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

Vegetative barley plants, grown with their root systems splitbetween two containers, were supplied with ¹¹CO₂ to the secondleaf. Both halves of the split root were monitored separatelywith scintillation detectors and a pre-treatment value for partitioningbetween them established. Mannitol or sorbitol was added tothe medium around one root-half to lower the solute potentialto –0?4 MPa, and partitioning of C-11 between the tworoot halves was followed continuously for 3 h. Alternatively,¹⁴CO₂ was supplied at, or 3 h or 24 h after, treatment withosmotica, and roots were harvested and counted 3 h later. Osmoticarapidly increased the import of isotope into the treated root-half,but this effect and a respiratory increase induced by the osmoticawas transient. Within 3 h of applying the osmoticum, partitioningbetween the root halves was little affected. Slow addition ofosmotica (0?12 MPa h^–1) did not affect partitioning betweenroot halves. We conclude that the solute potential of the apoplastcan have a major if transient effect on carbon partitioningbut that this is unlikely to be important in vivo as osmoregulationby the root can negate its effects. Key words: Carbon partitioning, barley, osmotica, split-root system