Sensitivity and strength of maize roots facing different physical conditions of the growth medium

The morphology of a plant's root is strongly affected by the compaction of the growth medium, the size of its particles, or the presence of non-movable obstacles. However, little is known about the effect of these characteristics on root anatomy and mechanical properties of the root tissues. Anatomical features of maize roots grown in media that varied in density and/or structure (soil, glass beads, vermiculite) were analyzed on cross-sections through the elongation and maturation zones of the roots of 14-day-old seedlings. The sections were stained for lignin and suberin to recognize the developmental stages of exodermis and endodermis. Cortex thickness, number of cortical cell layers, and diameter of the vascular cylinder (stele) were measured in both zones. The Young's modulus of the roots was determined using mechanical tensile tests. Assuming that the root can be considered a composite material, a model was used that allowed, for the first time, to estimate the mechanical properties of the stele and cortex. While the cell arrangement of roots grown in a medium with high density and fine movable particles (soil) was regular, roots grown in a medium with low density and light particles (vermiculite) and a medium with high density and large unmovable particles (glass beads) showed early damage of the rhizodermis and impaired cell arrangement in the cortex and vascular cylinder. In these roots, the exodermis and endodermis matured closer to the root tip than in roots from the soil. The vermiculite roots were the most outliers regarding morphometric parameters and mechanical properties. The Young's modulus of the stele was many times greater than the Young's modulus of the cortex in the roots of all variants. Of the media used in the experiment, the soil appears to be most favorable for the maize root growth and development.