How much is too much?—Influence of X-ray dose on root growth of faba bean (<i>Vicia faba</i>) and barley (<i>Hordeum vulgare</i>)

<div><p>X-ray CT is a powerful technology to study root growth in soil <i>in-situ</i>. Root systems can be studied in its true 3D geometry over time. Hence, the same plant can be scanned multiple times during development. A downside is the potential of X-rays to interfere with biological processes and therefore plant growth. The aim of this study is to evaluate the influence of cumulative X-ray dose on <i>Vicia faba</i> and <i>Hordeum vulgare</i> during a growth period of 17 days. One control treatment without X-ray scanning was compared to two treatments being scanned every two and four days, respectively. Scanned treatments received a maximum cumulative dose of less than 8 Gy. Plant species differed in their susceptibility to X-ray dose. For <i>Vicia faba</i>, mean total root length was reduced significantly. Leave growth was reduced as well. Number and length of second order laterals was reduced significantly, as well as length of first order laterals. <i>Hordeum vulgare</i> showed no negative impact of X-ray dose on any of the root parameters. Large differences between the two species investigated were detected in respect to susceptibility to X-ray dose. Results indicate that for X-ray CT studies involving temporal resolution a control treatment without scanning is required.</p></div

Mean root length in functional diameter classes of <i>Vicia faba</i> measured with WinRHIZO at the end of the experiment (17 DAP); FS = frequent scanning, MS = moderate scanning, C = control; smallest diameter class < 0.10 mm is discarded due to distorted values by influence of root hairs; standard errors are given as error bars.

<p>Mean root length in functional diameter classes of <i>Vicia faba</i> measured with WinRHIZO at the end of the experiment (17 DAP); FS = frequent scanning, MS = moderate scanning, C = control; smallest diameter class < 0.10 mm is discarded due to distorted values by influence of root hairs; standard errors are given as error bars.</p

Root length (bars) and number of first order laterals (circles) of <i>Vicia faba</i> over time measured by X-ray CT; FS = frequent scanning; MS = moderate scanning; C = no scanning (control); data is only shown for time steps when both treatments were scanned.

<p>Small letters refer to significant differences in root length and capital letters in number of laterals. Standard errors are given as error bars.</p

Time series of root system development of <i>Vicia faba</i>, acquired by X-ray CT.

<p>Representative 2D projections for both scanned treatments: a) frequent scanning (FS) and b) moderate scanning (MS). Root age is colour coded for 4 (black), 8 (green), 12 (orange) and 16 (purple) days after planting (DAP). Changes in position are also recorded; this is the reason for the green shade at the seed in b). Secondary thickening can also be seen by the purple shade around the upper part of both tap roots. Illustrating videos for those two samples are available in the supporting information (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193669#pone.0193669.s007" target="_blank">S1</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193669#pone.0193669.s008" target="_blank">S2</a> Videos).</p

Mean leaf area and shoot fresh weight of <i>Hordeum vulgare</i> for frequent scanning (FS), moderate scanning (MS) and no scanning control treatment (C) at the end of the experiment (17 DAP); standard errors are given as error bars.

<p>Mean leaf area and shoot fresh weight of <i>Hordeum vulgare</i> for frequent scanning (FS), moderate scanning (MS) and no scanning control treatment (C) at the end of the experiment (17 DAP); standard errors are given as error bars.</p

Mean leaf area and fresh weight of <i>Vicia faba</i> for frequent scanning (FS), moderate scanning (MS) and control treatment (C) at the end of the experiment (17 DAP); standard errors are given as error bars.

<p>Mean leaf area and fresh weight of <i>Vicia faba</i> for frequent scanning (FS), moderate scanning (MS) and control treatment (C) at the end of the experiment (17 DAP); standard errors are given as error bars.</p

Mean total root length of <i>Vicia faba</i> at the end of the experiment (17 DAP) measured with WinRHIZO; FS = frequent scanning; MS = moderate scanning; C = control (no scanning); standard errors are given as error bars.

<p>Mean total root length of <i>Vicia faba</i> at the end of the experiment (17 DAP) measured with WinRHIZO; FS = frequent scanning; MS = moderate scanning; C = control (no scanning); standard errors are given as error bars.</p

Length and number of second order laterals of <i>Vicia faba</i> measured with X-ray CT at 16 DAP; standard errors are given as error bars.

<p>Length and number of second order laterals of <i>Vicia faba</i> measured with X-ray CT at 16 DAP; standard errors are given as error bars.</p

Quantification of Root Growth Patterns From the Soil Perspective via Root Distance Models

The rhizosphere, the fraction of soil altered by plant roots, is a dynamic domain that rapidly changes during plant growth. Traditional approaches to quantify root growth patterns are very limited in estimating this transient extent of the rhizosphere. In this paper we advocate the analysis of root growth patterns from the soil perspective. This change of perspective addresses more directly how certain root system architectures facilitate the exploration of soil. For the first time, we propose a parsimonious root distance model with only four parameters which is able to describe root growth patterns throughout all stages in the first 3 weeks of growth of Vicia faba measured with X-ray computed tomography. From these models, which are fitted to the frequency distribution of root distances in soil, it is possible to estimate the rhizosphere volume, i.e., the volume fraction of soil explored by roots, and adapt it to specific interaction distances for water uptake, rhizodeposition, etc. Through 3D time-lapse imaging and image registration it is possible to estimate root age dependent rhizosphere volumes, i.e., volumes specific for certain root age classes. These root distance models are a useful abstraction of complex root growth patterns that provide complementary information on root system architecture unaddressed by traditional root system analysis, which is helpful to constrain dynamic root growth models to achieve more realistic results