Exodermis

[1]  The exodermis is a membrane of variable permeability responsible for the radial flow of water, ions, and nutrients.

[2] This layer is composed of a protein called suberin and is also hydrophobic meaning it also contributes to the ability of the exodermis to control water input.

Stage 3 includes the addition of cellulose and lignin with occasional deposition of suberin into the cell walls to strengthen them.

[1] Since suberin and the Casparian band are responsible for inhibiting nutrient and fluid uptake, it forces it across the exodermis and endodermis and into the root cortex.

In microenvironments, where macronutrient levels are low (such as phosphorus, nitrogen and potassium) development in the exodermis, Casparian strips and the suberin lamellae.

[9] In environments with low water supply, such as in drought or desert conditions, the deposit of tertiary layers in the plant's exodermis can be found up far higher in the apex of the root system.

It will also cause an increase in the selectivity of the ions that are able to cross the barrier and be absorbed, slowly becoming more susceptible to large osmotic changes.

The apoplastic nature of the exodermis means that selectivity should decrease with age not increase, however evidence and conflicting results between studies suggest otherwise and warrants further investigation.

[21] Lignin is a biopolymer that has been found to develop naturally in the Casparian strip to strengthen and thicken the cell wall of the plants.

[10][22] Xanthones are a type of specialized bioactive constituents that are found to accumulate in the root system Hypericum perforatum.

Xanthones are abundant in angiosperms with cDNA evidence suggesting that they are also present in Lusiaceae, Gentianaceae and Hypericaceae species.

Similar to other parts of the root system, the concentration of these molecules is dependent on the genetic variation and the environmental factors.

In the root system with bacterial infections and invasion from rhizogenes, the concentration of BPS increases to fight off the pathogens.