AM-fungi lead to better plant nutrient acquisition and drought tolerance in agricultural crops: A review

Today, use of arbuscular mycorrhizal fungi (AMF) in agricultural crops is the need of the hour to meet-out nutrient needs economically, rationalizing water-use and maintaining soil health under changing climate and impeding resource and production vulnerabilities. Adoption of AM-fungi as a biofertilizer in crop production might has proven as a very effective tool in mitigating various biotic and abiotic stresses. AMF is an inexpensive and eco-friendly input capable of enhancing phosphorus (P) and water use efficiency (WUE) of different crops and cropping systems. Besides P, AMF also supply other plant nutrients such as nitrogen (N), potassium (K), calcium (Ca), copper (Cu), zinc (Zn) etc. to the plants, which are absorbed by AMF from soil, leading to increased input economy as well as higher crop yields and quality. Several researchers have reported an increase in plant growth resulting from AMF association with crop plants and the same has been attributed by them to increased mineral element uptake mediated by AMF hyphae in the soil, improved soil-plant-water relations and plant tolerance to a variety of abiotic stresses, etc. Further, an economy of about 25% in soil test based fertilizer P has been registered following seed inoculation with AM fungi. AMF-plant root symbiosis often results into altered rates of water movement into, through and out of host plants with consequent effects on tissue hydration and leaf physiology. Mycorrhizal hyphae penetrate the soil pores which are inaccessible to root hairs thereby absorbing water which is unavailable to non-mycorrhizal plants. Studies have been shown that mycorrhizal plants maintained higher tissue water content imparting greater drought resistance to plants over non-mycorrhizal plants at moisture stress. Moreover, it enhances water- use efficiency of crops by about 15–30%. Studies conducted under Himalayan acid Alfisol has been indicated that integrated application of AM fungi and P at varying irrigation regimes improved the plant water relations vis-à-vis drought resistance, crop productivity, water productivity, soil aggregation and water holding capacity in different cropping system. Integrated use of AM fungi and P at varying irrigation regimes exhibited higher relative leaf water content and xylem water in comparison with plants without AMF inclusion. A number of workers have reported improvement in soil structure following AMF inclusion in different cropping systems. The AMF resulted in improvement in soil structure by way of binding of soil aggregates, involving their hyphal network and producing a glycoprotein called glomalin which is capable of binding soil particles leading to aggregate formation and in turn, improve soil structure/soil moisture retention capacity. Inclusion of AMF in different cropping systems exerted a significant impact on water holding capacity and mean weight diameter of soil particles. The AM fungi are capable of mineralizing organic-P and solublizing inorganic-P in soils by releasing various enzymes and organic acids. Overall, AMF use in crop production systems is of paramount importance in enhancing nutrient use efficiency and quality of produce. This is a low-cost farm input for resource-poor farmers, who ill afford expensive external inputs especially P. AMF also revealed a tremendous potential in enhancing water use efficiency, improved fruit succulence and imparting drought resistance to plants at moisture stress. Thus, AMF inoculation improves nutrient and water acquisition from soil significantly under nutrient and water stress situations.