Study of Root Growth: 7 Methods | Botany

The following points highlight the seven main methods used to study root growth. The methods are: 1. Trench Profile Method 2. Framed Monolith and Pin Board Method 3. Soil Moisture Depletion Method 4. Core Sampling Method 5. Minirhizotron Method 6. Radioactive Isotope Method 7. Allometry Method.

1. Trench Profile Method:

The trench profile method, which was introduced in the classic studies by Weaver (1926), is, with modification, still in common use. It involves excavating a trench perpendicular to the row or beside an individual plant and mapping or photographing the visible roots.

2. Framed Monolith and Pin Board Method:

The framed monolith method is the trench profile modified to quantify root extension and distribution. Pins inserted into holes equidistantly spaced in a board placed against the face of the trench in effect trap the roots in the soil profile in cubes. A soil monolith is cut and lifted from the trench.

After the prisms are soaked in water, the roots are carefully washed free of soil, described, and quantified as to length, weight, or other parameters. An application of Congo red stain stained differentially and permitted separation of living from dead roots. The framed monolith method can provide quantitative information, but it is la­borious, time-consuming, and costly.

3. Soil Moisture Depletion Method:

Soil moisture depletion can be measured either gravimetrically (weighing core samples) or by a neutron probe. It indicates depth of moisture depletion. The depletion front usually exceeded rooting depth by about 15 cm due to moisture migration in the soil. This method is rapid but is not reliable for determining root density or other root parameters because water extraction rates also depend on evaporative demand, soil water potential, and soil hydraulic characteristics.

4. Core Sampling Method:

Core sampling removes undistributed soil cores and roots contained therein from the root zone at predetermined locations relative to the plant and row. Tube or bucket samplers are commonly used. Several cores per point of reference are required. This method can be mechanized but is only semi-quantitative.

5. Minirhizotron Method:

A glass tube positioned in the soil profile can be used, with the aid of a light and mirror or TV camera, to observe rooting as roots grow against the side. The findings are only qualitative. A planting box with one tilted glass side or translucent tubes tilted at about 20° to 30° from upright and kept dark can indicate growth rate and depth penetration, as roots contact the glass and follow it downward.

6. Radioactive Isotope Method:

Isotope ³²P, placed at variable depths, can indicate the rooting depth from the counted and recorded isotope uptake. This method is not effective in determining root density. A similar method, consisting of placing perforated plastic bags of nitrogen (N) fertilizer at variable depths and observing the foliar N changes, yields comparable results, especially with grass plants.

7. Allometry Method:

This method assumes proportionality of the logarithms of weights of plant parts, in this case the shoot-root (S-R) ratio. Large top growth indicates proportionally large root growth. Measurements of plant height reliably indi­cated the depth of rooting of soybean on irrigated or rain-fed, barrier-free soils.

Rooting depth was twice plant height at the V₃ stage of growth (vegetative with three expanded leaves), and this relationship contin­ued until pod development, when it became 1.4 times plant height. Irrigated plants had approximately 15% more roots by weight in the 0 to 15-cm zone. The rooting of 22 strains of perennial grasses was allometric. This method can be misleading in that barriers in the soil profile do not significantly change the S-R ratios but drastically alter the locations of the root mass.

To characterize rooting most root studies have reported either fresh or dry weight, but root weight may be poorly correlated with water and nutrient uptake, the primary interest in root studies. The fine and young roots, princi­pally the root hair zone, function most effectively in nutrient uptake. Root hairs are confined to a section of a few millimeters, at most a few centimeters, of root near the tip. Root hairs form just after elongation of epidermal cells.

The number and rate of root hair formation was greater at 26°C than at 15°C. But the life span was 40 hr and 55 hr for the two tempera­tures, respectively. This suggests a somewhat constant rate of occurrence for a root segment of a given species. The length of the root hair zone depends to a degree on genotype and environment. Effec­tiveness of root uptake is better estimated by determination of root density (root length per soil volume) or by root surface area than by roots length.