Although both environment and genetics have been shown to affect the mycorrhizal colonization of host plants, the impacts of these factors on hosts that can be dually colonized by both ectomycorrhizal (EM) and arbuscular mycorrhizal (AM) fungi are less understood. We examined the influence of environment and host crosstype on the EM and AM colonization of cottonwoods (Populus angustifolia and natural hybrids) by comparing levels of colonization of trees growing in common gardens that differed in elevation and soil type. We also conducted a supplemental watering experiment to determine the influence of soil moisture on AM and EM colonization. Three patterns emerged. First, garden location had a significant impact on mycorrhizal colonization, such that EM colonization was 30% higher and AM colonization was 85% lower in the higher elevation garden than the lower elevation garden. Second, crosstype affected total (EM + AM) colonization, but did not affect EM or AM colonization. Similarly, a significant garden x crosstype interaction was found for total colonization, but not for EM or AM colonization. Third, experimental watering resulted in 33% higher EM colonization and 45% lower AM colonization, demonstrating that soil moisture was a major driver of the mycorrhizal differences observed between the gardens. We conclude that environment, particularly soil moisture, has a larger influence on colonization by AM versus EM fungi than host genetics, and suggest that environmental stress may be a major determinant of mycorrhizal colonization in dually colonized host plants.

Environmental and genetic effects on the formation of ectomycorrhizal and arbuscular mycorrhizal associations in cottonwoods

A single European aspen (Populus tremula) tree individual may potentially harbour dozens of Cenococcum geophilum ITS genotypes and hundreds of species of ectomycorrhizal fungi.

Earlier surveys of Populus deltoides Bartr. and Salix nigra x ? had suggested displacement of VA-mycorrhizal fungi by ectomycorrhizal fungi from the middle of soil moisture gradients. Succession from VAto ectomycorrhizal associations had also been observed previously in several plant species. Data were therefore analyzed for evidence of negative interactions among VAand ectomycorrhizal fungi inhabiting the same root system. Fine roots (< 2 mm diameter) were sampled from natural populations of cottonwood growing in the eastern USA, and from two field-infected cottonwood clones grown under uniform soil conditions in North Carolina. Percentages of non-mycorrhizal, VA-mycorrhizal, ectomycorrhizal, and dually colonized root lengths were determined for terminal (most distal branches of the fine roots) and non-terminal roots in each sampling unit. Dual mycorrhizae (VAand ectomycorrhizal fungi in the same cross section) in terminal roots of almost all Populus sampling units were significantly lower than expected from a random assortment model. Negative association with VA-mycorrhizal fungi varied among ectomycorrhizal morpho-types. The degree of negative association among mycorrhizal fungi was determined using a new statistical test based on linear regression. High levels of ectomycorrhizal colonization were related to depression of VA-mycorrhizal colonization in terminal as compared with non-terminal roots within the same sampling unit. Although these data are not conclusive, they suggest that ectomycorrhizal fungi might displace VA-mycorrhizal fungi from roots of VA-/ectomycorrhizal hosts.

Negative associations among VA-mycorrhizal fungi and some ectomycorrhizal fungi inhabiting the same root system.

summary

The ability of 28 monokaryons and 78 reconstituted dikaryons of Pisolithus arhizus (Scop, pers Pers.) Rauchert [Syn.: Pisolithus tinctorius (Pers.) Coker & Couch] to form ectomycorrhizas on Pinus pinaster (Ait.) Sol. and Pinus banksiana (Lamb.) was studied using the growth pouch method. The ability to form mycorrhiza and promote growth of P. pinaster differed both between and within monkaryons and dikaryons. Some monokaryons and dikaryons failed to form ectomycorrhizas. Monokaryons formed fewer ectomycorrhizas on P. pinaster than dikaryons. The heterokaryotic state was necessary for the full expression of ectomycorrhiza-forming ability. Growth of P. pinaster was more strongly correlated with ectomycorrhiza formation by dikaryons than by monokaryons.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1990.tb00501.x

Genetic variation in ectomycorrhiza formation by Pisolithus arhizus on Pinus pinaster and Pinus banksiana

Ectomycorrhizae, arbuscular mycorrhizae, and dark-septate fungi on Salix humboldtiana in two riparian populations from central Argentina

Seven clones of hybrid poplar, with parents from among four sections of the genus Populus, were potted in five Iowan soils. After a year they were sampled for mycorrhizae and endogonaceous spores. Eight species of Endogonaceae were recorded, seven known to form endomycorrhizae. Two ectomycorrhizal fungi, Ccnococcum geophilum and an unidentified species were noted. One poplar clone had fewer replicates in which Glomus fasciculatum spores were found, but otherwise there were no differences among the poplars in presence of spores. A soil from a P. grandidentata woodland had fewer pots with G. mosseae spores, while an agricultural soil had more occurrences of Gigaspora gigantea than other soils. No other significant differences among spores were noted for soils. Vesicles of endomycorrhizae occurred in all clones, but one was colonized more sparsely than the others. Two clones had ecto- and ectendo-mycorrhizae, two types of the former being recorded. One of these clones had significantly more C. geophilum colonization than the other. There were indications of differences in mycotrophy among the sections of the genus Populus.

C. Walker

Papers

Mycorrhizal symbionts associated with hybrid poplars from Iowa, USA