Reserve carbohydrate in maize stem : [C]glucose and [C]sucrose uptake characteristics

Maize (Zea mays L.) stem is thought to function alternately as a net importing and net exporting organ during ontogeny, depending on whole plant photosynthetic source and sink status. The [(14)C]sucrose and [(14)C]glucose uptake capacity of stem tissues was investigated to increase our understanding of the transport factors which may influence sink status.Uptake from solutions containing up to 200 millimolar radiolabeled sugar showed that d-glucose uptake consisted of saturable and nonsaturable components, while sucrose uptake was primarily nonsaturable during the kernel-fill stages. l-Glucose uptake lacked the saturable component but both d and l isomers apparently had similar slopes for the nonsaturable component. Uptake was sensitive to inhibitors and temperature, and was increased slightly by lowered pH.The seasonal chronology for saturable uptake by isolated vascular bundles and associated pith revealed highest rates between anthesis and early kernel growth, corresponding with the stage when net sugar accumulation rates were highest. For isolated pith, the rates increased at the final stages of plant development.The rate of labeled l-glucose movement from vascular bundles into pith in isolated stem segments was greater at the silking stage than at later developmental stages, suggesting a lower resistance to diffusive transport from vascular bundles into pith at silking. Studies with stem plus ear explants showed that the capability for sugar transport from pith to vascular bundles and for phloem loading and export from the stem region was present throughout the developmental period from early kernel fill (milk) to late kernel fill (dent).