Mycorrhizal network
Mycorrhizal relationships are most commonly mutualistic, with both partners benefiting, but can be commensal or parasitic, and a single partnership may change between any of the three types of symbiosis at different times.
[1] The formation and nature of these networks is context-dependent, and can be influenced by factors such as soil fertility, resource availability, host or mycosymbiont genotype, disturbance and seasonal variation.
Scientific understandings and thus publications utilize more specific definitions arising from the term common mycorrhizal network (CMN).
The keyword "common" requires that two or more individual plants are connected by the same underground fungal network, through which matter of various types and functions may flow.
However, tree species comprising the canopy of temperate and especially boreal forests in the Northern Hemisphere tend to associate with ectomycorrhizal fungi.
Evidence and potential mechanisms for a variety of plant-plant interactions mediated by the mycorrhizal symbiosis have been presented, but their validity and significance is still controversial.
[18] Numerous studies have reported that carbon, nitrogen and phosphorus are transferred between conspecific and heterospecific plants via AM and ECM networks.
[19][20][21][22] Other nutrients may also be transferred, as strontium and rubidium, which are calcium and potassium analogs respectively, have also been reported to move via an AM network between conspecific plants.
[25] Both signals and cues are important elements of communication, but workers maintain caution as to when it can be determined that transfer of information benefits both senders and receivers.
These studies strongly suggest that mycorrhizal networks increase the transfer of allelopathic chemicals and expand the range, called the bioactive zone, in which they can disperse and maintain their function.
[26] Furthermore, studies indicate increased bioactive zones aid in the effectiveness of the allelochemicals because these infochemicals cannot travel very far without a mycorrhizal network.
[26] The black walnut is one of the earliest studied examples of allelopathy and produces juglone, which inhibits growth and water uptake in neighboring plants.
[31] Spotted knapweed, an allelopathic invasive species, provides further evidence of the ability of mycorrhizal networks to contribute to the transfer of allelochemicals.
When plants are attacked they can manifest physical changes, such as strengthening their cell walls, depositing callose, or forming cork.
[34] They can also manifest biochemical changes, including the production of volatile organic compounds (VOCs) or the upregulation of genes producing other defensive enzymes, many of which are toxic to pathogens or herbivores.
When the plant is consumed, however, the composition of the VOCs change, which can then cause them to repel the herbivores and attract insect predators, such as parasitoid wasps.
Field observations cannot easily rule out the possibility that effects attributed to physical connection between plants via mycorrhizal networks could be happening due to other interactions.
Carbon transfer has been demonstrated by experiments using carbon-14 (14C) isotopic labeling and following the pathway from ectomycorrhizal conifer seedlings to another using mycorrhizal networks.
Further investigation of bidirectional movement and the net transfer was analyzed using pulse labeling technique with 13C and 14C in ectomycorrhizal Douglas fir and Betula payrifera seedlings.
[21] Several models have been proposed to explain the movement of nutrients between plants connected by a mycorrhizal network, including source-sink relationships, "market" analogies, preferential transfer and kin related mechanisms.
[2][20] These transfer mechanisms can facilitate movement of nutrients via mycorrhizal networks and result in behavioral modifications in connected plants, as indicated by morphological or physiological changes, due to the infochemicals being transmitted.
One study reported a threefold increase in photosynthesis in a paper birch transferring carbon to a Douglas fir, indicating a physiological change in the tree which produced the signal.
[45] Photosynthesis was also shown to be increased in Douglas fir seedlings by the transport of carbon, nitrogen and water from an older tree connected by a mycorrhizal network.
[59] The formation and nature of these networks is context-dependent, and can be influenced by factors such as soil fertility, resource availability, host or mycosymbiont genotype, disturbance and seasonal variation.
[60] It is hypothesized that fitness is improved by the transfer of infochemicals through common mycorrhizal networks, as these signals and cues can induce responses which can help the receiver survive in its environment.
[27] Although they remain to be vigorously demonstrated, researchers have suggested mechanisms which might explain how transfer of infochemicals via mycorrhizal networks may influence the fitness of the connected plants and fungi.
[20] This may happen in ecosystems where environmental stresses, such as climate change, cause fluctuations in the types of plants in the mycorrhizal network.
[37] Allelopathic chemicals transferred via CMNs could also affect which plants are selected for survival by limiting the growth of competitors through a reduction of their access to nutrients and light.
Douglas fir seedlings' growth expanded when planted with hardwood trees compared to unamended soils in the mountains of Oregon.
In burned and salvaged forest, Quercus rubrum establishment was facilitated when acorns were planted near Q. montana but did not grow when near arbuscular mycorrhizae Acer rubrum Seedlings deposited near Q. montana had a greater diversity of ectomycorrhizal fungi, and a more significant net transfer of nitrogen and phosphorus content, demonstrating that ectomycorrhizal fungi formation with the seedling helped with their establishment.
