Tripartite associations in snowbrush (Ceanothus velutinus): effect of vesicular–arbuscular mycorrhizae on growth, nodulation, and nitrogen fixation

Abstract

Symbiotic associations were established between nitrogen-fixing nonleguminous (actinorrhizal) snowbrush (Ceanothus velutinus Dougl.) seedlings and two categories of microorganisms: vesicular–arbuscular (VA) mycorrhizal fungi and a filamentous actinomycete capable of inducing nodule formation. The actinomycete is housed in nodules where fixation of atmospheric dinitrogen occurs and is made available to the host plant; the mycorrhizal fungus is both inter- and intra-cellular within the root tissue and may be found within the nodules. The two major nutrients, N and P, are made available and can be supplied to the host plant by these two symbiotic microorganisms. The root system of snowbrush seedlings was dually colonized by VA mycorrhizal fungi and a nitrogen-fixing actinomycete and the possibility of a direct interaction between the endophytes in the symbioses was investigated. Dually infected plants showed increases in total dry weight of shoots and roots, number of nodules, weight of nodular tissue, as well as higher levels of N, Ca2+, and P, and an increase in nitrogenase activity as measured by acetylene reduction.

Les mycorhizes des plantes actinorhiziennes

“…Ces myconodules ont une duree de vie tres courte puisque le cytoplasme de l'hote ainsi que les hyphes du champignon degenerent des que Ia cellule vegetale est completement envahie par ce dernier (Sequerra et al, 1995 On a montre que les nutriments apportes par les mycorhizes, en particulier P, etaient essentiellement utilises par Ia plante actinorhizienne pour developper les nodules formes avec Frankia et pour effectuer les processus biochimiques de Ia fixation deN,. Aussi, comme pour les Legumineuses, I ,-augmentation de Ia biomasse des nodules des plantes actinorhiziennes par suite de l'inoculation avec des champignons AM ou EM est-elle tres nette et rapportee dans de nombreuses publications (Diem et Gauthier, 1982 ;Rose et Youngberg, 1981 ;Visser et al, 1981 ;Chartapaul et at., 1989 ;Russo, 1989 ;Vasanthakrishna et Bagyaraj, 1989;Jha et al, 1993;Fraga-Beddiar et Le Tacon, 1990 ;Miller et al, 1992 ;Isopi et al, 1994 II faut enfin noter que les mycorhizes des plantes actinorhiziennes ne se bornent pas seulement, com me on vient dele voir, a stimuler l'activite fixatrice de N 2 de ces plantes. Parle reseau d'hyphes extraracinaires croissant au-dela de Ia rhizosphere de Ia plante-hote et par des connections hyphales avec les racines d' autres plantes, les champignons mycorhiziens peuvent aussi transferer, soit !…”

Symbiotic N2 - Fixation by Frankia: Actinorhizae Establishment on Disturbed Lands

“…Rose and Youngberg (1981) have examined the influence of actinorh~zal and VA mycorrhizae on Ceanothus velutinus in a greenhot.ise environment, and Gardner et al (1984) have examined the same interaction on Hippophae rhamnoides. However, wo·rk has not been reported for iosaceous plants and from sites in the field.…”

Biochemical, physiological and environmental aspects of symbiotic nitrogen fixation

“…The supportive role of mycorrhizal infections in mineral absorption by nodulated plants is currently the subject of much interest. For example, among actinorhizal species, ectomycorrhizal roots of Alnus viridis have been shown to absorb phosphate five times more rapidly than non-mycorrhizal roots (170) while increases in nodule number and weight and elevated levels ofN, Ca and P have been demonstrated in Ceanothus velutinus, infected with both vesicular-arbuscular mycorrhizae and Frankia, (172).…”