Growth and development of the facultative root hemiparasite Rhinanthus minor after removal of its host

Facultative plant hemiparasites exhibit optimal growth only when attached to a suitable host. After attachment, stomata of the parasite remain continuously open, thus, optimising the extraction of host xylem sap. When the host shoot was removed from the hemiparasitic Rhinanthus/barley association ~14 days after attachment, the resulting host-free attached Rhinanthus continued to grow and develop similarly well as the attached parasites. These plants, however, showed altered stomatal behaviour: their stomata were open at daytime and closed at night, whereas parasitising Rhinanthus has continuously open stomata all day and night and unattached single Rhinanthus has practically closed stomata throughout day and night. After removal of the host the root growth was strongly increased, thereby increasing the root-to-shoot ratio. Abscisic acid and cytokinin relationships became more 'normal' with the Rhinanthus roots becoming able to synthesise zeatin nucleotides and zeatin ribosides, thus, behaving much as non-parasitic plants in general. It is suggested that the degrading root system of the host plant produces signals that trigger this conversion. Two explanations for these changes are discussed, the supply of dissolved organic nitrogen by the degrading host root system and a possible strong growth of growth promoting soil microorganisms using the degrading host root system as a substrate.