Maize brace roots provide stalk anchorage

Abstract

Mechanical failure, known as lodging, negatively impacts yield and grain quality in crops. Limiting crop loss from lodging requires an understanding of the plant traits that contribute to lodging-resistance. In maize, specialized aerial brace roots are reported to reduce root lodging. However, their direct contribution to plant biomechanics has not been measured. In this manuscript, we use a non-destructive field-based mechanical test on plants before and after the removal of brace roots. This precisely determines the contribution of brace roots to establish a rigid base (i.e. stalk anchorage) that limits plant deflection in maize. These measurements demonstrate that the more brace root whorls that contact the soil, the greater their overall contribution to anchorage, but that the contributions of each whorl to anchorage were not equal. Previous studies demonstrated that the number of nodes that produce brace roots is correlated with flowering time in maize. To determine if flowering time selection alters the brace root contribution to anchorage, a subset of the Hallauer's Tusón tropical population was analyzed. Despite significant variation in flowering time and anchorage, selection neither altered the number of brace root whorls in the soil nor the overall contribution of brace roots to anchorage. These results demonstrate that brace roots provide a rigid base in maize and that the contribution of brace roots to anchorage was not linearly related to flowering time.

Keywords: anchorage; biomechanics; brace roots; maize; root lodging.

FIGURE 1

Plant Stiffness Changes with Time of Day and Developmental Stage. CML258 plants were tested at tasseling VT (103 days) and dent R5 (128 days) stages from two replicates (Plot A and B). At VT, within replicate A, each plant was tested three times as indicated by the colored dots (blue—first, red—second, and black—third tests). The Force‐Deformation slope is reduced throughout the day (p = 2.36E⁻⁰⁶). The effect of replication and repeat testing was not significant (p = .716 and .972, respectively). The same plants were tested at R5 and show higher Force‐Deflection slope compared to any time point at VT. Plant 11 and 12 in replicate A at R5 were not used due to weed control damage

FIGURE 2

Methods for Testing the Contribution of Brace Roots to Anchorage. The measure of plant biomechanics with all brace roots intact is labeled "A." For brace root removal experiments, the excision of sequential whorls starting at the top are labeled alphabetically. Here is an example of a plant with three whorls of brace roots, with two that enter the soil. The uppermost whorl of brace roots that enter the soil is excised and the measurement is labeled "B." The next whorl of brace roots that enter the soil is then excised and the measurement is labeled "C"

FIGURE 3

Brace Roots Contribute to Anchorage. (A) The number of brace root whorls in the soil was evaluated for CML258 plants in two replicates and there was a significant effect of replicate (p = .0308). (B)The Force‐Deformation slope was measured from CML258 plants upon sequential removal of brace root whorls. Test was performed with all whorls intact A, excision of the highest whorl B, the next highest whorl C, and so on. Plants with two whorls are represented by black and with three whorls by blue dots. There is an overall reduction in the slope upon removal of brace root whorls (p = 5.88E⁻¹¹). (C) This observation is extended to the individual plant, where the subsequent removal of brace root whorls results in a reduction of the Force‐Deflection slope. (D) Using the paired data measurements, the contribution of brace roots to anchorage is represented in two ways: 1) a ratio of the Force‐Deflection slope with no brace roots to the Force‐Deflection slope with all brace roots and 2) the difference in the Force‐Deflection slope with all brace roots and the Force‐Deflection slope with no brace roots. FDwo = Force‐Deflection slope without brace roots. FDw = Force‐Deflection slope with brace roots

FIGURE 4

Brace Root Whorls Have Differential Contribution to Anchorage. (a) The reduction of Force‐Deflection slope after removal of all brace root whorls is significantly correlated with the number of whorls in the soil (r = −.50, p = .02). (b) Each whorl of brace roots in the soil has a differential contribution to the Force‐Deflection slope. The effect of whorl is significant by ANOVA, p = 2.4431E⁻⁰⁶. The brace root whorl closest to the soil contributes the most and the additional whorls above contribute successively less. (c) The contribution of the lowest whorl is highly correlated with the overall contribution (r = .85, p = 1.302E⁻⁰⁶). Plants with two whorls are represented by black and with three whorls by blue dots. Lines represent a generalized linear model (glm) fit and shading indicates a 95% confidence interval. A lower ratio indicates a higher contribution of brace root whorls. WR = whorl

FIGURE 5

Selection for Early Flowering Alters the Force‐Deflection Slope, but not the Number of Brace Roots in the Soil or the Contribution of Brace Roots to Anchorage. (a) There is no effect of flowering time selection on the number of brace root whorls in the soil. (b) The contribution of brace roots to the Force‐Deflection slope is not affected through selection. (c) Selection for early flowering in the Tusón population has a significant effect on the Force‐Deflection slope (p = 7.44E⁻⁰⁶). WR = whorl