitness by reducing the cost of exploring soil by decreasing area of living tissue as fewer living cells requires less respiration per unit length (J. P. Lynch, 2011,2013). It is also reported that in maize, formation of root cortical aerenchyma or decreased living tissue area (Chimungu et al., 2014; Galindo-Castañeda et al., 2018) and senescence of root cortical cells in barley (Schneider et al., 2017) led to better adaptability under water and nutrient stress.
Production of suberin is also increased in primary roots under both water and salt stress, especially in exo- and endodermis layers where it controls loss of water (Karlova 2021). Role of suberin is generally known to be root-type specific, for example, it has been reported in grape that suberin deposition in roots provided increased tolerance to drought whereas presence of suberin in fine roots (approx. 2mm) led to increased drought susceptibility (Cuneo et al., 2020). As compared to domesticated cultivar, wild barley had increased suberin deposition in exodermis when exposed to drought, something which has also been observed in other species adapted to drought (Kreszies et al., 2020; Yang et al., 2020). Lignin deposition is also observed around mature xylem tissue which acts as a water-resistant barrier and promote drought tolerance (Xu et al., 2017; Liu et al., 2018; Sharma et al., 2020). Overexpression of lignin biosynthesis genes often produced elongated primary roots in mutants (Xu et al., 2020).