Table 2.

List of root phenotyping methods in various crops.

Crop	Trait	Method	Reference
Maize (Zea mays)	Root architectural traits of root crown - brace roots, number of brace roots, branching density of brace roots - number, angles, and branching density of crown root	At harvest, roots were excavated by removing a soil cylinder of 40 cm in diameter and 25-cm depth, with the plant base as the horizontal center of the soil cylinder. After root washing, clean roots were visually scored.	Trachsel et al. [170]
Arabidopsis (Arabidopsis thaliana)	Root system architecture - length, curvature, and stimulus-response parameters	Images were captured of Arabidopsis grown in the agarose gel condition contained in vertically-arranged plates to permit roots to grow on the surface of the medium.	French et al. [171]
Winter wheat (Triticum aestivum)	Root development and distribution - Number of total roots at different soil levels - Number of roots per observation depths	Field mini-rhizotrons were set up. Detailed images are available in the attached reference. Transparent rhizotubes were inserted into soil. Then, images were captured by the camera, which was located on both sides of the rhizotubes. The camera was positioned using an indexing handle at 20 observation locations in the tubes.	Cai et al. [172]
Maize (Zea mays)	Root morphology - axile - lateral root	Germinated seeds were transferred to moistened blotting paper in pouches. Root images were acquired by the scanner and then analyzed by WinRHIZO software.	Hund et al. [173]
Rice (Oryza sativa)	Root morphology - length - width - initiation angle - root tip	Rice seeds germinated in Petri plates were transplanted into glass growth cylinders containing 1.3 L of growth medium. The camera was placed in front of the growth cylinder. Image sequences were captured daily for each plant root system grown in the growth medium, consisting of 40 silhouette images taken every 9° for the entire 360° of rotation. RootReader3D software was used for the analysis of the 3D root images.	Clark et al. [174]
Sweet pea (Lathyrus odoratus), Sunflower (Helianthus annuus)	Analysis of soil aggregates to anticipate water flow toward the root	Sweet pea and sunflower seeds were planted on the surface and were grown for 30 days. An X-ray microtomography image was measured by high-resolution XMT beamline 8.3.2 at the Advanced Light Source (Lawrence Berkeley National Laboratory, USA). Transmitted X-ray light is converted to visible light using a CdWO4 single crystal scintillator, magnified by a Canon 2X lens, and imaged on a Cooke PCO 4000 CCD camera.	Aravena et al. [175]
Alfalfa (Medicago sativa)	Root system architecture - number of root tips - total root length - diameter - root angle orientation frequency	Alfalfa root crowns were separated from the aboveground foliage. Soil was brushed off the roots, which were then imaged in the laboratory using the RhizoVision Crown platform.	Mattupalli et al. [176]
Upland cotton (Gossypium hirsutum L.)	Root system architecture - total root length - average diameter of roots - number of root tips - maximum root depth - total explored area - maximum root width	Development of a root phenotyping platform, PhenoRoots, which allows for the non-invasive study of plant root system architecture. Substrate or soil-filled rhizotrons are used to grow plantlets, whose roots are directly visible through a glass plate. Pictures were taken using a digital camera and then analyzed by WinRHIZO and ImageJ software.	Martins et al. [177]
Soybean (Glycine max L. Merr.)	Root biomass and morphology - length - area	This study used “transparent soil” formed by the spherification of hydrogels of biopolymers. It is specifically designed to support root growth in the presence of air, water, and nutrients, and allows the time-resolved phenotyping of roots in vivo by both photography and microscopy. The roots developed by soybean plants in this medium were markedly more similar to those developed in real soil than those developed in hydroponic conditions and did not show signs of hypoxia.	Ma et al. [178]
Pea (Pisum sativum L.)	Root morphology - length of the tap root and lateral roots Root system architecture - number of lateral branches, branching angle representing the angle between the tap root and branched lateral roots	Measurements of root traits were performed on two phenotyping platforms. One system represented a typical high-throughput phenotyping platform for seedling root screening using agar-filled plates. The other system focused on mature root systems grown under more natural conditions (sand-filled columns) with less potential throughput. Images were analyzed using the software GrowScreen-Root	Zhao et al. [179]
Sorghum (Sorghum bicolor L. Moench)	Root system architecture - nodal root angle	The phenotyping platform consisted of 500 soil-filled root chambers (50 × 45 × 0.3 cm in size), made of transparent Perspex sheets that were placed in metal tubs and covered with polycarbonate sheets. Around 3 weeks after sowing, once the first flush of nodal roots was visible, roots were imaged in situ using an imaging box that included two digital cameras that were remotely controlled by two android tablets. Free software (openGelPhoto.tcl) allowed precise measurement of the nodal root angle from the digital images.	Joshi et al. [166]
Spring barley (Hordeum vulgare L.)	Destructive methods - total root length - root system surface - root volume - root diameter - number of tips Non‑destructive methods - root system depth - projected root surface - sum of the root lengths	The correspondence between a destructive (WinRHIZO scans) and non-destructive (RGB root imaging) method for root phenotyping using a described system was tested. The root images were analyzed after the staining of roots with powdered active charcoal. Root images were taken in the photographic room using an RGB camera. The images (JPG or TIFF files) of the plants taken in the photographic chamber were analyzed using ImageJ software. Root system scanning was performed using a specialized root scanner (STD4800 scanner) coupled with WinRHIZO Pro software (Regent Instruments, Quebec, Canada).	Slota et al. [180]