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 (CO2)-C-11 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 almost-equal-to 3 h. Alternatively, (CO2)-C-14 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.