The Physiology of Flowering Plants - KHAM PHA MOI

144 TRANSLOCATION OF ORGANIC COMPOUNDSFig: 5:4 Symplastic phloemloading by the polymer trapmechanism. Sucrose moves fromthe mesophyll cell to the companioncell (lacking wall ingrowths) throughplasmodesmata by diffusion andpossibly mass flow. In thecompanion cell the sucrose isconverted to oligosaccharide, whichdiffuses through branchedplasmodesmata into the sieve tube,but the plasmodesmata on themesophyll side do not permitback-flow.sieve elements must receive their supply via plasmodesmata fromthe companion cells since they do not have direct plasmodesmatalconnections to other cells. The companion cells lack transfer cellcharacteristics. The precise mode of movement of the solutesthrough the plasmodesmata is not certain. It has been noted thatsymplastic loaders translocate mainly galactose-containing oligosaccharidesrather than sucrose, predominantly raffinose and stachyose.Chemically, these are equivalent to sucrose with respectively one andtwo galactose residues added. It has therefore been suggested that, insymplastic loaders, sucrose diffusing into companion cells is thereconverted to oligosaccharides, and these are transferred to the sieveelement (Fig. 5.4). This would keep up a concentration gradient forsucrose from mesophyll to the companion cells. This ‘polymer trap’hypothesis implies that the oligosaccharides cannot back-flow intothe mesophyll through the plasmodesmata, which in the relevantcell walls must have rather low exclusion limits, permitting thepassage of sucrose (molecular mass 342 Da) but not of raffinose(504 Da) or stachyose (666 Da).In some symplastic loaders sucrose is the main carbohydratetranslocated. There are not many data on such species, but in twocases at least, willow (Salix babylonica) and poplar (Populus deltoides)there is evidence that the mesophyll cytosol has a high sucroseconcentration and there is a diffusion gradient from mesophyll cytosolto the SE–CC complex.Apoplastic loading is found in many herbaceous plants and infamilies generally regarded as more highly evolved: Asteraceae(= Compositae), Brassicaceae (= Cruciferae), Chenopodiaceae, Fabaceae(= Leguminosae) and Solanaceae. Apoplastic loading is considered moreefficient. It achieves higher loading rates, is relatively insensitive to lowtemperaturesandis found in temperate-zone species.Symplasticloadingis characteristic of the more primitive families, including tropical andsubtropical families with many tree species; it is cold-sensitive.