Quantification And Visualization Of Three-dimensional Root System Architecture Of Field-grown Maize

Root system architecture (RSA), the topology and geometry of root segments, plays a key role in supporting shoot growth and in plant water and nutrients uptake. RSA has large variation among species and gemontypes in the same species. Roots have strong plasticity which can changes their architecture with the variation of soil environment, In several instances, RSA determines the productivity (such as yield, water use and nutrient capture) as the overlap of QTLs for root traits and those for productivity. Hence, understanding the root architecture has significant meaning on advancing the cultivation and plant breeding processing. However, due to the opacity of soil, it is difficult to observing and measuring root system, and thus impeding our thorough understanding on root architecture, limiting the parameterization of root architectural model, and hindering its application on plant breeding.To address these, we developed a suit of method on the quantification of the root architecture of mature field-grown maize. For measuring axile root architecture, we used a three-dimensional(3D) digitizer to measure the3D trajectory of axile root in situ in the field, in parallel with an image processing method for measurement of diameter along the same roots. For measuring the topology and geometry of single nodal roots, we developed an integrated method includes a custom-made whole-root sampling system for extracting intact root systems of individual maize plants, a combination of proprietary software WinRHIZO Pro and a novel Excel VBA program used for collecting individual RSA information. The whole-root sampling system includes a sampling cylinder (50cm diameter and55cm deep), an electric hammer module (2.2m high,1.1m long,1m wide), a lifting module (1.2m wide,2m high) and root washing system. The whole-root sampling system was applied on measurement of root architecture of two maize cultivars (Zhangdan958and Xianyu335). To reconstructe the root architecture, we developed a viRoot software. The software was written using C++language and used Boost library, SQLite relational database and Visualisation Toolkit library. Visualisation was executed using ParaView software. viRoot was used for reconstruction of3D axile root architecture and2D nodal root architecture, and for matching lateral roots on axile roots to construct a whole3D root architecture.Our calculations provide evidence that neither unequal spacing nor unequal soil volumes play a role in determining root trajectory of field-grown maize of both cultivars. The two cultivars had different initial angles from the vertical and horizontal root spread, and presented slightly different patterns of root angle distribution. Root diameter decreased sharply along an axile root arising from the higher whorls. The reorientation of an axile root downwards was related to its angle and diameter. Second-and higher order laterals are found to contribute substantially to total lateral root number and length of a30cm long axile root of nodal root, up to85%in total number of lateral roots. The length of laterals of distinct orders varies significantly. A log-normal distribution is suit to describe the probability density of lateral root length for distinct order. Lateral root unit (LRU) indicates a first-order laterals and higher-order laterals derived from it. LRU rank was determined by highest branching order of LRU. Second-and higher rank LRU occupied most of total lateral root length and number, up to90%in total lateral root length. Abundant higher order laterals can arise from a single first-order lateral and one lateral root unit can occupy up to18%of total lateral root length of one nodal root. Most total lateral root length and number distributes in the proximal5cm long axile branching zone.Combination of3D digitizer and image processing is suitable for accurate determination of the3D architecture of axile roots of mature maize plants under field conditions. Integration of root sampling system and image processing is suitable for measurement of topological and geometrical structure of nodal roots of field-grown maize. Based on above two methods, a unique estimation on root architecture of maize could be obtained, which should serve as a valuable resource for the parameterisation of root architecture models and as a benchmark for evaluating other less demanding, field sampling methods and for testing whether the screened root architecture based on laboratory method also express the same under field conditions.