Carbon Partitioning in Split Root Systems of Barley: The Effect of Osmotica

Vegetative barley plants, grown with their root systems split between two containers, were supplied with 11CO2 to the second leaf. Both halves of the split root were monitored separately with scintillation detectors and a pre-treatment value for partitioning between them established. Mannitol or sorbitol was added to the medium around one root-half to lower the solute potential to −0·4 MPa, and partitioning of C-11 between the two root halves was followed continuously for ≈3 h. Alternatively, 14CO2 was supplied at, or 3 h or 24 h after, treatment with osmotica, and roots were harvested and counted 3 h later. Osmotica rapidly increased the import of isotope into the treated root-half, but this effect and a respiratory increase induced by the osmotica was transient. Within 3 h of applying the osmoticum, partitioning between the root halves was little affected. Slow addition of osmotica (0·12 MPa h−1) did not affect partitioning between root halves. We conclude that the solute potential of the apoplast can have a major if transient effect on carbon partitioning but that this is unlikely to be important in vivo as osmoregulation by the root can negate its effects.