Mycorrhizal Fungi Can Reduce the Effects of Drought
VAM fungi colonize the roots of host plants and perform absorption services for the plant. Various studies have demonstrated that plants associated with VAM fungi show increased uptake of various materials from the soil, including water, and macro and micronutrients compared to non-VAM plants. As a result, VAM fungi improve their host plants’ ability to grow under conditions of drought stress or in mineral deficient soils.
One of the major limiting factors for plant growth is water availability. Drought affects many aspects of plant physiology and tends to shut down plant growth and reduce photosynthesis. This impacts the flow of sugars from the host plant to its fungal partner. Therefore, VAM fungi have a vested interest in reducing drought stress for their host plants.
How VAM (Endo) Fungi Address Drought Stress
VAM fungi actively reduce drought stress symptoms in their host plants. Research has shown various ways this is accomplished. These are:
- VAM fungi expand the roots by adding their own expansive network of absorbing strands to mine the soil for water and the dissolved minerals carried therein.
- VAM fungi affect the opening or closure of the breathing pores in leaves. These pores are called “stomates.” Under conditions of drought stress, the plant will close the stomates to reduce water loss. VAM fungi can affect the closure of these pores and help provide more efficient water conservation.
- VAM fungi increase water pressure (turgor) in plant tissue (via 1 and 2 above), thereby preventing or delaying wilting. This supports cell function, allowing growth and photosynthesis to continue.
As a result of these effects, under conditions of drought stress, plants that are associated with VAM fungi outperform non-VAM plants in regard to growth, mineral content in tissues, reduced or delayed wilting, and overall metabolism.
Implications for Agriculture
In areas where agriculture is practiced without regular irrigation, VAM fungi can have a dramatic impact on crop yield. VAM fungi can significantly increase plant growth and crop yield in areas where regular irrigation is impossible, impractical, or too costly.
Limitations of VAM Use in Agriculture
The principal limitation is cost. While inoculants containing VAM fungi are available, their cost needs to be compared to the overall value of the crop. In many cases, the cost of inoculation is too high compared to the crop’s value. In such cases, one may still be able to activate naturally-occurring, native VAM fungi that are already in the soil.
Alternatives to Soil Inoculation
In situations where direct application of VAM fungi is too costly or not practical, you can harness the services of the native VAM fungi already in the soil by applying a stimulant that can increase the level of root colonization by these fungi. Certain root extracts have been shown to have such an effect. Among these is a plant extract called “formononetin.” Formononetin is naturally-occurring in a wide range of plants including clover and soy bean. Application of formononetin to soils containing VAM fungi produces an increase in the physical connection between VAM fungi and their host plants. In the presence of formononetin, VAM fungi will produce significantly more points of contact and penetration into host root tissue. This often results in increased crop yield, and this effect is more dramatic under harsh environmental conditions, particularly drought stress and reduced soil fertility. Of course, use of such stimulants is only effective in soils that already contain some VAM fungi. Fumigated fields would be devoid of live organisms, and therefore, would not respond to such a treatment.
Commercially-Available VAM Stimulant
Extraction of formononetin directly from plants is costly and very inefficient. Fortunately, scientists have devised methods for manufacturing formononetin from commercially-available substances using a patented process. As a result, formononetin can be mass-produced efficiently and at a much lower cost compared to direct plant extraction.
Mycorrhizal fungi can significantly increase water uptake by their host plants, and can provide a measurable degree of drought stress for plants grown under droughty conditions where irrigation is not available. Fields without irrigation can produce increased crop yields by applying mycorrhizal fungi inoculants, or by stimulating root colonization of native VAM fungi. The latter method tends to be less costly, and therefore, has more application in agriculture.
Aside from reducing water stress, mycorrhizal fungi also improve mineral absorption. As a result, VAM fungi can also produce increased or sustained yields with reduced fertilizer application. This can reduce farm expenses and cut down on pollution of surface and ground water. The magnitude of these effects varies with different crops and different farm practices.
There are hundreds of published scientific papers addressing the beneficial effects that mycorrhizal fungi provide to their host plants in regard to water relations. A limited bibliography is provided here to illustrate this.
Aboul-Nasr, A. 1998. Effects of inoculation with Glomus intraradices on growth, nutrient uptake and metabolic activities of squash plants under drought stress conditions. Ann Agric Sci Cairo 1:119-133.
Al-Karaki, G.N. 1998. Benefit, cost and water-use efficiency of arbuscular mycorrhizal durum wheat grown under drought stress. Mycorrhiza 8: 41-45. Aliasgharzad, N., Neyshabouri, M.R., Salimi, G. 2006. Effects of arbuscular mycorrhizal fungi and Bradyrhizobium japonicum on drought stress of soybean. Biologia-Bratislava 61(19): 324-328.
