Analysis and Modeling of the Variations of Root Branching Density Within Individual Plants and Among Species.

Loïc Pagès

doi:10.3389/fpls.2019.01020

Abstract

Branching density (or the reciprocal: inter-branch distance) is an important trait which contributes to defining the number of roots in individual plants. The environmental and local variations in inter-branch distance have often been stressed, and simulations models have been put forward to take them into account within the dynamics of root system architecture (RSA). However, little is known about the interspecific and intra-plant variations of inter-branch distance. In this paper, we present an analysis which draws on 40 samples of plants belonging to 36 species collected in homogeneous soils, to address how the variations in inter-branch distance are structured within individual plants, and how this structure varies from one species to another. Using measurements of inter-branch distance on various roots of the same species and our knowledge of the branching process, we defined a simple and generic model dedicated to the simulation of the observed variations. This model distinguishes between two sub-processes: i) the longitudinal location of potential branching sites and ii) the effective emergence of lateral roots at these sites. Thus, it represents the variations in distance between the potential sites (with two parameters), and the probability of emergence of a lateral root at each site (one parameter). We show the ability of this model to account for the main variations in inter-branch distances with a limited number of parameters, and we estimated them for the different species. These parameters can be considered as promising traits to characterize—in a comprehensive and simple way—the genetic and environmental variations in the whole branching process at plant level. Based on the results, we make recommendations for carrying out comparable measurements of the branching density in developed plants. Moreover, we suggest the integration of this new model as a module in future RSA simulators, to improve their capacity to account for this important and highly variable characteristic of plant species.

Highlights

The linear branching density for a given root can be defined as the number of lateral branches per length unit along this root
A given initial step may define the potential location for branching, whereas a further step may confirm the branching in that particular place. Another complementary approach to the topic is to take a macroscopic view of the emergent result of these sequences leading to patterns of inter-branch distances, and to quantify expression variations within the whole root system, taking into account several roots from the same plants and looking at variations among species. We present such an analysis based on 40 different samples (36 species) observed in homogeneous soils to investigate how inter-branch distance varies within individual plants and from one species to another
Pot-grown plants were cultivated in greenhouses using long PVC tubes

Summary

Introduction

The linear branching density for a given root can be defined as the number of lateral branches per length unit along this root It can be characterized by the reciprocal variable, the inter-branch distance (IBD), and is often measured on the branching parts of roots, i.e., excluding the distal end where laterals have not emerged yet (Dubrovsky and Forde, 2012). It is a very important trait at the root system level, because it largely contributes to defining the total number of roots in the whole plant. Physiologists usually characterize branching density based on the young radicle (Dubrovsky et al, 2006; Lavenus et al, 2013), whereas ecologists propose global average evaluations on mature plants (Kong et al, 2014). Wu et al (2016) observed intraplant structured variations, with a dependence of branching density on the diameter of the parent root in field-grown maize plants

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