| Abstract: | Peterson, C. A. 1987. The exodermal Casparian band of onionroots blocks the apoplastic movement of sulphate ions.—J.exp. Bot. 38: 2068–2081. It has previously been established that the exodermal Casparianbands in the roots of onion and many other angiosperm speciesare impermeable to fluorescent, apoplastic dyes. The permeabilityof the onion root exodermal Casparian band to ions was testedby measuring sulphate free space in the walls using compartmentalanalysis by elution. Results of this analysis revealed fourcompartments for sulphate. The first was a surface film, between15 and 35 µm thick, which had a half-time of elution tooshort to be measured (i.e. less than 15 s). The second was awall compartment with an average half-time of elution of 1·0min. The third and fourth compartments were the cytoplasm andvacuole which had half-times of elution of 12·4 min and4·9 d, respectively. Experiments in which the temperatureand concentration of sulphate in the treatment solution werevaried indicated that the compartment identified as the wallwas a free space. Segments were excised from roots in regionswith a mature exodermal Casparian band. The sulphate free spacein the walls of ‘whole’ segments (i.e. segmentswith their cut ends sealed) was compared with that of ‘bisected’segments (i.e. segments which had been bisected and their stelesremoved). The free space in the walls of the whole segmentswas consistently smaller than the comparable free space in thebisected segments; the ratio whole/bisected ranged from 0·16to 0·26. This ratio compared favourably with predictedvalues obtained by taking the ratio of the wall volumes, i.e.walls external to the exodermal Casparian band/walls internaland external to the exodermal Casparian band. The ratios predictedfrom wall measurements varied from 0·25 to 0·27.Furthermore, the volume of the sulphate free space in the wholesegments was less than the volume of the walls external to theCasparian band. The results of the present study show that theexodermal Casparian band of onion is impermeable to sulphateions, which have a hydrated ionic radius of 0·378 nm. Key words: Allium cepa, exodermis, sulphate |