Allen M.F., Boosalis, M.G. 1983. Effects of two species of VA mycorrhizal fungi on drought tolerance of winter wheat. New Phytol. 93:67-76.
Allen, M. 2007. Mycorrhizal Fungi: Highways for Water and Nutrients in Arid Soils. Vadose Zone J. 6: 291-297.
Aroca R., Vernieri, P., Ruiz-Lozano, J.M. 2006. How does arbuscular mycorrhizal symbiosis regulate root hydraulic properties and plasma membrane aquaporins in Phaseolus vulgaris under drought, cold or salinity stresses? New Phytologist 173( 4): 808 – 816.
Aroca, R., Porcel, R., Ruiz-Lozano, J.M. 2007. How does arbuscular mycorrhizal symbiosis regulate root hydraulic properties and plasma membrane aquaporins in Phaseolus vulgaris under drought, cold or salinity stresses? New Phytologist 173:808-816.
Aroca, R., Vernieri, P., Ruiz-Lozano, J.M. 2008. Mycorrhizal and non-mycorrhizal Lactuca sativa (lettuce) plants exhibit contrasting responses to exogenous ABA during drought stress and recovery. J Exp Bot. 59(8): 2029–2041.
Auge, R.M., Stodola, A.J.W., Brown, M.S., Bethlenfalvay G.J. 1992. Stomatal response of mycorrhizal cowpea and soybean to short-term osmotic stress. New Phytologist 120(1):117-125.
Auge, R.M., Stodola, A.J.W., Tims, J.E., Saxton, A.M. 2001. Moisture retention properties of a mycorrhizal soil. Plant and Soil 230: 87-97.
Beltrano, J., Ronco, M.G. 2008. Improved tolerance of wheat plants (Triticum aestivum L.) to drought stress and rewatering by the arbuscular mycorrhizal fungus Glomus claroideum: effect on growth and cell membrane stability. Braz. J. Plant Physiol. 20(1): 29-37.
Bethlenfalvay, G.J., Brown, M.S., Mihara, K., Stafford A.E. 1987. Glycine-Glomus-Rhizobium symbiosis. V. Effects of mycorrhizae on nodule activity and transpiration in soybeans under drought stress. Plant Physiol. 85:115-119.
Boomsma, C., Vyn, T. 2008. Maize drought tolerance: Potential improvements through arbuscular mycorrhizal symbiosis? Field Crops Research 108(1): 14-31.
Bryla, D.R., Duniway, J.M. 1997. Effects of mycorrhizal infection on drought tolerance and recovery in safflower and wheat. Plant Soil 197(1): 95-103.
Bryla, D.R., Duniway, J.M. 1998. The influence of the mycorrhiza Glomus etunicatum on drought acclimation in safflower and wheat. Physiol. Plant. 104(1):87-96. Busse, M.D., Ellis, J.R. 1985. Vesicular-arbuscular mycorrhizal (Glomus fasciculatum) influence on soybean drought tolerance in high phosphorus soil. Can. J. Bot. 63: 2290-2294. Cui, M., Nobel, P.S. 1992. Nutrient status, water uptake and gas exchange for 3 desert succulents infected with mycorrhizal fungi. New Phytol 122: 643-649. Davies, F.T., Potter, J.R., Linderman R.G. 1992. Mycorrhiza and repeated drought exposure affect drought resistance and extraradical hyphae development of pepper plants independent of plant size and nutrient content. J. Plant Physiol. 139:289-294.
Davies, F.T., Potter, J., Linderman R.G. 1993. Drought resistance of mycorrhizal pepper plants independent of leaf P-concentration - response in gas exchange and water relations. Physiol Plant. 87:45-53.
Davies, F.T., Svenson, S.E., Henderson, J.C., Phavaphutanon, L, Duray, S.A., OlaldePortugal, V., Meier, C.E., Bo, S.H. 1996. Non-nutritional stress acclimation of mycorrhizal woody plants exposed to drought. Tree Physiol. 16:985-993. Dell’Amico, J., Torrecillas, A., Rodriguez, P., Morte, A., Sanchez-Blanco, M.J. 2002. Responses of tomato plants associated with the arbuscular mycorrhizal fungus Glomus clarum during drought and recovery. Journal of Agricultural Science 138: 387-393. Ellis, J.R., Larsen, H.J., Boosalis, M.G. 1985. Drought resistance of wheat plants inoculated with vesicular-arbuscular mycorrhizae. Plant Soil 86:369-378.
