Radial force development during root growth measured by photoelasticity

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.