Arbuscular Mycorrhizal Fungal Symbiosis Enhances Crop Photosynthetic Traits Under Drought Stress—A Meta-Analysis

The benefits of arbuscular mycorrhizal fungi (AMF) in alleviating plant abiotic and biotic stresses have been well documented; however, how AMF modulate photosynthesis-related processes under different drought intensities is poorly understood. This study quantified the impact of different AMF formulations on the photosynthetic traits in different host plant types under different intensities of drought stress. A total of 52 published studies were included in a meta-analysis with a random-effects model. Synthesizing the research findings revealed that, under drought stress, AMF significantly improved plant photosynthetic rates and nutrient absorption, with the strongest effect on phosphorus absorption (the effect size Hedges’ g = 3.85, 95% CI: 2.76–4.95, p < 0.001). Overall, the between-study heterogeneity was moderate to high (I2 = 64.7%, τ2 = 0.38), indicating variability among the included studies. As drought intensity increased, the effect of AMF on the net photosynthetic rate decreased, with the transpiration rate (Tr) and stomatal conductance (Gs) first increasing and then diminishing. Drought intensity exceeding the ‘moderate’ threshold inhibited both Tr and Gs. The AMF effect on chlorophyll content differed among the plant types, with Hedges’ g being 1.656, 2.715, and 3.231 for herbaceous, grass, and woody plants, respectively. Inoculation with multiple AMF species provided greater benefits than single AMF strains in promoting chlorophyll content (Hedges’ g = 1.949 for single vs. 3.217 for mixture) and net photosynthetic rate (Hedges’ g = 2.242 for single vs. 3.986 for mixture). We conclude that the AMF–plant symbiotic association alleviates drought stress by adjusting the net photosynthetic rate, transpiration rate, and stomatal conductance. The magnitude of these responses varies depending on plant functional type, drought intensity, and AM fungal formulation.