Here we look at two recent discoveries showing how mycorrhizal inoculation can help plants better survive drought and salinisation.
In one study (Marulanda, A, et al. Drought tolerance and antioxidant activities in lavender plants colonized by native drought-tolerant or drought-sensitive Glomus species. MICROBIAL ECOLOGY), researchers looked at drought-tolerant species of the mycorrhizal fungi Glomus (namely drought-tolerant strains of Glomus intraradices and Glomus mosseae ) and their effects on drought-tolerant Lavandula spica (lavender). Drought tolerant strains of Glomus intraradices showed 35% greater root mass growth in the lavender compared to the regular strains of G. intraradices. And the drought tolerant Glomus mosseae showed 100% greater root mass compared to regular strains of G. mosseae.
Other beneficial effects included an increase in water content in the plant and decreases in antioxidants which would hurt plant health in times of drought. Plants with the drought tolerant strain also had higher levels of nitrogen and potassium compared to the non-drought tolerant Glomus species.
I think one could reasonably expect that the less drought tolerant strains would still be better than an absence of any mycorrhizal fungi. Nevertheless, the drought resistant strains would be a very useful and welcome addition to arid and semi-arid systems.
The following is an excerpt from the study’s abstract:
This study compared the effectiveness of four arbuscular mycorrhizal (AM) fungal isolates (two autochthonous presumably drought-tolerant Glomus sp and two allochthonous presumably drought-sensitive strains) on a drought-adapted plant (Lavandula spica) growing under drought conditions. The autochthonous AM fungal strains produced a higher lavender biomass, specially root biomass, and a more efficient N and K absorption than with the inoculation of similar allochthonous strains under drought conditions. The autochthonous strains of Glomus intraradices and Glomus mosseae increased root growth by 35% and 100%, respectively, when compared to similar allochthonous strains. These effects were concomitant with an increase in water content and a decline in antioxidant compounds: 25% glutathione, 7% ascorbate and 15% H2O2 by G. intraradices, and 108% glutathione, 26% ascorbate and 43% H2O2 by G. mosseae. Glutathione and ascorbate have an important role in plant protection and metabolic function under water deficit; the low cell accumulation of these compounds in plants colonized by autochthonous AM fungal strains is an indication of high drought tolerance.
The second study on the effects of Glomus fasciculatum on the salt tolerance of Acacia nilotica (Giri, B, et al. 2007.Improved tolerance of Acacia nilotica to salt stress by arbuscular mycorrhiza, Glomus fasciculatum may be partly related to elevated K/Na ratios in root and shoot tissues. MICROBIAL ECOLOGY), higher nutrient levels were observed in trees inoculated with the mycorrhizal fungi Glomus fasciculatum where soil was salinated.
The United States Department of Agriculture considers soil over 4 dS/m to be “saline soil.”The study looked at uninoculated trees and inoculated trees at salt levels of 1.2, 4.0, 6.5, and 9.5 dS per metre. In the inoculated plants, higher biomass in root and shoot was observed, showing that fungi assisting in plant growth. Inoculated trees had higher levels of phosphorus, zinc and copper than their uninoculated counterparts. And interestingly, in the lower levels of salt, inoculated trees took up more sodium (1.2 and 4.0 dS/m) than the control trees. At higher levels (6.5 and 9.5 dS/m), the sodium levels decreased whereas the control trees took up more sodium. And as salinity increased, the absorption of potassium in the inoculated trees increased. These results show that Glomus fasciculatum fungi increases the health of Acacia nilotica in saline conditions when compared to uninoculated Acacia nilotica. It is reasonable that this species of Glomus and possibly others could benefit other species of trees in saline conditions as well.
They found that at the higher level of salt (9.5 dS/m), the mycorrhizae had a harder time being established. Designers might offset this somewhat with swales. This would allow fresh rainwater to hold in the soil, reducing the salt content over time. Where salty water tables are a problem, appropriate trees can be used to pump down the water table, thereby removing salt from the upper levels. Whether or not Glomus spp. could help tree species used in this way to pump down salty water tables remains to be seems; but it is very plausible.
A pot experiment was conducted to examine the effect of arbuscular mycorrhizal fungus, Glomus fasciculatum, and salinity on the growth of Acacia nilotica. Plants were grown in soil under different salinity levels (1.2, 4.0, 6.5, and 9.5 dS m(-1)). In saline soil, mycorrhizal colonization was higher at 1.2, 4.0, and 6.5 dS m-1 salinity levels in AM-inoculated plants, which decreased as salinity levels further increased (9.5 dS m-1). Mycorrhizal plants maintained greater root and shoot biomass at all salinity levels compared to nonmycorrhizal plants. AM-inoculated plants had higher P, Zn, and Cu concentrations than uninoculated plants. In mycorrhizal plants, nutrient concentrations decreased with the increasing levels of salinity, but were higher than those of the nonmycorrhizal plants. Mycorrhizal plants had greater Na concentration at low salinity levels (1.2, 4.0 dS m-1), which lowered as salinity levels increased (6.5, 9.5 dS m-1), whereas Na concentration increased in control plants. Mycorrhizal plants accumulated a higher concentration of K at all salinity levels. Unlike Na, the uptake of K increased in shoot tissues of mycorrhizal plants with the increasing levels of salinity. Our results indicate that mycorrhizal fungus alleviates deleterious effects of saline soils on plant growth that could be primarily related to improved P nutrition. The improved K/Na ratios in root and shoot tissues of mycorrhizal plants may help in protecting disruption of K-mediated enzymatic processes under salt stress conditions.
The moral of the story reaffirms what we already know: Healthy soils with mycorrhizal fungi allow for healthier plants, particularly in difficult situations.