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Destructive root assessment

John Foulkes, University of Nottingham

Root washing 

After sampling above-ground plants, the roots are extracted for assessment of root system architecture traits. The soil samples in the soil core are taken in 10 cm depth horizons (or whole pot sampled if plants not grown in pots rather than soil columns). The soil samples should be placed in plastic bags and stored in a cold room (4 °C) for not more than 2 weeks prior to root extraction. Plant roots may be extracted from the soil either by using: (i) an ‘automated root washer’ (e.g. Delta-T root washer (RWC-UM-2, Delta-T Devices LTD, Cambridge, UK (http://www.delta-t.co.uk/product-support-material.aspor) or Gillison’s Variety Fabrication, Inc. Hydropneumatic Elutriation System Rootwasher, Michigan, USA  (http://gillisons.com/products/hydropneumatic_elutriation_system.htm) or (ii) manual root washing using sieves. For automated root washing, the root washer should provide the correct pressure for root extraction (0.34 to 0.48 KPa). For large volumes of soil samples, for optimum extraction the soil sample may need to be divided into two sub-samples for running in the root washer. For manual root washing, carefully use water to separate root tissue from soil and other debris within the soil core. Wash and clean each sample for the same duration, and in the same manner to make samples comparative. Each sample may take up to 0.5-1.0 hour to process. Add water to the soil samples whilst still in their plastic bag, mix gently, tie and leave overnight. Transfer the soil and water mix to a tray, stir gently by hand, wait a few minutes, and decant water through a 500 μm sieve to recover the roots. Remove large plant material and debris by hand. Collect the roots into a plastic tube. Repeat the washing process at least twice as roots may remain in the soil at the bottom of the tray. Add a solution of 15% alcohol to tube (to preserve the root sample). Store roots at 4–6oC. Alternatively, if root cleaning and scanning is not possible immediately, the samples may be stored in a freezer (-20 °C). The fine cleaning can then be performed prior to scanning after leaving the sample to thaw over night.

Root cleaning

Hand clean the root samples carefully using forceps/ tweezers. Remove all of the material that is not live roots, especially dead roots which can be identified from their darker colour and their lack of elasticity and flexibility which is characteristic of living roots. The sample of roots and debris may be spread in an acrylic box with water, and the live roots (white to light brown colour) are then manually selected using fine forceps.

Root scanning (using WinRhizo Software)

Cleaned roots samples are spread in an acrylic box (size A4) with tap water to

minimize the number of overlaps, and digitalized at 400 dpi resolution and 256

greys contrast (Tiff file format) with a scanner with a transparency adapter (e.g. WinRHIZO STD 1600+, Regent Instruments Inc., Quebec, Canada). When the root sample is too large to complete in one scan, two or more scans should be performed. On the scanned images of the root systems the total length, mean diameter, total area and volume are measured, using the WinRHIZO regular V.2002c software (Regent Instruments Inc., Quebec, Canada). After scanning the root system to scanning the root system, dry weight is recorded to 3 dp after drying for 48 h at 80ºC.

Root System Architecture Traits

The following RSAT can be calculated:

Root dry weight (RDW) per plant (g plant-1)

Total root length (TRL) per plant (cm plant-1)

Root length density (RLD) = root length / soil volume (cm cm-3).

Root volume density (RVD) = root volume /soil volume (cm

Specific root length (SRL) = root length / root dry weight (cm g-1)

The cumulative distribution of RLD with depth (β) is:

where Y is the fraction of the root system accumulated from the soil surface to depth, d, and β is a parameter that describes the shape of the cumulative distribution with depth.