Sucrose and Hexose Release by Excised Stem Segments ofVicia fabaL.: The Sucrose-Specific Stimulating Influence ofCuscutaon Sugar Release and the Activity of Acid Invertase

Abstract

By washing out solutes in 0.5 mM CaSO⁴at 25 °C during a period of 5–6 h, the release of sugars by excised stem segments of Vicia faba L. was measured. The stem parasite Cuscuta europaea strongly stimulated the release of sucrose into the efflux medium; this effect was most marked during the last hours of each experiment but this stimulating effect of the parasite could not be detected for glucose and fructose. The fact that parasitized stem segments released higher than normal hexose amounts during the last hours of several experiments, could be explained as the result of extracellular hydrolysis of sucrose by free space invertase. A high free space acid invertase activity was present in young stem segments of Vicia faba and in tissues of Cuscuta. The stimulating influence of Cuscuta on sugar release by cells of stem segments appears sucrose-specific, supporting the idea that the stimulating influence of Cuscuta on sugar release is restricted to the sieve-tube system. When metabolic inhibitors were added to the washing solution or when segments were incubated at low temperature, no clear effect of the parasite could be observed and for all segments (parasitized and non-parasitized) a strongly enhanced release of sucrose into the efflux medium was found during the last hours of an experiment. These data support the idea that an intensive resorption of sucrose occurs within stem segments, after its release into the apoplast.