Gas and Liquids in Intercellular Spaces of Maize Roots

Oils are spontaneously absorbed by gas-filled intercellular spaces (IS) in maize root cortex. The network of these spaces in living root sections was imaged by confocal laser scanning microscopy using a fluorescent solution of Nile red in oil. The gas volume fraction (GVF) of root segments was quantified by the increase in weight (differentiated zones) or tissue density (2–3 mm root tips) due to complete vacuum infiltration. Cooling to 6 °C or inhibition of oxidative phosphorylation diminished the GVF of root tips but did not significantly affect the GVF of differentiated root zones. The threshold pressure difference for measurable infiltration of isolated root segments is lower (10 to 15  kPa) than the threshold for infiltration of comparable zones of attached roots or of detached roots with the cut surface sealed (>60 kPa). In the absence of an open cut, pressure-driven infiltration of the root cortex is accelerated by microscopic fissures within the epidermal/hypodermal barrier. The GVF of the root cortex was reduced after transferring roots from sugar solutions (0.1 to 0.3m ) to water. This points to efficient water transport from the medium to sugar-containing cortical cell walls through epidermal and hypodermal protoplasts. When 2-cm-long primary roots were vacuum infiltrated in situ and then allowed to grow on aerated mineral medium for a further 5 d, cortical IS of the originally infiltrated root bases remained filled with liquid but the subsequently grown apical root zones had a normal GVF.