Mycorrhizal dependency of a representative plant species in Mediterranean shrublands (Lavandula spica L.) as a key factor to its use for revegetation strategies in desertification-threatened areas

Lavender plants (Lavandula spp.) are small woody shrubs which belong to the natural succession in certain plant communities of semiarid mediterranean ecosystems in the southeast of Spain and they thrive in areas which are threatened by desertification. Lavender plants form arbuscular mycorrhizas and have been described as mycorrhizal-dependent species, but they are also known to display a stimulated root phosphatase activity in conditions of low-nutrient supply. Consequently, a series of microcosm experiments were carried out in order to identify the relative importance of the mycotrophic habit in comparison with phosphatase production as mechanisms which account for plant establishment and nutrient uptake, using four soils typical of the degraded area of study. It was found that lavender plants were obligatorily mycorrhizal species under the prevailing conditions of their natural habitat. The specific activity of root-associated acid phosphatase in lavender was enhanced in P-deficient, non-mycorrhizal, plants, but apparently this stimulated phosphatase activity did not result in an increased ability of the plant to take up P. Low organic phosphate content in the test soils may account for the absence of plant response to acid phosphatase activity. However, it is much more likely that this ineffectiveness is due to the fact that P-deficiency is concomitant, in the degraded test habitats, with deficiencies in other nutrients. Thus plant growth is being limited by an overall lack of nutrient supply. Mycorrhizal inoculation largely improved P, N and K uptake thereby restoring the biochemical cycling of plant nutrients. In conclusion, lavender plants must be mycorrhizal in order to thrive in the degraded soils from desertification threatened areas in typical mediaterranean ecosystems. (C) 1997 Elsevier Science B.V.