When you think of things that can help seedlings thrive at their earliest stages of development, a fungus is probably at the bottom of that list, but a team of researchers recently hypothesized that maybe cannabis cultivators should be looking to a specific type of fungus to aid in their growing efforts.
Rhizohagus irregularis is a type of fungus called an arbuscular mycorrhiza fungus (AMF). Fungi in this family have previously been shown to have symbiotic relationships with plants and are even capable of boosting plant growth. The researchers tested several different concentrations of R. irregularis spores on cannabis seedlings in a hydroponic setup and found that the fungus did, in fact, help the seedlings grow. 
Increase in Seedling Biomass
Of the three concentrations of spores tested, the two lower amounts (40 and 80 spores per liter of nutrient solution) didn’t yield much in the way of statistically significant results, but the third and highest concentration of spores (120 spores/L) undeniably increased several key indicators of plant health.
When compared with the control group that was not exposed to AMF spores, the plants in group AMF3, exposed to 120 spores per liter of nutrients, had an increased overall root length of approximately 34%.
The dried stems from plants in group AMF3 were also significantly impacted by their exposure to the spores. The group showed an overall increase in dried stem weight of 21% over the control.
Altogether, this paints a picture of plants that are growing at a faster rate than those which are not infected with R. irregularis, meaning that the plants’ symbiotic relationship with the fungus is leading to increased growth.
Increase in Nutrient Acquisition
It would make sense that, if the plants are growing larger and more rapidly, that they’d also be acquiring more nutrients to do so. The researchers looked into nutrient acquisition levels of two key nutrients, nitrogen and phosphorus. While they were unable to find a significant increase in nitrogen in the plant tissue, they did find a marked increase in phosphorus. The team hypothesized that the plants could be taking in more nitrogen as well, but it was concentrated in the root network.
The researchers also questioned whether or not the increased levels of phosphorus could be aiding the plants in photosynthesis, but did not conduct any further tests.
Greater Survival Rates
The greatest indicator of overall health is probably the survival of the plants, and this is another marker that the team accounted for. Prior to this study, there was evidence that AMF fungi could increase transplant shock resistance in seedlings. The team tested this out as well and found that the seedlings in group AMF3 had a roughly 5% higher survival rate than the control group. They also measured plant health using Dickinson’s quality index (DQI), a common benchmark of overall plant quality. They found that the cannabis seedlings in group AMF3 that had been exposed to the highest number of fungal spores scored significantly higher than the control, an increase of 12%.
The evidence is clear that, however counterintuitive it may seem, infecting cannabis seedlings with R. irregularis fungus is actually beneficial to their overall health and increases their chances of survival.
Reference Kakabouki I, Mavroeidis A, Tataridas A, et al. Effect of Rhizophagus irregularis on growth and quality of Cannabis sativa seedlings. Plants (Basel). 2021;10(7):1333. Published 2021 Jun 29. doi:10.3390/plants10071333. [journal impact factor = 3.935; times cited = 4]