STRUCTURE AND FUNCTION OF TRANSFER CELLS IN THE SPOROPHYTE HAUSTORIUM OF FUNARIA-HYGROMETRICA HEDW .2. KINETICS OF UPTAKE OF LABELED SUGARS AND LOCALIZATION OF ABSORBED PRODUCTS BY FREEZE-SUBSTITUTION AND AUTORADIOGRAPHY

Excised sporophytes of the moss F. hygrometrica Hedw. absorb externally applied sugar through their basal haustrorium. Influx of [3H]sucrose is inhibited by metabolic uncouplers, darkness and by the photosynthesis inhibitor DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea]. The kinetics of uptake of glucose and sucrose suggest a biphasic mechanism of absorption. Uptake of 3-O-methyl [3H]glucose shows no saturation characteristics and a passive mechanism is indicated. Externally applied glucose is rapidly converted to sucrose. Good retention of products of short-term absorption and metabolism of [3H]glucose was achieved by freeze-substitution. Autoradiography showed dense and uniform labeling of the transfer cells of the haustorium. Vmax values for uptake of sucrose and glucose, expressed in terms of the weight and external surface area of haustorium, are considerably greater than typical values from other plant systems. If the surface area amplification that is brought about by the development of wall ingrowths in the transfer cells is taken into account, fluxes per unit area of plasma membrane are reduced into the range of typical values. The hypothesis that the surface area amplification that characterizes transfer cells is related functionally to processes of solute transport is supported.