Helianthemum—Terfezia relations in different growth media

The Israeli desert truffle, Terfezia leonis Tul., failed to enter into association with its plant symbiont, Helianthemum sessiliflorum (Defs.) Pers., when cultivated together on an agar medium consisting of inorganic salts. Addition of activated charcoal to the growth medium brought about rapid and extensive association. Activated charcoal did not enhance fungal growth in the absence of the plant seedling. The stimulating effect of the charcoal is probably related to its adsorption characteristics and may not be due to contact or abrasion between seedling roots and charcoal particles.