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Abstract
A pot experiment was conducted under two soil conditions, namely (i) Original paddy soil and (ii) Diluted soil (paddy soil diluted with andosol subsoil; Paddy soil: Andosol subsoil=1:4) to investigate the effects of arbuscular mycorrhizal fungi (AMF) and water regimes on dry matter production, grain yield, AMF colonization, AMF sporulation, and mineral nutrition of wetland rice (Oryza sativa L.). Plants were either inoculated ( + AMF) or not (-AMF) under four water regimes, namely (i) Flooded up to maturity (F); (ii) Non-flooded (kept at 60% moisture of maximum water holding capacity) up to maturity (NF); (iii) Flooded for 3 weeks after sowing and then non-flooded up to maturity (F-NF); and (iv) Non-flooded for 3 weeks after sowing and then flooded up to maturity (NF-F). There were 2×2×4=16 treatment combinations. Generally, plants grown in paddy soil showed a higher plant dry matter content and higher N, P, K concentrations than in diluted soil. Grain yield was higher and grain formation was improved under flooded (F and NF-F) than under non-flooded conditions (NF and F-NF) and grain formation was retarded in the latter resulting in a low harvest index especially in inoculated plants. Mycorrhizal treatment resulted in a decrease in the amount of shoot dry matter under flooded conditions and also in the production of unhulled grains under non-flooded conditions. Though mycorrhizal colonization was highest under non-flooded conditions, a 2–12% colonization level was observed even under flooded conditions at 60 d after transplanting, and 3% remained in the NF-F treatment whereas no colonization occurred under continuously flooded conditions (F) at the maturation stage. AM establishment before flooding had a positive impact on AMF colonization and sporulation in wetland rice cultivation at the vegetative stage. N and P concentrations in unhulled grain were significantly increased due to AMF inoculation whereas AMF significantly decreased shoot N concentration and did not affect shoot P concentration at the maturation stage. These findings suggest that AMF may accelerate N and P transfer from shoots and/or soils to rice grains even under flooded conditions along with the tendency to increase the harvest index.