El-Tohamy, W., Schnitzler W.H., El-Behairy, U., El-Beltagy, M.S. 1999. Effect of VA mycorrhiza on improving drought and chilling tolerance of bean plants (Phaseolus vulgaris L.). Angewandte Botanik. 73: 178-183. Fitter, A.H. 1988. Water relations of red clover Trifolium pratense L. as affected by VA mycorrhizal infection and phosphorus supply before and during drought. J. Exp. Bot. 39:595-603. Gemma, J.N., Koske, R.E., Roberts, E.M., Jackson, N, De Antonis, K. 1997. Mycorrhizal fungi improve drought resistance in creeping bentgrass. Journal of Turfgrass Science 73: 15-29.
Goicoechea, N., Antolin, M.C., Sanchez-Diaz, M. 1997. Influence of arbuscular mycorrhizae and Rhizobium on nutrient content and water relations in drought stressed alfalfa. Plant and Soil. 192(2): 261-268. Goicoechea, N., Merino, S., Sanchez-Diaz, M. 2004. Contribution of arbuscular mycorrhizal fungi (AMF) to the adaptations exhibited by the deciduous shrub Anthyllis cytisoides L. under water deficit. Physiologia Plantarum 122: 453-464. Hardie, K, Leyton, L. 1981. The influence of vesicular-arbuscular mycorrhiza on growth and water relations of red clover. I. In phosphate deficient soil. New Phytol. 89:599-608.
Johnson, C.R., Hummel, R.L. 1985. Influence of mycorrhizae and drought stress on growth of Poncirus x Citrus seedlings. HortScience 20(4):754-755. Lu, J.Y., Mao, Y.M., Shen, L.Y., Peng, S,Q, Li, X.L. 2003. Effects of VA mycorrhizal fungi inoculated on drought tolerance of wild jujube (Zizyphus spinosus Hu) seedlings. Acta Horticulture Sinica 30(1): 29-33.
Qiang S.W., Ying, N.Z., Ren, X.X. 2006. Effects of water stress and arbuscular mycorrhizal fungi on reactive oxygen metabolism and antioxidant production by citrus (Citrus tangerine) roots. European Journal of Soil Biology 42( 3): 166-172.
Runjin, L. 1989. Effects of vesicular-arbuscular mycorrhizas and phosphorus on water status and growth of apple. J Plant Nutrition 12: 997-1017.
Safir, G.R, Boyer, J.S, Gerdemann, J.W. 1972. Nutrient status and mycorrhizal enhancement of water transport in soybean. Plant Physiol. 49:700-703. Sieverding, E. 1983. Influence of soil water regimes on VA mycorrhiza, II. Effect of soil temperature and water regime on growth, nutrient uptake, and water utilization of Eupatorium odoratum L. Z. Acker- und Pflanzenbau (J. Agron. Crop Sci.) 152:56-67. Simpson, D, Daft, M.J. 1990. Interactions between water-stress and different mycorrhizal inocula on plant growth and mycorrhizal development in maize and sorghum. Plant Soil121:179-186.
Simpson, D, Daft, M.J. 1991. Effects of Glomus clarum and water stress on growth and nitrogen fixation in 2 genotypes of groundnut. Agric. Ecosystems Environ. 35:47-54.
Subramanian, K.S.; Charest, C. 1995. Influence of arbuscular mycorrhizae on the metabolism of maize under drought stress. Mycorrhiza 5:273-278.
Subramanian, K.S., Charest, C, Dwyer, LM, Hamilton RI. 1997. Effects of arbuscular mycorrhizae on leaf water potential, sugar content, and P content during drought and recovery of maize. Canadian Journal of Botany. 75(9): 1582-1591.
Subramanian, K.S., Charest, C. 1999. Acquisition of N by external hyphae of an arbuscular mycorrhizal fungus and its impact on physiological responses in maize under drought-stressed and well-watered conditions. Mycorrhiza 9(2): 69-75. Tarafdar, J.C.; Rao, A.V. 1997. Response of arid legumes to VAM fungal inoculation. Symbiosis 22(3): 265-274. Waterer, D.R., Coltman, R.R. 1989. Response of mycorrhizal bell peppers to inoculation timing, phosphorus, and water stress. HortScience 24(4): 688-690.
Wu, Q., Xia, R., Hu, Z. 2005. Effect of Arbuscular Mycorrhiza on the Drought Tolerance of Poncirus trifoliata seedlings. Chinese Journal of Applied Ecology. 16(3): 100-104.