ROOT PHENOTYPING METHODS
Phenotyping presents many challenges in front of plant breeders for
enhancing stress tolerance in crop plants as screening roots for complex
genetic traits is extremely difficult under field conditions (Sharma et
al 2016). Root phenotyping methods involves combination of imaging and
image processing with automation. Image-based phenotyping is a
non-destructive method involving optical analysis of plant traits
(Furbank et al 2011; Fiorani and Schurr 2013), and it mainly involves
characterization of biochemical, physiological and anatomical properties
of roots (Guo and Zhu 2014).
Minirhizotrons involves non-destructive analysis in which a clear tube
is positioned in ground and root growth adjoining the tube is detected
and image is captured using camera which is installed down the tube
(Smit et al., 2000). However, it has certain limitations, as tube and
soil interface provides an unnatural environment for the growth of
roots, which results in false assessments of characteristics of growth
in plants (Wasson et al., 2012). Electrical resistance tomography (ERT)
is another approach used for analyzing physical properties of soil
(Basso et al., 2010) and how soil water is being exploited by roots in
different crops (Srayeddin and Doussan, 2009) both are the measures to
detect root behavioral patterns indirectly. Various software such as
WinRhizo, Image J, EZ-Rhizo, Root system analyzer, Smart Root, Optimas
analysis, Root trace, and Root Nav are popular for image-based analysis
of root phenotyping (Sharma et al., 2016). For measuring growth kinetics
and branching angles Smart Root software can be used. DART is another
software that can be used for studying root structure. It is a manual
software coded in JAVA (Bot et al., 2010) and produces flexible datasets
of individual parameters of root. Manual softwares like WinRHIZOTM, DART
etc., helps to study root lifespan by monitoring color of roots manually
(Li et al., 2022). X-ray computed tomography (CT) can be used for
assessment of roots within soil profile and for visualizing 3D
configuration of roots which helps to decide which character of roots is
useful under drought conditions for obtaining high crop yield (Wasaya et
al. 2018). The 3D magnetic resonance imaging (MRI) is a non-destructive
method that is being practised commonly as a 3D root phenotyping process
(Metzner et al., 2014). Image quality of MRI is known to be influenced
by substrate type and water content (Rogers & Bottomley 1962). Daniel
et al. (2022) has recently used this method for phenotyping roots of 288
wheat seedlings using a new pipeline which is designated as
medium-throughput phenotyping. Electrical capacitance (EC) is another 3D
root phenotyping method used in the field, which analyzes alternating
current of low frequency (less than 1kHz) between the soil and base of
stem and then evaluate the dielectric properties for re-establishing
root architecture (Dalton, 1995). This method has been used for
phenotyping roots of different crops, including maize (Imre et al.,
2018), soybean (Cseresnyés et al., 2017) and wheat (Cseresnyés et al.,
2021). Ground-penetrating radar (GPR) is another non-destructive
geophysical device which uses electromagnetic waves for detecting
surfaces which is based on physical and dielectric properties of
materials. It is used for analyzing features of RSA and estimation of
its biomass (Lorenzo et al 2010). When compared to soil, roots have
distinct dielectric permittivity thereby, GPR is useful to detect
number, diameter and depth of coarse roots by measuring the reflected
signals produced by hyperbolic reflections on radargrams (Wu et al.
2014; Lombardi et al., 2021). For example, GPR was used to measure
coarse roots in plants, including, cassava (Delgado et al., 2017),
citrus (Zhang et al., 2019) and willow (Li et al., 2015).
For observing roots in controlled environment, various soil-less growth
media are being employed, such as gels, paper rolls, and aerated aqueous
solutions. Hydroponics techniques are practiced commonly for growing
plants using transparent plexiglass nail board sandwiches packed with
glass beads of about 1.5 mm size for maintaining proper flow of
nutrients (Zhu et al 2005). These systems help to measure total root
length, root traits, root branching angles through imaging technique or
manually.