The Exodermal Casparian Band of Onion Roots Blocks the Apoplastic Movement of Sulphate Ions

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Abstract
Peterson, C. A. 1987. The exodermal Casparian band of onion roots blocks the apoplastic movement of sulphate ions.—J. exp. Bot. 38: 2068–2081. It has previously been established that the exodermal Casparian bands in the roots of onion and many other angiosperm species are impermeable to fluorescent, apoplastic dyes. The permeability of the onion root exodermal Casparian band to ions was tested by measuring sulphate free space in the walls using compartmental analysis by elution. Results of this analysis revealed four compartments for sulphate. The first was a surface film, between 15 and 35 μm thick, which had a half-time of elution too short to be measured (i.e. less than 15 s). The second was a wall compartment with an average half-time of elution of 1·0 min. The third and fourth compartments were the cytoplasm and vacuole which had half-times of elution of 12·4 min and 4·9 d, respectively. Experiments in which the temperature and concentration of sulphate in the treatment solution were varied indicated that the compartment identified as the wall was a free space. Segments were excised from roots in regions with a mature exodermal Casparian band. The sulphate free space in the walls of ‘whole’ segments (i.e. segments with their cut ends sealed) was compared with that of ‘bisected’ segments (i.e. segments which had been bisected and their steles removed). The free space in the walls of the whole segments was consistently smaller than the comparable free space in the bisected segments; the ratio whole/bisected ranged from 0·16 to 0·26. This ratio compared favourably with predicted values obtained by taking the ratio of the wall volumes, i.e. walls external to the exodermal Casparian band/walls internal and external to the exodermal Casparian band. The ratios predicted from wall measurements varied from 0·25 to 0·27. Furthermore, the volume of the sulphate free space in the whole segments was less than the volume of the walls external to the Casparian band. The results of the present study show that the exodermal Casparian band of onion is impermeable to sulphate ions, which have a hydrated ionic radius of 0·378 nm.