Role of Arbuscular Mycorrhizae in the Alleviation of Ionic, Osmotic and Oxidative Stresses Induced by Salinity in Cajanus cajan (L.) Millsp (pigeonpea)

Salinity stress causes ion toxicity and osmotic imbalances, leading to oxidative stress in plants. Arbuscular mycorrhizae (AM) are considered bio-ameliorators of saline soils and could develop salinity tolerance in crop plants. Pigeonpea exhibits strong mycorrhizal development and has a high mycorrhizal dependency. The role of AM in enhancing salt tolerance of pigeonpea in terms of shoot and root dry weights, phosphorus and nitrogen contents, K+ : Na+, Ca2+ : Na+ ratios, lipid peroxidation, compatible solutes (proline and glycine betaine) and antioxidant enzyme activities was examined. Plants were grown and maintained at three levels of salt (4, 6 and 8 dSm(-1)). Stress impeded the growth of plants, led to weight gain reductions in shoots as well as roots and hindered phosphorus and nitrogen uptake. However, salt-stressed mycorrhizal plants produced greater root and shoot biomass, had higher phosphorus and nitrogen content than the corresponding uninoculated stressed plants. Salt stress resulted in higher lipid peroxidation and membrane stability was reduced in non-AM plants. The presence of fungal endophyte significantly reduced lipid peroxidation and membrane damage caused by salt stress. AM plants maintained higher K+ : Na+ and Ca2+ : Na+ ratios than non-AM plants under stressed and unstressed conditions. Salinity induced the accumulation of both proline and glycine betaine in AM and non-AM plants. The quantum of increase in synthesis and accumulation of osmolytes was higher in mycorrhizal plants. Antioxidant enzyme activities increased significantly with salinity in both mycorrhizal and non-mycorrhizal plants. In conclusion, pigeonpea plants responded to an increased ion influx in their cells by increasing the osmolyte synthesis and accumulation under salt stress, which further increased with AM inoculation and helped in maintaining the osmotic balance. Increase in the antioxidant enzyme activities in AM plants under salt stress could be involved in the beneficial effects of mycorrhizal colonization.