Water in aerenchyma spaces in roots. A fast diffusion path for solutes

During a study of the diffusivity of sulphorhodamine G in the cortical apoplast of maize roots widely discrepant rates were found between different samples. In roots which had developed large aerenchyma spaces, the diffusion in some regions was very fast, indistinguishable from the rate in water. In other regions the rate was as much as 100 times slower. Examination of frozen intact roots with the cryo-scanning electron microscope showed the presence of liquid filling some of the aerenchyma spaces, while other spaces of the same root contained air. X-ray microanalysis of the liquid (for oxygen) showed that the liquid was water with few detectable ions. Similar liquid was present in small intercellular spaces within the spoke-like radial files of cells between the large spaces, or between remnants of collapsed cell walls at the edges of the large spaces. It is proposed that regions of roots with high diffusivity are those in which some of the aerenchyma spaces are filled with water. In seeking the origin of this liquid, the progress of aerenchyma formation could be followed in the frozen tissues. The first change observed in a group of contiguous cells was a loss of vacuolar solutes and of cell turgor. Next the walls broke apart and collapsed back onto the surrounding turgid cells leaving a volume of ion-poor liquid. The liquid was probably not that found in some aerenchyma spaces of the mature roots, because the final stage of space formation was a loss of the liquid, leaving an air filled cavity surrounded by a composite lining formed from the collapsed walls of the broken cells. It is likely that the liquid in the spaces of mature aerenchyma is exuded from the remaining living cortical cells at times when the root turgor is high. This would be consistent with several recent studies which have shown periodic exudation of water from the surface of turgid roots. The spasmodic occurrence of root cortex tissue with enhanced diffusivity would have important implications for the transport of nutrient ions across the root.Abbreviations CSEM cryo-scanning electron microscope - EDX energy dispersive X-ray microanalysis - SR-G sulphorhodamine G