Increased Drought tolerance and Resistance to Salinity Through Fungi

Past articles have looked at evidence of mycorrhizal fungi helping plants tolerate salinity, heavy metals and arsenic, and have increased tolerance to acid rain. 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.

The following is the abstract from the study:

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.

Click for information on Acacia nilotica subsp nilotica.

Mycorrhyzae help plants survive heavy metals/salt

[Updated Oct. 27]

It seems like the more we learn about mycelium, the more we learn how beneficial it is to all sorts of life. Research published in the African Journal of Biotechnology (on the Mycorrhyza Literature Exchange site) shows that mycorrhyzae can help plants survive higher levels of zinc and cadmium:

...From a number of physiological indices measured in this study, microsymbionts significantly increased dry weight, root : shoot ratios, leaf number and area, plant length, leaf pigments, total carbohydrates, N and P content of infected plants as compared with non infected controls at all levels of heavy metal concentrations. Tolerance index of cowpea plants was increased in the presence of microsymbionts than in their absence in polluted soil. Microsymbionts dependencies of cowpea plants tended to be increased at higher levels of Zn and Cd in polluted soil. Metals accumulated by microsymbionts-infected cowpea plant were mostly distributed in root tissues, suggesting that an exclusion strategy for metal tolerance widely exists in them.

This news from Scientia Horticulturae (also on the Mycorrhyza Literature Exchange site) shows that mycorrhyzae can benefit tomatoes grown in saline conditions:

This study was conducted to determine if pre-inoculation of transplants with arbuscular mycorrhizal (AM) fungi alleviates salt effects on growth and yield of tomato (Lycopersicon esculentum Mill. Cv. Marriha) when irrigated with saline water. Tomato seeds were sown in polystyrene trays with 20 cm(3) cells and treated with AM fungi (AM) or without (nonAM) Glomus mosseae. Once the seedlings were reached appropriate size, they were transplanted into nonsterile soil in concrete blocks (1.6 m x 3 m x 0.75 m) under greenhouse conditions. The soil electrical conductivity (ECe) was 1.4 dS m(-1). Plants were irrigated with nonsaline water (ECw = 0.5 dS m(-1)) or saline water (ECw = 2.4 dS m(-1)) until harvest. These treatments resulted with soil EC at harvest 1.7 and 4.4 dS m(-1) for nonsaline and saline water treatments, respectively. Root colonization with AM fungi at flowering was lower under saline than nonsaline conditions. Pre-inoculated tomato plants with AM fungi irrigated with both saline and nonsaline water had greater shoot and root dry matter (DM) yield and fruit fresh yield than nonAM plants. The enhancement in fruit fresh yield due to AM fungi inoculation was 29% under nonsaline and 60% under saline water conditions. Shoot contents of P, K, Zn, Cu, and Fe were hi-her in AM compared with nonAM plants grown under nonsaline and saline water conditions. Shoot Na concentrations were lower in AM than nonAM plants grown under saline water conditions. Results indicate that pre-inoculation of tomato transplants with AM fungi improved yield and can help alleviate deleterious effects of salt stress on crop yield.

Update: As discussed elsewhere on these pages, Glomus mosseae helps plants resist the uptake of arsenic and thereby increase the uptake of phosphorus. Glomus mosseae has also been shown to reduce the uptake of copper, zinc, lead and cadmium.

A glasshouse pot experiment was conducted to investigate effects of the arbuscular mycorrhizal fungus Glomus mosseae on the growth of Vicia faba and toxicity induced by heavy metals (HMs) (Cu, Zn, Pb and Cd) in a field soil contaminated by a mixture of these metals. There was also uninoculation treatment (NM) simultaneously. Mycorrhizal (GM) plants have significantly increased growth and tolerance to toxicity induced by heavy metals compared with NM plants. P uptake was significantly increased in GM plants. Mycorrhizal symbiosis reduced the transportation of IlMs from root to shoot by immobilizing HMs in the mycorrhizal, shown by increasing the ratios of HMs from root to shoot. Oxidative stress, which can induce DNA damage, is an important mechanism of heavy metal toxicity. GM treatment decreased oxidative stress by intricating antioxidative systems such as peroxidases and non-enzymic systems including soluble protein. The DNA damage induced by heavy metals was detected using comet assay, which showed DNA damage in the plants was decreased by the GM treatment. [Journal of Environmental Sciences-China also see University of Ljubljana]

Oyster mushrooms, a type of saprophytic mushroom (not mycorrhizal), have been used to detoxify soils contaminated with cadmium. However, these mushrooms would not be fit for human consumption. When using mycelium to detoxify land or help plants resist toxins, do not eat any mushrooms that fruit.