Evidence for Symplastic Involvement in the Radial Movement of Calcium in Onion Roots

The pathway of Ca^2+ movement from the soil solution into the stele of the root is not known with certainty despite a considerable body of literature on the subject. Does this ion cross an intact, mature exodermis and endodermis? If so, is its movement through these layers primarily apoplastic or symplastic? These questions were addressed using onion (Allium cepa) adventitious roots lacking laterals. Radioactive Ca^2+ applied to the root tip was not transported to the remainder of the plant, indicating that this ion cannot be supplied to the shoot through this region where the exodermis and endodermis are immature. A more mature zone, in which the endodermal Casparian band was present, delivered 2.67 nmol of Ca^2+ mm^-1 treated root length d–1 to the transpiration stream, demonstrating that the ion had moved through an intact endodermis. Farther from the root tip, a third zone in which Casparian bands were present in the exodermis as well as the endodermis delivered 0.87 nmol Ca^2+ mm^-1 root length d^-1 to the transpiration stream, proving that the ion had moved through an unbroken exodermis. Compartmental elution analyses indicated that Ca^2+ had not diffused through the Casparian bands of the exodermis, and inhibitor studies using La3+ and vanadate (VO43–) pointed to a major involvement of the symplast in the radial transport of Ca^2+ through the endodermis. It was concluded that in onion roots, the radial movement of Ca^2+ through the exodermis and endodermis is primarily symplastic.