Optimizing mycorrhizal spore application for effective colonization and early seedling establishment in rice

Arbuscular mycorrhizal (AM) fungi form a symbiotic association with plant roots, promoting plant growth and fostering resilience. However, the efficacy of this symbiotic association highly depends on the nature and abundance of spores, but there is no clear evidence regarding the relationship between the quantity of spores in the inoculum and colonization efficiency. Therefore, the present study was conducted to establish quality standards for AM inoculum and to investigate the effect of varying mycorrhization levels of Rhizophagus intraradices on root colonization and early seedling vigor of rice (Oryza sativa L., var. CO51) under semi-dry cultivation conditions. The experiment was set up with seven treatments (T1-T7), with spore inoculation ranging from 0 to 6 spores per seed, arranged in a completely randomized design (CRD) with three replications. Results evidenced a clear dose-dependent promotive effect of increasing AM spore density on germination percentage, chlorophyll content, seedling biomass and root colonization. Notably, the study also confirmed that a threshold effect of 2 or more spores per seed is required to achieve optimal germination and effective AM colonization. Furthermore, the treatments T6 and T7 (5 and 6 spores per seed respectively) recorded the highest shoot and root lengths, chlorophyll concentration and biomass accumulation throughout the observation period (30 days of plant growth). T7 also exhibited the fastest and highest level of colonization by mycorrhizal fungi, recording efficient early symbiotic establishment. However, results were not strictly linear among treatments, underscoring the significance of establishing threshold and optimal inoculum dosages rather than depending solely on higher doses. These findings highlight the potential of low-dose AM fungal seed biotization in enhancing early rice vigor and mycorrhizal colonization.