# Evaluation of root lodging resistance throughout the growth phase at the plant level in maize

### Experimental design and culture management

Field experiments were conducted at Chengyang Agricultural Experiment Station, Qingdao, China (36°18′ 11″/N, 120°21′ 13″/E) in 2019 and 2020. The soil type in the field was a brown loam that contained 22.76 g kg^{−1} organic matter, 82.39 mg kg^{−1} Alkaline hydrolyzable N, 25.10 mg kg^{−1} Olsen-P and 94.89 mg kg^{−1} K exchangeable. The maize cultivars tested were Jinhai5 with high lodging resistance and Xundan20 with low lodging resistance, which were repeated four times in plots arranged in random blocks. Planting density was 7.5 plants/m^{2} with a line spacing of 60 cm. the plot consisted of 8 rows with a length of 15 m. Two to three seeds per hole were sown manually at 5 cm on April 20, 2019 and April 24, 2020, and the seedlings were thinned to the target planting density at V2, and harvested on September 10 and September 14, respectively. Fertilization and irrigation management followed local maize production practices.

### Sampling and measurement

Plant samples were taken at V8, V12, R1, R2 and R6. Ten typical plants of each cultivar tested were selected to be subjected to aerial mechanical and morphological measurements at each sampling. The other three maize plants were used to measure root morphological traits. Xundan20 was severely damaged due to the storm at the end of maize growth in 2020, resulting in missing data on physiological maturity.

### Determination of vertical leaf area distribution

The leaf area of the leaves deployed each was calculated by the method of coefficients: area of a single leaf = length * width * 0.75. The leaf area of undeveloped leaves was estimated by the leaf weight method. Leaf area per plant was the sum of all individual green leaf areas. Leaf height is the height between the ground and the crown position of the corn leaves.

### Determination of maximum resistance to root pullout

Sample plants were surrounded by watertight steel devices inserted into the subsoil and watered to saturation soil moisture one day prior to mechanical testing. When measuring, due to the limited space, all the leaves of the sample plants are removed to improve the accuracy of the measurement. The defoliated stems were immobilized by a pair of longitudinal steel clamps to prevent the stems from bending (Fig. 7). After the digital pole dynamometer^{18} with a slider 1.5 m long and a main unit was tied to the stems at a height of 80 cm from the ground, the hand operator pulled at a slow and even speed until the roots were pulled out . Records of load force, declination angle and sensor position were automatically stored in the main unit during this operation. The maximum value of the forces, extracted from the recordings, was taken as the maximum lateral tensile strength of the root.

### Anti-root dwelling index

Based on the method of Cui et al.^{6}the force value comparison is replaced by the moment value comparison to calculate the root anti-lodging index:

$${text{AL}}_{root} = M_{root} / , M_{wind} = F_{root} / , F_{wind}$$

(1)

where *M *_{root} is the root failure moment, *M *_{wind} is the moment resulting from the wind. The root-lodging index indicates the ability of plants to resist root-lodging. The greater its value, the stronger the resistance and vice versa.

$${text{M}}_{root} = F, *d$$

(2)

where *F* is the maximum lateral tensile strength of the root, *D* is the moment arm, i.e. the length of the force arm. As a component of the root anti-lodging index, the root breaking moment represents the ability of the root system to resist lateral traction. The greater its value, the better the resistance and vice versa.

With the base of the rod as a fulcrum,

$${text{M}}_{vent} = sum 0. {5}CA_{i} rho V^{2} h_{i}$$

(3)

where *VS* is the coefficient of air resistance, *ρ* is the density of air,*V* is the wind speed, *A*_{I} is the area of a single leaf, *h*_{I} is the height of the sheet, ∑ represents the sum over all the sheets. *VS* the value is set to 0.2^{19}. When encountering a grade 6 or higher wind speed, corn is more prone to lodging. Unless explicitly stated, the following analysis was limited to the upper wind speed for grade 6 wind^{20}.

### Morphological traits of roots

The number and length of all primary nodal roots were measured. Balls of root soil from each of the two or three plants tested were obtained after a lateral root pulling test. Images of the three frontal faces, 120 degrees apart, of the root clods were taken using a digital camera. The ball volumes were then evaluated considering them to be rotationally symmetric. Average volumes were used for further analysis.

### Single root tensile strength

The roots after counting the number of nodal roots were used to measure the tensile strength of a single root. First, clean the dust from the roots. Then, the root diameters were determined with a vernier caliper. The tensile strength of a single root was measured by an HF-500 digital push-pull device. The upper and lower ends of the root were fixed, then one end moved slowly and evenly, the other end was still until the root broke. The maximum tensile force displayed by the instrument was taken as the single root tensile strength.

### statistical analyzes

Based on analysis of variance, Tukey’s method was used to compare differences between means. The logarithmic transformation of the variables was performed to improve the homogeneity of the error variance where applicable.

The substantial effect or influence of various factors on the response variable can be expressed by the factor effect size, which can be calculated as part of the analysis of variance. The effect size is the proportion of the effect of a certain factor in the total effect, which is a dimensionless number^{21,22.23}.

The formula for calculating the effect size of the factors is as follows:

$$omega^{2} = frac{{df_{effect} times left( {MS_{effect} – MS_{error} } right)}}{{SS_{total} + MS_{error} } }$$

(4)

where *df* is the degree of freedom, MS represents the mean square.

Two conceptual models were used to address effect size. A model was made up of components, i.e. taking the logarithm of both sides of the equation. (1):

$${text{LOG}}left( {{text{AL}}_{{{text{root}}}} } right) , = {text{LOG}}left( { {text{M}}_{{{text{root}}}} } right) , + {text{LOG}}left( {{text{M}}_{{{text {wind}}}} } right)$$

(5)

where LOG denotes a logarithmic transformation.

The other was the factorial model, i.e.

$${text{factors affecting AL}}_{{{text{root}}}} = {text{ wind grade }} + {text{ cultivar }} + {text{ growth stage}} $$

(6)

### Experimental research and field studies on plants, including the collection of plant material

The authors declare that the cultivation of plants and the performance of studies at the Chengyang Agricultural Experiment Station complies with all relevant institutional, national and international guidelines and treaties.

### Declaration of authorizations and/or licenses for the collection of plant specimens or seeds

The authors state that the seed specimens used in this study are publicly available seed and we have received explicit written permission to use them for this research.