Chemical composition of apoplastic transport barriers in relation to radial hydraulic conductivity of corn roots (Zea mays L.)

The hydraulic conductivity of roots (Lp(r)) of 6- to 8-d-old maize seedling s has been related to the chemical composition of apoplastic transport barr iers in the endodermis and hypodermis (exodermis), and to the hydraulic con ductivity of root cortical cells. Roots were cultivated in two different wa ys. When grown in aeroponic culture, they developed an exodermis (Casparian band in the hypodermal layer), which was missing in roots from hydroponics . The development of Casparian bands and suberin lamellae was observed by s taining with berberin-aniline-blue and Sudan-III. The compositions of suber in and lignin were analyzed quantitatively and qualitatively after depolyme rization (BF3/methanol-transesterification. thioacidolysis) using gas chrom atography/mass spectrometry. Root Lp(r) was measured using the root pressur e probe. and the hydraulic conductivity of cortical cells (Lp) using the ce ll pressure probe. Roots from the two cultivation methods differed signific antly in (i) the Lp(r) evaluated from hydrostatic relaxations (factor of 1. 5), and (ii) the amounts of lignin and aliphatic suberin in the hypodermal layer of the apical root zone. Aliphatic suberin is thought to be the major reason for the hydrophobic properties of apoplastic barriers and for their relatively low permeability to water. No differences were found in the amo unts of suberin in the hypodermal layers of basal root zones and in the end odermal laver. In order to verify that changes in root Lp(r) were not cause d by changes in hydraulic conductivity at the membrane level, cell Lp was m easured as well. No differences were found in the Lp values of cells from r oots cultivated by the two different methods. It was concluded that changes in the hydraulic conductivity of the apoplastic rather than of the cell-to -cell path were causing the observed changes in root Lp(r).