The root-hemiparasite Thesium humile Vahl (Santalaceae) causes important damage to cereal crops in Mediterranean countries. It maintains lower osmotic potentials than its host and the main osmotically active solutes involved are identified and measured in host and hemiparasite under well-watered conditions or water stress.
Mineral ions account for more than 50% of the osmolarity in the unstressed host (Triticum vulgare), K+ and Cl- playing the major role. Organic solutes contribute to a limited extent and, among these, free amino acids are present at higher concentration than carbohydrates (glucose, fructose, sucrose) or carboxylic acids (mainly as malate).
Inorganic ions (especially K+, Cl- and Na+) account for the greatest part of the osmolarity in unstressed Thesium. Organic solutes contribute to a lower extent; among them, carbohydrates (mainly as mannitol) and carboxylic acids (mainly as malate, ketoglutarate and citrate) are present at an higher concentration than free amino acids.
Host and parasite respond to water stress by decreasing their osmotic potentials through accumu- lation of species-specific sets of solutes. Increases in inorganic ion concentration in the stressed host result from water loss. Organic solutes are also strongly accumulated, mostly as carbohydrates (mainly fructose). Contents of free amino acids (compatible solutes) and carboxylic acids are about four times higher than under unstressed conditions. Thesium shows less dehydration under water deficits than does wheat and the mineral ion content is still slightly enhanced. Free amino acids and carbohydrates are present at about twice the concentration as under well-watered conditions. The increase in carboxylic acid content relates principally to an accumulation of citrate and glycerate.
Soluble carbohydrates, identified as oligofructans, may be also involved in the osmoregulation of both wheat and Thesium under stressed or unstressed conditions, but the extent of their contribution to osmolarity is unknown. Among carbohydrates, the compatible solute mannitol occurs in Thesium at highest concentration. The biosynthetic pathway of this polyol may be a possible biochemical target for chemical control of Thesium.
