PHYSIOLOGICAL-CHANGES IN A ROOT HEMIPARASITIC ANGIOSPERM, THESIUM HUMILE (SANTALACEAE), BEFORE AND AFTER ATTACHMENT TO THE HOST-PLANT (TRITICUM VULGARE)

Thesium humile, an obligate root hemiparasitic angiosperm, can live in an autotrophic-like way during several weeks before attachment to the host. About 6 weeks after germination, a decreasing growth rate and symptoms of senescence of the seedling are observed. Before attachment, Thesium is able to take up water and inorganic ions from the soil, but its mineral content is characterized by very high Na and low P levels. During the same period, rates of net photosynthesis and dark respiration as well as RuBPc and PEPc activities decrease continuously. The expression of a photosynthetic gene like psbA also decreases and the amount of transcripts of the rbcL gene is very low. After attachment to the host, Thesium recovers a more physiological balance by deriving inorganic ions from the host xylem sap. Increasing growth rate, photosynthetic carbon fixation, carboxylase capacities and expression of a photosynthetic gene like rbcL are observed and become fairly similar to those found in non-parasitic Cg plants. A strong K accumulation in leaves also occurred and that might be an explanation for the observed anomalous stomatal behaviour. Before attachment, the poorly developed root system of Thesium seems unable to selectively absorb inorganic ions from the soil and to maintain a physiological mineral balance. Consequential to the very high Na and the low P contents in the seedling of the parasite, metabolic perturbations affecting also gene expression might result.