Carbon Acquisition and Metabolism in a Root Hemiparasitic Angiosperm, Thesium humile (Santalaceae) Growing on Wheat (Triticum vulgare)

Thesium humile Vahl (Santalaceae) is a root-hemiparasite which causes economic losses in cereal crops in the Mediterranean countries. O2 exchange measurements showed that photosynthesis does occur in Thesium. Experiments using 14CO2 gave additional evidence of the photosynthetic capacity of T. humile at both preparasitic and parasitic stages; roots of T. humile, unlike those of non-parasitic plants, exhibited a low sink strength for photosynthates. The end product of photosynthesis in T. humile is mannitol which is also the main carbohydrate translocated in the phloem. Nevertheless, feeding host leaf with [3H]glucose clearly demonstrated that the parasite, in spite of its photosynthetic capacity, remained dependent upon its host for an additional supply of reduced carbon. Moreover, in isolated T. humile shoots supplied with [14C]sucrose via the transpiration stream, the absorbed sucrose was rapidly converted into mannitol. Thus the parasite would be able to convert sucrose derived from the host plant into mannitol. The present work gives clear evidence that mannitol can be regarded as the major soluble carbohydrate in the carbon metabolism and in the translocation of reduced carbon in T. humile. The mannitol synthesis pathway might be a metabolic target for the chemical control of this parasite in cereals, in which this polyol has never been detected.