Radial force development during root growth measured by photoelasticity
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Background and aims
The radial growth of roots largely affects and reorganizes the porous or crack networks of soils and substrates. We studied the consequences of a radial steric constriction on the root growth and the feedback force developed by the root on the solid phase.
We developed an original method of photoelasticity to measure in situ root forces. By changing the gap width (0.5 to 2.3 mm) between two photoelastic disks we applied variable radial constrictions to root growth and simultaneously measured the corresponding radial forces. Changes in morphology and forces of primary roots of chick pea (Cicer arietinum L.) seedlings were recorded by time-lapse imaging every 24 min up to 5 days.
The probability of root entering the gap depended on the gap size but was also affected by circumnutation. Compared to non-constrained root controls, no significant morphological change (elongation, diameter) was measured outside the gap zone. Inside the gap zone, outer cortex cells were compressed, the central cylinder was unaffected. Radial forces were increasing with time but no force levelling was observed even after 5 days.
Radial constrictions applied to roots did not significantly reduce their growth. The radial force was related to the root strain in the gap.
KeywordsRadial force development Root growth Photoelasticity Mechanical stress Cicer arietinum L. (chick pea)
Non entering root
Gap size (mm)
Radial strain (no unit)
Radial stress (Pa)
Root vertical velocity (mm.h−1)
- dA, dB
Root diameters at 2.9 mm above (dA) and below (dB) the gap (mm)
is the averaged value of the measured variable x
We thank Laure-Emmanuelle Lecoq, Henri de Cagny, Harold Gouet, Simon Cabello-Aguilar and Lucie Guignier who worked on this subject during their undergraduate lab-training, as well as Laurent Quartier, who carefully designed the root chambers, Guillaume Clermont, who cut the photoelastic disks and Thierry Darnige, who automated one part of the experimental setup.
We are particularly grateful to Professor Tom Mullin (Manchester Centre for Non Linear Dynamics-UK) who gave us our first photoelastic disks as well as Professor Robert P. Behringer (Duke Physics, USA) who introduced us to the photoelastic technique and who constantly gives us relevant and kind advice. We also thank Anette Hosoi (MIT, USA) and her students Dawn Wendell and Katharin Luginbuhl for very fruitful interactions through our commun MIT-France Seed Fund project on “Flexible Objects in Granular Media”.
We also want to thank Professor Arezki Boudaoud (ENS Lyon, France) for our helpful discussions, Isabelle Bonnet (Institut Curie, France) and François Graner (Institut Curie and MSC, Paris 7, France) for the careful comments on the manuscript and the continuous and kind support, and Laurette S. Tuckerman (PMMH, ESPCI, France) for the English corrections. Thanks also to the friendly review of Eugénie Carnero-Diaz and the efficient help of Laurence Goury (IRD) in providing many articles and book chapter cited here.
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