Anatomical Structures of the VA Mycorrhiza in the Apocynaceae (Gentianales)

Abstract

AbstractRoots of 19 Apocynaceae species were studied anatomically with respect to their symbiosis with VAM‐fungi. In plants collected from the field, VAM‐fungi were established in the root cortex. Also, inoculations with different Glomus species on the cultured plants are very successful in the infection and colonization of the root cortex.After penetration of the rhizodermis, the special exodermal short cells become colonized by winding hyphae. Then, in the root cortex of many Apocynaceae species, the VAM‐fungi produce intercellular running hyphae which leads to extensive colonization of the root. Arbuscules develop on intracellular running hyphae, whereas vesicles develop mainly on intercellular hyphae. Except for some special details, this is the most common type of colonization of VAM fungi in flowering plants.But in Amsonia tabernaemontana, Nerium oleander, and Thevetia peruviana, another type of colonization could be observed. In these species, the colonization of the hyphae within the root cortex is only possible by intracellular growth. Intercellular running hyphae in the root are lacking. Therefore, after penetration the colonization in the cortex is cluster‐like and strictly limited. Only by additional penetrations from hyphae in the soil, will roots show heavy infestations. This type of growth of the VAM fungi in the root is well known from the Gentianaceae and was explained as a structural incompatibility.In Catharanthus roseus, Pachypodium lamerei, and Trachelospermum jasminoides, intermediate stages of both types of colonization could be described. The results are discussed in the search for possible stimulants for structural incompatibility.