Keywords: cell wall thickenings, endodermis, heavy metal hyperaccumulators, root anatomy, Thlaspi arvense, Thlaspi caerulescens, root structure, heavy metals, environmental pollution
Difference in the root structure of hyperaccumulator Thlaspi caerulescens and non-hyperaccumulator Thlaspi arvense
The primary root structure of Zn/Cd hyperaccumulator Thlaspi caerulescens was compared with the root anatomy of closely related non-accumulator Thlaspi arvense. The most striking structure formed close to the root tip in T. caerulescens, but not in T. arvense, is the peri-endodermal layer of cells with irregularly thickened inner tangential walls. This layer observed in some species of Brassicaceae by early anatomists and described as 'reseau sus-endodermique' is composed of secondary cell walls impregnated by lignin, and it forms a compact cylinder encircling the endodermis. The differences in the proportions of individual tissues ? rhizodermis, cortex, stele and endodermis ? of the two species are statistically insignificant. However, there are differences in the cell arrangement. The precise function of the lignified cell wall thickening of the peri-endodermal layer of T. caerulescens is unclear at present. Some possible roles are discussed and its potential relation with metal hyperaccumulation and/or tolerance should be investigated.