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 o f Nile red in oil. The gas volume fraction (GVF) of root segments was quant ified by the increase in weight (differentiated zones) or tissue density (2 -3 mm root tips) due to complete vacuum infiltration. Cooling to 6 degrees C or inhibition of oxidative phosphorylation diminished the GVF of root tip s but did not significantly affect the GVF of differentiated root zones. Th e threshold pressure difference for measurable infiltration of isolated roo t segments is lower (10 to 15 kPa) than the threshold for infiltration of c omparable zones of attached roots or of detached roots with the cut surface sealed (> 60 kPa). In the absence of an open cut, pressure-driven infiltra tion of the root cortex is accelerated by microscopic fissures within the e pidermal/hypodermal barrier. The GVF of the root cortex was reduced after t ransferring roots from sugar solutions (0.1 to 0.3 M) 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 pri mary roots were vacuum infiltrated in situ and then allowed to grow on aera ted mineral medium for a further 5 d, cortical IS of the originally infiltr ated root bases remained filled with liquid but the subsequently grown apic al root zones had a normal GVF. (C) 1999 Annals of Botany Company.