Role of Arbuscular Mycorrhizal Fungi in Enhancing Thermotolerance andGrowth Performance of Onion (Allium cepa L.)

Global warming and increasing heat stress adversely affect crop development. This study
assessed the potential of Arbuscular Mycorrhizal Fungi (AMF) to mitigate high temperature
(>35°C) stress in thermosensitive onion (Allium cepa). A completely randomized design with
four treatments—Control (CT), AMF, high temperature (HT), and HT+AMF—each with three
replicates, revealed significant differences (p < 0.05). HT-stressed plants showed the lowest
shoot height (9.33 cm), root length (474.58 cm), and biomass (2.28 g), whereas HT+AMF
improved these traits by >200%, >140%, and 75%, respectively, demonstrating AMF’s
mitigating effect. Plants with AMF alone produced the highest growth (shoot 35.50 cm, root
1613.43 cm). Enzyme activity (acid and alkaline phosphatase) that peaked under HT, was
reduced by AMF and was intermediate in plants with HT+AMF treatment, indicating
moderation of stress responses. Nutrient acquisition (N, P, K) increased by 188%, 49%, and
24% in plants with HT+AMF treatments relative to HT-stressed plants, with mycorrhizal
dependency highest (47.17%). Mechanistically, AMF improved nutrient uptake and
phosphorus-related enzymatic functions, enhancing physiological stability. PCA and
correlation analyses highlighted biomass, shoot, and root length as primary contributors to
treatment differences and strong associations among nutrient uptake, enzyme activity, and
growth. In conclusion, AMF inoculation enhances thermotolerance in onion, offering a
sustainable strategy for maintaining productivity under rising temperatures.