The exodermis: a variable apoplastic barrier

The exodermis (hypodermis with Casparian bands) of plant roots represents a barrier of variable resistance to the radial flow of both water and solute s and may contribute substantially to the overall resistance. The variabili ty is a result largely of changes in structure and anatomy of developing ro ots. The extent and rate at which apoplastic exodermal barriers (Casparian bands and suberin lamellae) are laid down in radial transverse and tangenti al walls depends on the response to conditions in a given habitat such as d rought, anoxia, salinity, heavy metal or nutrient stresses. As Casparian ba nds and suberin lamellae form in the exodermis, the permeability to water a nd solutes is differentially reduced. Apoplastic barriers do not function i n an all-or-none fashion. Rather, they exhibit a selectivity pattern which is useful for the plant and provides an adaptive mechanism under given circ umstances. This is demonstrated for the apoplastic passage of water which a ppears to have an unusually high mobility, ions, the apoplastic tracer PTS, and the stress hormone ABA. Results of permeation properties of apoplastic barriers are related to their chemical composition. Depending on the growt h regime (e.g. stresses applied) barriers contain aliphatic and aromatic su berin and lignin in different amounts and proportion. It is concluded that, by regulating the extent of apoplastic barriers and their chemical composi tion, plants can effectively regulate the uptake or loss of water and solut es. Compared with the uptake by root membranes (symplastic and transcellula r pathways), which is under metabolic control, this appears to be an additi onal or compensatory strategy of plants to acquire water and solutes.