Mycorrhizae Enhance Rooting of Cuttings
R. G. Linderman
Project Chairman, U.S.D.A. Ornamental Plants Laboratory, Corvallis, Oregon
From a paper given at the 1977 Breeders Roundtable, Eugene, Oregon
Healthy rootlets of most vascular plants grown in natural soil are inhabited by fungi living in a state of symbiosis with the plant root. Roots so inhabited are called mycorrhizae (myco = fungus, rhiza = root). Such mycorrhizal associations are the rule, not the exceptions. In this association,, which for some plants is essential, the fungus derives its carbohydrates from the host photosynthate. In return, the fungus, whose hyphal network extends out into the soil away from the root, absorbs water and nutrients which it translocates back into the host root. In the course of this association between host root and fungus, significant metabolic exchanges occur, not the least of which involve growth regulators and hormones. Certain of these fungi are known to produce such substances, and many of the physiological and morphological changes that occur in the mycorrhizae are in response to such substances.
Three main groups of mycorrhizae are known based on their morphology. Ecto-mycorrhizae are formed by fungi that can be grown in culture, and occur mainly in the Pinaceae, Betulaceae, and Fagaceae. These fungi form a heavy external mantle of fungal material around the root tip, and penetrate between the cortical cells. Endomycorrhizae, in contrast, are obligate symbionts, form no mantle, and penetrate into the cortical cells. Some 90% of the plants on the earth form endomycorrhizae.
A third group, ectendomycorrhizae, exhibit characteristics of both ecto- and endomycorrhizae by having a loose mantle, penetrating both intra- and intercellularly in the cortex, and being culturable. These mycorrhizae occur largely in the Ericaceae, but are formed by some of the same fungi that form ectomycorrhizae in the Pinaceae.
We hypothesized that since many of the ectomycorrhizal fungi are known to produce growth regulators and hormones that influence the morphology and physiology of roots, rooting of cuttings may be enhanced. We tested this hypothesis on bearberry [ Arctostaphylos uva-ursi (L.) Spreng.] and demonstrated a striking enhancement of rooting percentage as well as root ball size when mycorrhizal inoculum was added in a 1:5 dilution of a standard 1:1 peat-perlite rooting medium (1). Of the several fungi used, certain ones were very effective on one cultivar and less so on others. When the roots were examined for formation of mycorrhizae, only Thelephora terrestris had actually formed mycorrhizae. Thus, we concluded that the rooting enhancement was a pre-penetration phenomenon, presumably due to something produced by the fungi in the medium.
In subsequent experiments conducted by graduate student C. A. Call, we learned (1) that the inoculum could be diluted 1 :10 and still get enhancement of rooting; (2) that culture filtrate also worked but less so than the actual mycelium; (3) that the enhancement was more in well-aerated than waterlogged rooting medium; (4) that infections do occur and mycorrhizae form after transplant of rooted cuttings; and (5) that there is a significant seasonal variation in the enhancement phenomenon, December sticking being the most striking period in that without mycorrhizal fungi, rooting is very poor. The physiology of the stock plant apparently plays a key role as the cuttings interface with the fungi. In the presence of mycorrhizal fungus inoculum, cuttings maintained a generally healthier look, buds broke earlier and new growth was more vigorous.
Some of the above observations have led us to again hypothesize as to the mechanism of this phenomenon. Since so many different fungi have enhanced rooting of bearberry, we feel that some common factor must be involved. Not all the fungi are known to produce hormones, growth regulators, vitamins, etc. We have tended to lean more toward the involvement of substances possibly produced by all the fungi that may influence the endogenous hormone levels in the cuttings, i.e. substances like ethylene and/or auxin synergists (polyphenols). Our future experiments will be aimed at the elucidation of the mechanism of this phenomenon, as well as to see how widespread it is. We hope to test our fungi for their effect on rhododendrons in the near future.
Reference - Linderman, R. G., and C. A. Call, 1977, Enhancement of rooting of woody plant cuttings by mycorrhizal fungi, J. Amer. Soc. Hort. Sci. (in press).